S

sediment particles settle slowly to the bottom, settling

SALT MARSH ACCRETION
out is aided by natural meandering streams; conversely,
if straight channels are dug into the marsh to aid drainage
Peddrick Weis (as occurs with ditching for mosquito “control”), then the
Department of Radiology, Rutgers – New Jersey Medical tide ebbs faster with less time for settling out. If the
School, Newark, NJ, USA upstream materials are prevented from reaching an estu-
ary, as occurs with dams on and dredging of rivers, and
Definition by construction (urban development), a marsh has less
Salt marsh accretion is defined as growth by deposition of material to accumulate for the accretion process.
suspended particles during flooding (allochthonous
growth) and by accumulation of plant material, both roots Vegetation
and decomposed material from plants growing in the Since high and low marsh have different species of vege-
marsh (autochthonous growth) (Schuerch et al., 2012). tation and since different species are characteristic of these
marsh segments on the several continents, it follows that if
Process the several species cause different rates of accretion to
Salt marsh accretion consists of the addition of both sed- occur, then different marsh areas and geographic regions
iment and detritus from above and adding root tissue will have different accretion rates. The phenomena relat-
below the salt marsh surface. The sediment components, ing to accretion include stem density, plant biomass
both organic and inorganic suspended solids, are water- (both above and below the marsh surface), and rate of
borne particles that have been trapped in the salt marsh detritus formation following autumnal senescence. In fact,
vegetation. The detritus is decaying plant material from it has been suggested, on the basis of both observations
dieback following the growing season. Added together, and modeling, that vegetation actively engineers its land-
the peaty soil of a salt marsh was reported to be >80 % scape, building its environment to soil heights that opti-
organic matter in a Chesapeake Bay salt marsh (Langley mize its stability (Marani et al., 2013).
et al., 2009) and, in a Rhode Island salt marsh, 91 %
and 96 % in low and high marsh, respectively (Bricker- Stem density
Urso et al., 1989). There are many factors influencing The flow of water during tidal flooding is somewhat
the rate and quality of accretion, as shown in Figure 1. impeded by the plant stems. The physical transport of
The result of the many interacting factors is that salt suspended solids is thus slowed down – reduced 16 %
marsh accretion is keeping ahead of sea-level rise in many according to Wolanski et al. (2000) – allowing more time
but not all regions. for (1) adhering to plant stems and (2) settling out of the
water column to the marsh surface.
Suspended solids When observing a salt marsh at low tide, the high tide
Materials washing downstream or resuspended from line on plant stems can be seen because sediments have
under open water tend to settle out in salt marshes, where adhered to the intertidal portion of the vegetation. How-
the water movement is slow. These materials include both ever, Li and Yang (2009) quantified the amount of sedi-
organic and inorganic components. Since suspended ment adhering to vs. trapped by three species of plants

M.J. Kennish (ed.), Encyclopedia of Estuaries, DOI 10.1007/978-94-017-8801-4,

© Springer Science+Business Media Dordrecht 2016
514 SALT MARSH ACCRETION

Storms

+
Flooding
Ice
shearing
Ice rafting
_
+ +

+
+
Depositional Vegetative
processes growth
+

+ + +
CO2
increase

_ NET
Autocompaction VERTICAL
ACCRETION

Salt Marsh Accretion, Figure 1 Modified from Chmura et al., 2001.

in the Yangtze River Delta. They found that 5–10 times as Flooding/Sea-level rise
much weight of sediment settled into joints of leaves and In a greenhouse experiment designed to replicate another
stems than adhered directly to the leaves and stems; the result of climate change, flooding stimulated root growth
total amount of biomass correlated directly with the (Nyman et al. 2006), thus adding to the organic portion
amount of adherent sediment; the amount of adherent sed- of the marsh soil.
iment decreased with elevation from the marsh surface
and with the distance from the marsh edge toward the high Ice
marsh (both of these obviously reflect the amount of time
When wind-driven ice flows are marooned on a marsh, the
of water contact during tidal ebb and flow); and the inva-
debris in the ice can add to the marsh surface. On a marsh
sive (in that region) species Spartina alterniflora trapped
in Maine, this process added anywhere from 0 % to 100 %
significantly more sediment than the native Phragmites
of the total surficial accumulation (Wood et al., 1989).
alterniflora and Scirpus mariqueter. All of these plant
This phenomenon is obviously more relevant at higher lat-
species die back in the fall, adding their sediment-laden
itudes and thus with global warming can be expected to
detritus to the marsh surface.
decrease in significance. On the other hand, ice shearing
can be destructive, removing both organic and inorganic
CO2 increase material from the marsh surface (see Figure 1).
As atmospheric CO2 increases, its effect on stimulating
plant growth has been studied in many areas. With salt Autocompaction
marsh vegetation in particular, a mesocosm experiment As marsh soils continue to accrete, the accumulated
in which CO2 was nearly doubled (simulating expecta- weight squeezes out the water from deeper layers, lower-
tions for the year 2100) enhanced growth, thickening the ing the overall height of the marsh (Orson et al., 1998),
soil by 4.9 mm yr
1 vs. the control plot increase of thus decreasing the height of the apparent accretion.
0.7 mm yr
1, in part due to stimulation of fine root growth
(Langley et al., 2009). This effect is greater in C3 plants Meteorological forcing
such as the sedge Schoenoplectus americanus than in C4 In low marsh areas, which normally have more time under
plants such as the grasses Spartina spp. and Distichlis water, storm strength was determined to be a major factor
spicata (Cherry et al., 2009; Langley et al., 2009). In the in enhancing accretion. In high marsh areas, which nor-
latter, it was not just the CO2 fertilization effect that was mally have less time under water, it is storm frequency that
observed but also an interaction between CO2 and salinity is relevant (Schuerch et al., 2012). This was determined
(amelioration of salt stress). with cores of marshes in Germany’s Wadden Sea where
 SALTMARSHES 515

geochronology was done by radioisotope measurements states. Proceedings of the National Academy of Science, 110,
(see “Methods of measuring accretion”, below) and 3259–3263.
comparing depths with hydrologic data going back Nyman, J. A., Walters, R. J., Delaune, R. D., and Patrick, W. H., Jr.,
2006. Marsh vertical accretion via vegetative growth. Estuarine
to 1938. and Coastal Shelf Science, 69, 370–380.
Orson, R. A., Warren, R. S., and Niering, W. A., 1998. Interpreting
Methods of measuring accretion sea Level rise and rates of vertical marsh accretion in a southern
New England tidal salt marsh. Estuarine and Coastal Shelf Sci-
Vertical cores taken through salt marsh soils are sliced ence, 47, 419–429.
into disks. These disks can then be measured for the radio- Schuerch, M., Rapaglia, J., Liebetrau, V., Vafeidis, A., and Reise,
isotopes 137Ce and 210Pb. These radioisotopes fall to earth K., 2012. Salt marsh accretion and storm tide variation: an exam-
with precipitation. The 137Ce was most heavily deposited ple from a barrier island in the North Sea. Estuaries and Coasts,
from the atmosphere in 1963, the final year of atmospheric 35, 486–500.
nuclear weapon testing, so the depth at which the greatest Wolanski, E., Jamilton, L. J., and Shi, Z., 2000. Near-bed currents
and suspended sediment transport in saltmarsh canopies. Journal
amount is found indicates when that level was deposited. of Coastal Research, 16, 909–914.
On the other hand, 210Pb forms steadily in the upper atmo- Wood, M. E., Kelley, J. T., and Belknap, D. F., 1989. Patterns of sed-
sphere as a result of interaction between atmospheric iment accumulation in the tidal marshes of Maine. Estuaries, 12,
Pb and cosmic radiation; perturbations in the amount of 237–246.
this steady deposition can be correlated with historical
knowledge of meteorological events, both wet and dry.
The 137Ce measurement is useful at the yearly level, Cross-references
while 210Pb measurement is useful at the decadal level. Climate Change
The analysis of pollen grains in the soils is useful at the Mosquito Ditching
Sediment Resuspension
century level if local changes in plant cover are known, Sediment Transport
such as the conversion of forested land to agriculture. Tides

Summary
As seen above, there are many factors that interact in the
accretion process. CO2 increase enhances plant growth, SALTMARSHES
but more in C3 salt marsh plants, which are less salt toler-
ant, than in C4 salt marsh plants, which tend to be more salt Paul Adam
tolerant. Flooding, on the other hand, as is occurring more School of Biological Earth and Environmental Sciences,
often with sea-level rise, can bring higher salinity to University of New South Wales, Sydney, NSW, Australia
a given marsh. In many areas, salt marsh accretion is keep-
ing up with global climate change-induced sea-level rise.
Definition
However, this resilience, as represented by stable equilib-
ria, may be susceptible to “changes in the relative sea-level The coastal saltmarsh ecosystem is defined by both habitat
rise. . . with consequent reductions in the associated biodi- and biota. It is intertidal, occurring on soft shores, and the
versity” (Marani et al., 2013). plant communities are comprised of herbaceous flowering
plants, both monocotyledonous and dicotyledonous, and
small shrubs. The ecosystem can be distinguished from
Bibliography mangroves which are dominated by trees and in which
Bricker-Urso, S., Nixon, S. W., Cochran, J. K., Hirschberg, J. J., and herbaceous understory is absent or rare (Janzen, 1985)
Hunt, C., 1989. Accretion rates and sediment accumulation in and from sea grass communities which are predominantly
Rhode Island salt marshes. Estuaries, 12, 300–317. subtidal or, if intertidal, occur lower on the shore than
Cherry, J. A., McKee, K. L., and Grace, J. B., 2009. Elevated CO2
enhances biological contributions to elevation change in coastal
saltmarsh (see Mangroves).
wetlands by offsetting stressors associated with sea-level rise.
Journal of Ecology, 97, 67–77. Introduction
Chmura, G. L., Heimer, L. L., Beecher, C. B., and Sunderland, Saltmarshes are conspicuous features of many estuaries and
E. M., 2001. Historical rates of salt marsh accretion on the outer
Bay of Fundy. Canadian Journal of Earth Science, 38, are one of the most intensively studied ecosystems in the
1081–1092. world (Chapman, 1960; Ranwell, 1972; Long and Mason,
Langley, J. A., McKee, K. L., Cahoon, D. R., Cherry, J. A., and 1983; Adam, 1990; Pennings and Bertness, 2001; Perillo
Megonigal, J. P., 2009. Elevated CO2 stimulates marsh elevation et al., 2009; Saintilan, 2009a) (see Coastal Wetlands).
gain, counterbalancing sea-level rise. Proceedings of the Coastal saltmarsh is found on low-energy coasts, pre-
National Academy of Science, 106, 6182–6186. dominantly in estuaries, but also in the shelter of barriers
Li, H., and Yang, S. L., 2009. Trapping effect of tidal marsh vegeta-
tion on suspended sediment, Yangtze Delta. Journal of Coastal or where wave energy is dissipated before reaching the
Research, 25, 915–924. marsh. Saltmarsh may be found on the shores of coastal
Marani, M., Da Lio, C., and D’Alpaos, A., 2013. Vegetation engi- lagoons, both permanently open lagoons and ICOLLs
neers marsh morphology through multiple competing stable (Intermittently Closed and Open Lakes and Lagoons;
516 SALTMARSHES

Haines, 2012). ICOLLs are of widespread occurrence, and deposited by high tides at different levels within the marsh
some are very large. While lagoon ecosystems have many during the course of the year (Beeftink, 1979). Some
distinctive features (Barnes, 1980), the fringing material may subsequently be carried back to the sea, but
saltmarshes are, at least floristically, little different from much will eventually accumulate at the level of the highest
those found in permanent estuaries in the same region. tides of the year. At localities close to human settlement,
Saltmarshes occupy the interface between terrestrial the wrack may be collected as fertilizer for local farms or
and marine environments (Valiela et al., 2001) and have gardens, but, increasingly, it is seen as unsightly and
many features and attributes of both, as well as some a source of odors and is removed to “tidy up” the coast.
unique features. The boundaries of saltmarsh may be Nevertheless, wrack provides a significant habitat, and
abrupt and easily defined (Figure 1), or their recognition decomposition of wrack provides an important pathway
may require setting an arbitrary limit along a continuum for nutrient recycling.
of change (Pratolonga et al., 2009). Even if, to the human
eye, the landward boundary is sharp, there may be fresh
groundwater flowing under a marsh, affecting water rela- Global variation
tions of saltmarsh plants and determining biogeochemical Saltmarshes occur at suitable locations on all continents
processes (Valiela et al., 2001; Boorman, 2009). Valiela except Antarctica. They are generally regarded as
et al. (2001) suggest that more attention should be given a feature of temperate coasts being replaced in the same
to studying exchanges into, out of, and within marshes positions in the landscape in the tropics and subtropics by
than to boundaries per se. mangroves (Wolanski et al., 2009). While mangroves are
At their seaward edge, saltmarshes may abut sand/mud- restricted to tropics and subtropics with only minor incur-
flats, sea grass beds, or, in tropical and subtropical regions, sions into temperate zones (most extensively in Austral-
mangroves. At the landward side of saltmarshes, there are asia), saltmarshes occur in the tropics in two situations:
often artificial boundaries to embankments, built develop- (1) on coasts with reliable rainfall, evenly distributed
ment, or transitions to shingle/cobble beaches (Bertness, throughout the year where they may be found as a narrow
2007), sand dunes, or some form of swamp forest or fresh- fringe above mangroves, and (2) as much more extensive
water wetland. The upper boundary of saltmarsh is saltmarsh stands occurring on drier, often markedly sea-
frequently marked by the presence of a driftline of wrack sonal, coasts where hypersalinity in the upper intertidal pre-
and other material (increasingly plastic; Adam, 1990; cludes mangrove development (Adam, 1990; Costa et al.,
Wolanski et al., 2009). The natural components of wrack 2009). As salinity increases, vascular plants in the upper
may originate from within the marsh itself or be carried intertidal become very sparsely distributed, although
into the estuary from the catchment (timber, leaf litter) or microalgal and microbial mats may be extensive; any
from the sea (algae and sea grasses). Material may be boundary between flats and sparse saltmarsh is arbitrary.

Saltmarshes, Figure 1 Saltmarsh abutting the limestone cliff of Humphrey Head, Morecambe Bay, northwest England.
 SALTMARSHES 517

Saltmarshes as ecosystems support a wide range of

organisms, but it is the vascular plants which are the most
obvious component. The distribution of plant taxa permits
the recognition of different types of saltmarsh at the global
scale (Chapman, 1953; Chapman, 1960; Chapman, 1977).
Chapman’s classification was adopted by Ibanez
et al. (2013). Adam (1990) proposed a variant of this
model, suggesting broad similarities between temperate
marshes in both hemispheres. With one conspicuous
exception, Spartina dominated marshes on the West
Atlantic coast of the Americas, the distribution of the
major types of saltmarsh can be related to latitude. In terms
of floristic richness, saltmarshes provide an exception to
the general rule that within biomes, species richness is
greatest in the tropics and declines with increasing lati-
tude. Tropical saltmarshes have very low species diversity,
and the highest diversity is shown in high latitude temper-
ate marshes. This pattern is shown both in the northern
hemisphere (Adam, 1990) and in Australia (Saintilan,
2009b; Boon et al., 2011) and South America (Issach
et al., 2006), although in this latter case, at the highest lat-
itudes in Tierra del Fuego, there are floristic links to north-
ern hemisphere temperate marshes. Saltmarshes on coasts
experiencing Mediterranean climates share strong similar-
ities in floristics, vegetation structure, and physiognomy.
Whether biogeographic patterns of saltmarsh fauna at
a global scale are congruent with floristics has not been
fully investigated.

Variation within estuaries

Environmental conditions vary in space and time within
estuaries, and this is associated with changes in the fring-
ing saltmarshes (Ranwell, 1968). Some changes relate to
the stage of geomorphological evolution of estuaries
(Roy, 1984; Roy et al., 2001), others to the gradient in Saltmarshes, Figure 2 Zonation of saltmarsh vegetation at
salinity from the mouth to the head of the estuary. In gen- Humphrey Head, Morecambe Bay, northwest England. Low
eral, conditions within saltmarshes are more saline marsh (to left) dominated by Puccinellia maritima, mid-marsh
towards the mouth of an estuary, while at the limits of dominated by Festuca rubra, and upper marsh dominated by
incursion of saline water, freshwater tidal marshes may Juncus maritimus.
occur (Odum, 1988; Temmerman et al., 2003). The head
of estuaries was often the location of the first established
tidal limit (Figure 2). The proportion of the tidal range
human settlements, so that there is a very long history of
occupied by saltmarsh varies between sites; where Spar-
destruction of freshwater tidal marshes, which in many
tina spp. occupy the lower zone they are able to endure
parts of the world are now rare. Between the saline and
more frequent inundation than other species. In Victoria
the freshwater zone, there may be brackish marshes,
(Australia), the introduced Spartina anglica occurs below
which are variable in their species composition (Sainty
the mangrove zone (Avicennia marina) (Boon et al.,
et al., 2012). Species characteristic of more saline
2011). The boundaries between zones are often (but not
sites may occur in the brackish zone, but the zonation
necessarily) sharp even if the topographic gradient is
patterns may be inverted (Gillham, 1957), with the saline
slight (Marani et al., 2013) (see Species Zonation).
zone species being restricted to the higher, less fre-
Two principal interpretations of zonations have been
quently flooded parts of marshes, where evapotranspira-
advanced (Davy, 2000): that they are a spatial expression
tion between flooding tides results in more saline
change over time (succession) or that the distribution of
conditions.
species is controlled by environmental factors and/or com-
petitive interactions between species. Davy (2000) warned
Variation within marshes against too ready assumption of successional interpreta-
One of the almost universal features of saltmarshes is tion and emphasized that succession and zonation should
the zonation of species from low marsh to the upper not be treated as synonymous concepts.
518 SALTMARSHES

Static zonations are widespread; for example, on rocky systems (e.g., Mississippi, Nile, Yangtze, and Venice
intertidal shores or as the bands of vegetation related to lagoon), saltmarshes are now suffering “sediment starva-
altitude on mountains. The position of boundaries tion,” compromising the ability of saltmarshes to respond
between species, or between communities, is determined to disturbance or to future rises in sea level. Mudd (2011)
by species’ responses to environmental factors and by has suggested release of pulses of sediment into estuaries
interactions between species. The patterns could be to preserve marshes.
redrawn by climate change, major tectonic events, or the
invasion of introduced species but, in the absence of these
external forces, will remain stable. There are also many Succession on saltmarshes
well-documented examples of zonations which reflect The development of saltmarshes is affected by a range of
succession, for example, the plant communities on physical processes and interactions between these pro-
moraines deposited by retreating glaciers. cesses and the biota, both flora and fauna (Allen, 2000;
Are the zonations on saltmarshes expressions of suc- Reed, 2000). The position of saltmarsh relates to that of
cession? The zonation may continue beyond the tidal sea level, which over geological time has varied consider-
limit, for example, the zones of swamp forest ably. Following the last glacial maximum, sea level rose to
(Casuarina, Melaleuca, and Eucalyptus) found inland of reach approximately its present position 6000 years ago.
saltmarsh in eastern Australia (Pidgeon, 1940). While Since then, there have been considerable changes in the
Pidgeon (1940) suggested that this zonation reflected suc- occurrence and extent of intertidal marshes. There is
cession, there is little evidence for progression from ample empirical evidence that saltmarshes can develop,
saltmarsh to nontidal communities. In many parts of the or erode away, in short periods (Oliver, 1906; Oliver,
world, the most seaward saltmarsh communities today 1907; Packham and Liddle, 1970; Pringle, 1995; Adam,
are dominated by introduced species of Spartina. The 2000; Davy, 2000). As a marsh develops, it can both pro-
zonation of communities above the Spartina zone reflects grade (extend farther seaward) and accrete (increase in
succession from the original (pre-Spartina) pioneer, but, if surface elevation). The two processes frequently
the marshes continue to prograde and succession from co-occur, but accretion can be maintained, or even
Spartina occurs, it is unlikely that there will be increased, if, when, a marsh front is eroding, sediment
a recapitulation of historic succession, rather a new range released by erosion is deposited on the remaining marsh.
of communities may develop (Adam, 1990; Davy, 2000). Low- and mid-marsh zones can develop over short
During the course of marsh development, management periods – decades (Adam, 2000) or one or two centuries
regimes may change. The introduction or the removal of (Pethick, 1980; Pethick, 1981). Upper marshes may be
livestock, or changes in the abundance of native grazing stable for very much longer – 2,000 + years (Pethick,
animals, can change the occurrence and abundance of 1980; Pethick, 1981) in East Anglia, United Kingdom,
plant species, so that, even if zone boundaries remain at and 4,000 + years in northeastern United States
the same elevation, the species composition within zones (Redfield, 1972). Upper marsh zones are frequently not
may change, so the communities now occurring are not homogeneous, but are complex patterns of species and
those present at the same elevations earlier in the marshes’ communities reflecting the operation of a range of ecolog-
development. ical factors rather than simple succession.
In northern temperate regions at the end of the last gla- Accretion requires an increase in surface height, gener-
ciations, the newly exposed land surface would have been ally as a result of accumulation of sediment, either
prone to erosion, but sea level was much lower than at pre- minerogenic (allochthonous) – sand, silt, or clay carried
sent so deposition would have occurred at locations now in by the tide – or autochthonous accumulation of organic
beneath the sea. When sea level reached its present posi- material (from plant roots or incorporated stems and
tion about 6000 years ago, the landscape was well vege- leaves) or varying combinations of the two. Marsh growth
tated and rates of erosion would have declined. commences with the establishment of pioneer plants,
Clearance of forests and the development of agriculture which promotes sedimentation. In the earlier stages of
would have initiated a new phase of increased erosion. marsh development, sediment accumulation is not neces-
In Europe, this would have occurred more than 1,000 sarily continuous in space or time. Sediment deposited
years ago, but in North America, major clearing following may be eroded. However, as the density of vegetation
European colonization was only a few centuries ago increases, more sediment is retained, and the marsh sur-
(Kirwan et al., 2011). In North America, the increased sed- face rises. With increasing elevation, the rate of accretion
iment input into estuaries stimulated a phase of saltmarsh will decline as the number of flooding tides decreases
growth (Kirwan et al., 2011). Recognition of the damage and the flooding water will have already passed through
caused by erosion in catchments and the initiation of soil the vegetation of the lower marsh where sediment will
conservation measures has reduced sediment input into have been deposited. The rate of allochthonous sedimen-
rivers. Construction of dams and other water management tation would be expected to fall close to zero at the tidal
works has reduced sediment transport into estuaries limit. Studies in eastern England (The Wash – Kestner,
throughout the world (Walling, 2006; Walling, 2008). 1975; North Norfolk – Pethick, 1981) showed that, while
A consequence of this is that in a number of major estuary accretion in the lower marsh declined as elevation
 SALTMARSHES 519

increased, as the model predicted, the upper marsh surface and pans are absent from many sites, although shallow
reached an asymptote lower than that of the highest bare areas may occur (Figure 4) (Adam, 1997). Pan devel-
predicted tide. Allochthonous sedimentation on vegetated opment occurs early in succession, with patchy distribu-
surfaces occurs in two ways: through capture of sediment tion of pioneer plants leading to sediment deposition
onto leaves and stems and through settling out of sediment around plants, and depressions between them
from water stilled by the vegetation (Mudd et al., 2010). (Yapp et al., 1917; Goudie, 2013) (Figure 3a). As the veg-
Input of sediment is not the only factor determining marsh etated surface continues to rise and consolidate, the
elevation; Cahoon (2006) identified eight processes which depressions form pans (Figure 3b). In addition, localized
influence changes in elevation, both positively and nega- bank slumping produces blockages in creeks resulting in
tively. Sediment input could be countered by, for example, chains of channel pans (Yapp et al., 1917). If pans were
compaction or shrinkage (see Salt Marsh Accretion). formed only early in succession, then their density would
The dropping of sediment or accumulation of organic be similar regardless of marsh age. Pethick (1974) demon-
material creates a topographic gradient. The separation strated that in eastern England the density of pans
of species along this gradient to give rise to zonation is increased in high mature marsh, so the Yapp et al. (1917)
a response to a variety of factors. Conventional wisdom model is not a complete explanation of pan formation.
suggests that the lower (seaward) limit of species is deter- There are a number of different possible mechanisms of
mined by responses to physical factors, while interaction formation of additional pans. Pethick (1974) suggested
between species is more important in the upper marsh. that local deposition of wrack could smother vegetation,
However, even at the lowest limits, marsh biotic interac- leading to death followed by erosion to create pans. On
tions can be important. Doubts that the lower limit of the some coasts, ice scour might also be a mechanism of pan
pioneer Salicornia europaea in southeast England was formation in high marshes. Importantly, the upper marsh
set by tidal action were raised by Gerdol and Hughes pans described by Pethick (1974) have the same general
(1993), who showed that the lower limit of Salicornia form as those discussed by Yapp et al. (1917) – steep sided
coincided with the upper limit of the amphipod and generally water filled (as illustrated by Figure 2.3 in
Corophium volutator. Removal of Corophium permitted Steers, 1977). Also widespread in some high-marsh zones
seedlings transplanted to lower levels to survive, are extensive shallow depressions, either bare or sparsely
suggesting that disturbance of the sediment by the amphi- vegetated, which are variously referred to as pans, pannes,
pod prevented Salicornia from reaching its physically or, as in Clarke and Hannon (1967), “rotten spots.” These
determined lower limit. might develop as a consequence of poor drainage
(Ewanchuk and Bertness, 2004) or as a result of the devel-
opment of a hypersalinity through evapotranspiration in
Creeks and pans summer. The extent of these bare areas may vary over
Saltmarshes are rarely simple inclined planes; generally, time. Hamilton (1919) described more widespread and
they have complex internal topographic variation, with extensive bare areas in the marshes of the Sydney region
creek and pan systems present. The form of creek systems (Australia) in the early twentieth century than are present
varies considerably. On very sandy marshes, the creek sys- in the early twenty-first century. Loss of vegetation may
tems are simple and less dense than those on muddy sub- also be caused by the use of recreational vehicles
strates (Chapman, 1960). Pye (2000) recognized six (Kelleway, 2005), and naturally bare areas may be
different arrangements of creek forms in southeast extended, or have recolonization prevented, by vehicle
England. use (Figure 4).
Creeks, as well as providing drainage, represent an
extension of the estuary water body and mudflat habitats
into the marsh. Creeks also generate additional topo- The environment
graphic complexity within marshes. As flooding tides Environmental conditions within saltmarshes are very
overflow the creek banks, sediment is deposited to create much determined by the tide. The tidal regime varies
levees and, consequently, basins between creeks between sites, and estuarine saltmarshes experience the
(Beeftink, 1977; Temmerman et al., 2004). The well- full range of tidal regimes. Tidal ranges vary from macro,
drained soils of the levees provide a contrast to the poorly including locations with the largest ranges in the world
drained, frequently anoxic, conditions of the basins. This (Bay of Fundy, the Bristol Channel), to negligible. In
environmental difference is reflected either by the occur- some regions, there is a seasonal variation in water level,
rence of different species assemblages on the levees or in for example, in the Baltic (Gillner, 1965), or the estuaries
the basins (Beeftink, 1977; Adam, 1990) or, in species of southwest Western Australia (Brearley, 2005). ICOLLs
poor zones, by taller growth on the levees. may, when closed, be nontidal for extended periods, but,
As well as creek systems, the marsh surface may be nevertheless, the conditions established during opening
interrupted by pans, steep-sided pools. The density of pans periods determine the structure and function of any fring-
varies considerably between marshes and in some sites is ing saltmarshes.
low. In southeast Australia, where marshes occur as The number of tides reaching particular positions on the
a zone on the landward side of mangroves, both creeks marsh surface varies with elevation. The lower marsh may
520 SALTMARSHES

Saltmarshes, Figure 3 (a) Pioneer saltmarsh in the Duddon Estuary, northwest England. Sedimentation in patches of Puccinellia
maritima creates hummocks, with bare areas in between. (b) Pans in a saltmarsh in the Leven estuary, northwest England. A heavily
grazed marsh with the vegetation 2–5 cm tall.

be reached by nearly every tide (although where man- establishment, newly germinated seedlings being vulnera-
groves are found as a seaward zone even the lowest levels ble to uprooting. The timing of tides and of rainfall
of marsh may be flooded by relatively few tides). The (germination of many halophytes is promoted by salinity
upper levels of marsh are reached by spring tides, at reduction) means opportunities for seedling establishment
the highest levels possibly only once or twice a year. In are limited.
the lower marsh, with frequent flooding, soil salinity and Explanations of the patterns of species and community
soil aeration are reasonably constant; at higher elevations, distributions within saltmarshes have traditionally empha-
there is greater variability. Conditions are strongly sized the importance of physical factors and assigned
influenced by climate. Rainfall can lower soil salinity, a limited role to biotic interactions, an example being the
and dry periods can result in hypersalinity. The tidal range description of the holocoenotic complex by Clarke and
will determine the depth to which the lower marshes flood Hannon (1969). Nevertheless, the role of plants was rec-
and importantly, the velocity of the tidal current. The ognized very early in scientific investigations of saltmarsh
speed of the current can influence the success of seedling geomorphology. Similarly, the role of grazing by livestock
 SALTMARSHES 521

Saltmarshes, Figure 4 A large bare area in a saltmarsh on the Kurnell Peninsula, Botany Bay, Australia. This panne may be of natural
origin, but any regeneration has been limited by the use of the area by off-road bicycles.

in determining the species composition of vegetation has grazers which have been increased by reduction in preda-
long been recognized in Europe (Adam, 1978). However, tor pressure. Osgood and Silliman (2009) discount eutro-
until recently, the importance of invertebrates and the phication as a major factor in dieback, although
effects of human activities in estuarine ecosystems were acknowledging that it contributes to the general stresses
not fully appreciated (Silliman et al., 2009a). In the United on marshes.
States, there have now been several convincing demon- Eutrophication is widespread in estuarine and coastal
strations that key consumers can alter vegetation, for waters in the United States (Scavia and Bricker, 2006)
example, following increases in populations of snails and elsewhere in the world. Deegan et al., (2012) propose
(Silliman et al., 2009a) and crabs (Holdredge et al., that eutrophication is a driver of saltmarsh loss. The
2008; Altieri et al., 2012). The increases in grazers were mechanism suggested is that nutrient enrichment pro-
most probably initiated by reduction of predator numbers motes increased aboveground productivity and reduction
as a result of harvesting by both professional and recrea- (or maintenance of) below ground biomass so that
tional fishers. there is a reduction in the binding of sediment, leading
to collapse of creek sides and erosion of the lower
Dieback marsh zone.
In the 1950s, extensive areas of Spartina anglica marshes The mechanisms suggested as contributing to both die-
in southern England exhibited dieback (Goodman back and losses from eutrophication are not unique to
et al., 1959), although despite considerable investigation, North America. Why have similar phenomena not been
no single causal factor was isolated. In the early twenty- reported from saltmarshes in other continents? Are there
first century, dieback of thousands of hectares of Spar- features of Spartina marshes which render them particu-
tina-dominated marshes on the Gulf and Atlantic coasts larly vulnerable?
of the United States also occurred (Alber et al., 2008;
Osgood and Silliman, 2009). A single causative factor is
unlikely and Osgood and Silliman (2009) suggest that Biota
interactions between several factors are involved: (1) cli- The biota of saltmarshes can be categorized in a number of
mate, with severe drought being a trigger; (2) drought, different ways – by conventional taxonomy, by whether
which could be responsible for changes in soils, including species are of marine or terrestrial origin, and by pattern
increased salinity, acidity, and bioavailability of metals, of occupancy of saltmarsh habitats (permanent residence,
the stresses either directly killing Spartina rendering it residence for particular stages of life cycle, migrant, or
more susceptible to pathogens; and (3) populations of opportunistic).
522 SALTMARSHES

Saltmarshes, Figure 5 A diversity of salt glands in saltmarsh plants. (a). Spartina anglica. (b). Samolus repens. (c). Frankenia pauciflora.

Vascular plants appears to have evolved independently on a number of

The vascular plants growing in the intertidal zone face two occasions (Flowers et al., 2010). Viewed in isolation, there
particular challenges: salinity and the consequences of are no features which immediately identify a plant as
inundation. Of these, salinity has been the most studied, being a saltmarsh species. This differs from the much
and there is now a detailed understanding of the mecha- smaller pool of species which constitute mangroves,
nisms conferring salt tolerance (Flowers et al., 1977; where features such as possession of aerial roots or
Flowers et al., 1986; Flowers and Colmer, 2008; Flowers production of viviparous seedlings are characteristic.
et al., 2010; Munns and Tester, 2008). Plants which tolerate A feature shown by many saltmarsh species is succulence,
saline conditions are referred to as halophytes, in contrast to but this is also displayed by species in a number of other
non-salt-tolerant species, the glycophytes. Halophytes are habitats and on its own would not be sufficient to identify
found not only in intertidal saltmarshes but in a range of species as being from saltmarsh. One feature that might
other salt-affected habitats. As yet there is not a complete indicate that a species is a halophyte is the possession of
list of halophytes, in part because there are parts of the salt-excreting glands (Figure 5) which can be detected
world where saline habitats have not been fully investigated by the occurrence of salt crystals on the surface of leaves.
but also because of lack of agreement on the definition of Salt glands are also found in a number of mangrove
halophyte. There is a continuum of salinity conditions and species. In terms of structure salt glands are similar to
where within the transition from brackish to freshwater con- the range of other secretory structures found in plants.
ditions a limit is drawn between the habitats of halophytes Saltmarsh plants lack the obvious external root modifi-
and glycophytes is arbitrary. Adopting the definition of cations displayed by mangroves, but like many other wet-
ability to complete the life cycle in at least 200 mM salt land plants possess abundant aerenchyma tissue
(Flowers and Colmer, 2008; Flowers et al., 2010), the num- (Armstrong 1978), so that aerobic conditions are
ber of halophytes is probably between 500 and 1,000. In maintained within tissues even when the surrounding sed-
some cases, an entire taxon might be described as halo- iment is anoxic.
phytic, but there are a number of widespread species where Saltmarshes and mangroves today are overwhelmingly
only some genotypes exhibit halophytic traits (Adam, dominated by flowering plants. Nevertheless, the estua-
1990). Some of the species-rich upper saltmarsh communi- rine landforms they occupy existed on earth long before
ties in northwest Europe (Adam, 1981) may be composed, the evolution of the flowering plants some 65 million
at the species level, of species also found in a range of years ago. Did earlier land plants form saltmarshes and
nonsaline habitats. Nevertheless, the saltmarsh forms of mangroves?
the species are adapted to tolerate and thrive under moder- The fossil record of plants is patchy. Many do not have
ately saline conditions and can thus be regarded as halo- structures which are candidates for fossilization, and many
phytic genotypes. This emphasizes the point made by land plant habitats are unlikely to be environments where
Flowers et al. (2010) that halophytes are not unique; all in situ fossilization could occur. Nevertheless, the world’s
plants possess the same features, but the pattern and degree major coal deposits represent the former vegetation of vast
of expression of particular traits are different in halophytes wetlands, many of them occurring in deltas under brackish
from those in glycophytes. or saline conditions. Fossils in these deposits reveal the
Halophytes include species from a number of different occurrence of large arborescent early vascular plants,
families and orders; the ability to tolerate saline conditions which formed communities analogous to mangroves.
 SALTMARSHES 523

It is curious that today none of the survivors of those plant saltmarshes in North America and numerous localities in
lineages occur in mangroves, with the exception of the Europe, larger fucoids contribute substantially to marsh
fern Acrostichum aureum, which is widespread through- productivity (Roman et al., 1990) creating protective
out the tropics (Tomlinson, 1986). There is no indication microclimates and providing habitat for intertidal inverte-
from the pre-flowering plant record of saltmarsh ana- brates (Tyrrell et al., 2012). Eutrophication of waterways
logues. One of the earliest land plants was Rhynia, which can promote overgrowth and smothering of saltmarshes
occurred in what Channing and Edwards (2009) by green macroalgae (McComb and Lukatelich, 1995),
interpreted as a hot mineral-rich spring similar to those particularly by species of Ulva.
found at Yellowstone National Park today. Some at least
of the earliest land plants therefore lived in harsh environ-
Microorganisms
ments, and the high mineral content of the springs might
suggest that tolerance of salinity would have been possi- Microorganisms play essential roles in ecosystem pro-
ble. However, there is no support from the fossil record cesses in saltmarshes, playing important roles in nitrogen,
for early occurrence of extensive saltmarsh. sulfur, and other biogeochemical cycles and in breakdown
Bryophytes are likely to have been early components of of detritus. Microbial films on detritus are an important
the terrestrial flora, but are not generally recognized as food source for browsing invertebrates.
components of saltmarsh floras. However, a number of Recently, there has been considerable interest in the
bryophyte species occur in tidal saltmarshes and in inland interactions between halophytes and microorganisms in
saline habitats (Adam, 1976; Adam, 1990; Garbary et al., the rhizosphere (Ruppel et al., 2013). Bacteria increasing
2008); therefore, while bryophytes are frequently absent the availability of phosphorus and iron have been isolated
and are rarely abundant in saltmarshes, they demonstrate from the rhizosphere of several halophytes (Ruppel et al.,
a capacity to tolerate salinity and might have been more 2013). Arbuscular mycorrhizae, which enhance nutrient
abundant in the past. uptake by plants, were first reported from the roots of hal-
The present-day vascular flora of saltmarshes is ophytes early in the twentieth century (Adam, 1990; Davy
diverse. Barbier et al. (2011) refer to saltmarshes as grass- et al., 2000), but only recently has there been investigation
lands, but while large areas of the world’s saltmarshes are of their functions (Fuzy et al., 2008; Evelin et al., 2009).
grasslands or dominated by physiognomically similar Studies so far have concentrated on individual species;
graminoids (sedges and rushes), there are also large areas placing mycorrhizae in the context of the functions of
dominated by herbs or shrubs, particularly by genera of saltmarsh ecosystems is a research challenge for the
Amaranthaceae (Figure 6). future.
Most saltmarsh communities are dominated by peren-
nial species. Annual plants are found in the low marsh Fauna
zone in those parts of the world where Salicornia occurs The majority of animal phyla can be found in saltmarshes,
and in microhabitats in the mid and upper marsh where although many groups have been little studied. A review
disturbance creates openings in which annual plants are of the fauna of saltmarshes was provided by Daiber
able to germinate. (1982). The majority of the literature assessed in that study
was from North America; the number of studies outside
America has subsequently increased, but there are still
Algae
extensive geographic regions and many faunal groups
Algae are important components of saltmarshes. for which few data are available.
Microalgae are not always apparent to the human eye,
but microalgal films contribute to marsh formation and
ecosystem productivity. Many microalgae have mucus Marine fauna
coatings, and this is important in binding sediment Saltmarsh creeks are an extension of the estuarine water
(Coles, 1979; Underwood, 2000). Establishment of body, and for nekton (species within the water column)
microalgae on mudflats may promote development of movement between the main estuary and creeks is readily
microtopography (van de Koppel et al., 2012), facilitate possible. On high tides when creeks are bank full and
the establishment of pioneer vascular plants (Coles, overflow to flood the marsh surface, nekton can move
1979), and protect the surface of the lower marsh from from the creeks onto the marsh surface. There are, how-
erosion. Mason et al. (2003) showed that microalgae were ever, differences in the age and size structure of nekton
susceptible to even low concentrations of triazine herbi- assemblages in marshes compared with those in the main
cides, suggesting that runoff of herbicides from agricul- estuary. Saltmarsh creeks are important nursery habitats
tural land into estuaries might initiate erosion of for many fish and crustacean species and are thus essential
saltmarshes. for supporting the estuary ecosystem. As a consequence,
Macroalgae may be conspicuous components of they also sustain many commercial fisheries upon which
saltmarshes. In western Ireland and western Scotland, local communities may depend for employment and from
very dense swards of dwarf fucoids (only 1 or 2 cm tall) which the larger human community gains part of its food
dominate lower marshes (Adam, 1981), and on Atlantic supply.
524 SALTMARSHES

Saltmarshes, Figure 6 Saltmarshes are not always grasslands. (a) Saltmarsh at Bosham, southern England. The large grey-foliaged
shrub is Atriplex (Halimione) portulacoides. (b) Saltmarsh at the Sydney Olympic Park, Homebush Bay, Australia, dominated by the
succulent subshrub Sarcocornia quinqueflora (Amaranthaceae (Chenopodiaceae)). The marsh is fringed by swamp oak (Casuarina
glauca) forest.

Upper saltmarshes are visited by nekton when flooded Individual larvae are very small, but they are extremely
by spring tides; these flooding events are brief and may abundant. They represent a high-quality food resource
not occur on every spring tide cycle during the year. As for fish, which consume them in large numbers. Thus,
such, the utilization of the upper marsh by nekton has been although high marshes are limited in extent and only
considered to be accidental and probably of little value to accessible for short periods of time, they may play
the visitors. Studies in eastern Australia challenge this a disproportionately large role in estuarine fish ecology,
assumption (Hollingsworth and Connolly, 2006; both directly for species which access the marshes and
Mazumder et al., 2006; Mazumder, 2009; Platell and indirectly higher up the food chain in estuaries and adja-
Freewater, 2009; Mazumder et al., 2011). Breeding of cent coastal waters.
saltmarsh crabs is linked to the tidal cycle, with release The marine benthic fauna is diverse and occupies
of zoea larvae coincident with high spring tides. a range of habitats. The regularly inundated unvegetated
 SALTMARSHES 525

banks and sides of creeks support a similar range of spe- Terrestrial fauna
cies to those of intertidal mudflats. Marsh surfaces are The terrestrially derived fauna of saltmarshes is large, but
habitat for a range of species including large numbers of many components of it have not been as well studied as
crabs and marine gastropod molluscs. Within the sedi- the marine component.
ment, there is an abundance and diverse range of
meiofauna, whose survival in anaerobic mud may be
enabled by the creation of an oxygenated rhizosphere Invertebrates
(only a few millimeters thick), sheathing the roots of vas- For many members of the public, the terrestrial fauna which
cular plants as a result of oxygen loss from aerenchyma are of most concern are mosquitoes and biting insects.
tissues (Teal and Kenwisher, 1966; Teal and Wieser, There are a number of diseases which can be transmitted
1966; Osenga and Coull, 1983). Interactions between to humans by mosquitoes, and with growing human
larger benthic species and flora may be important. The populations close to saltmarshes and the possibility of
sediment input into marshes may contain a high propor- increased insect populations and greater incidence of path-
tion of fecal and pseudofecal pellets (Frey and Basan, ogens as a consequence of global warming, there is likely to
1985), which, as well as influencing particle size, may be greater pressure for implementation of control measures
be an important source of bioavailable nitrogen and phos- (Dale and Breitfuss, 2009). There has been a long history of
phorus for plant growth (Long and Mason, 1983). Crab ditch construction in saltmarshes in the United States with
species may be present in large numbers and, through their the object of reducing insect populations (Gedan et al.,
burrowing, may influence microtopography, sediment aer- 2009), but the consequences of the ditching for the whole
ation, and chemistry and drainage (Bertness and Miller, ecosystem were not initially given consideration. In sub-
1984). tropical saltmarshes in Australia, shallow ditches, referred
The interactions between the fiddler crab Uca pugnax to as runnels, have been dug and have apparently been suc-
and the dominant vascular plant Spartina alterniflora cessful in reducing mosquito populations but with few other
were described by Bertness (1985). The crabs are absent effects on the marshes (Dale and Breitfuss, 2009).
from the mudflats in front of the marsh, as the sediment Many of the insects in saltmarshes do not encounter the
is insufficiently consolidated to permit burrows to be impacts of tidal flooding, as they utilize tall vegetation
maintained. They are also absent from areas of the short which remains emergent even at high tide. However, her-
form of Spartina alterniflora where the dense root map bivorous species (grazers and sap suckers) must be
precludes burrow formation. However, they are abundant adapted to process plant tissues with high salt content
in stands of tall Spartina alterniflora where root densities and low water potentials.
are lower. Bertness (1985) proposed that there was There are other species which are behavioral and
a facultative mutualism between the crabs and tall Spar- physical adaptations to either survive or avoid the impacts
tina such that the plant root density was sufficient to pro- of tidal submergence (Treherne and Foster, 1979;
vide firm sediment to maintain burrows but not so dense Foster WA, 2000). Some of the most challenging condi-
as to prevent burrowing, while improved drainage and aer- tions for insects on saltmarshes are experienced by species
ation created by burrows promotes plant productivity and of aphids which live on plant roots (Foster WA, 2000).
maintains the tall growth form of the grass. Not all Beetles can utilize saltmarsh pools, although not neces-
marshes support such abundant crab populations, but the sarily at all stages of their life cycle (Foster GN, 2000).
example demonstrates the ability of invertebrates to struc- There is also a diversity of ground-living beetles in upper
ture the environment, and even in the absence of crabs, marsh grasslands (Luff and Eyre, 2000). The food plants
other taxa may play a similar, if quantitatively lesser, role. of the larvae of a number of larvae can survive occasional
The spatial scale over which individual invertebrates on submergence by seawater (Agassiz, 2000). Some
saltmarshes feed may be small. Saintilan and Mazumder saltmarsh lepidopterans are rare and of conservation con-
(2010) took advantage of the difference in photosynthetic cern (Agassiz, 2000; Relf and New, 2009), but others are
pathway in the two dominants of the saltmarsh in eastern very abundant, for example, in Australia the small Sam-
Australia, the C4 grass Sporobolus virginicus and the C3 phire Blue Theclinesthes sulpitius whose larvae feed on
succulent subshrub Sarcocornia quinqueflora, to investi- samphires (succulent shrubby chenopods) (Orr and
gate the feeding ecology of two grazing crab species and Kitching, 2010).
a marine gastropod. The different photosynthetic path- Anthills occur in other saltmarshes in northern Europe
ways result in the plant tissues having different carbon iso- and, although higher than the surrounding marsh surface,
tope signatures which can be traced from the plants to may be completely submerged during spring tides (Kay
herbivores. The two plants occur in a mosaic of virtually and Woodell, 1976). Anthills provide a habitat for
monospecific patches. The study showed that Sporobolus a number of plant species which are otherwise restricted
was the species directly consumed and that individual in the distribution in marshes (Woodell, 1974). There are
grazers had territories of at most a few square meters. In a number of reports of ants occurring in saltmarshes else-
the Sarcocornia-dominated patches, the major energy where. The role of ants in processes such as seed distribu-
source for the fauna was fine organic material, rather than tion and pollination in saltmarshes has not been
living plant tissues. investigated (Adam, 1990).
526 SALTMARSHES

There is a considerable diversity of spiders in saltmarshes,

and they are major predators in the ecosystem (Barnes, 1953;
Heydemann, 1979; Grimshaw, 1982; Petillon et al., 2007).
Some species escape flooding tides, but a number of species
can survive inundation (Petillon et al., 2009).
Terrestrial invertebrates on saltmarshes exhibit
a zonation comparable to that of plants (Heydemann,
1979). In some cases, this is due to a direct relationship
with particular plant species, while in other cases, it is
due to the animals responding to a similar environmental
gradient which determines plant distribution.

Terrestrial vertebrates
The most obvious vertebrates utilizing saltmarshes are
birds. Many species fly over saltmarshes or are occasional
visitors, but there are species which are particularly asso-
ciated with saltmarshes.
In the northern hemisphere, large flocks of waterfowl –
ducks, geese, and swans – are a feature of many
saltmarshes for at least part of the year. Many of the spe-
cies are migratory, breeding in the Arctic in summer and Saltmarshes, Figure 7 Black-winged stilt Himantopus
overwintering farther south. The populations of himantopus foraging amongst Sarcocornia quinqueflora at
a number of these species, particularly geese, have Sydney Olympic Park, Homebush Bay, Australia. Black-winged
increased considerably in recent decades primarily in stilts occur in all continents except Antarctica. In Australia, most
response to changed agricultural practices which have birds are resident but are mainly migrants in the tropical north.
increased the availability of winter food, leading to higher
survival rates. In North America, the lesser snow goose
(Chen caerulescens caerulescens) population breeding secure undisturbed roosting sites is important to minimize
adjacent to Hudson Bay in northern Canada has caused energy loss in birds preparing for long-distance flights. An
extensive damage to coastal marshes (Jefferies et al., essential attribute of roosting sites is that they provide
2006), with loss of vegetation results from the grubbing uninterrupted sightlines so birds can detect potential dan-
up of roots and rhizomes; the development of ger; waders will not roost in tall vegetation. Tall vegeta-
hypersalinity in the newly bare areas and the compaction tion, such as Spartina alterniflora or Phragmites
of the soil through trampling by numerous geese prevent australis, is habitat for secretive and rarely observed rail
recolonization (Jefferies et al., 2006; Henry and Jefferies, species (see Shorebirds).
2009). On the wintering grounds, grubbing by geese can Waterfowl and shorebirds are potential vectors for the
be a major but localized disturbance (Smith and Odum, transport, either internally or externally, of seeds
1981), but geese also utilize extensive areas of cropland, (Proctor, 1968) and invertebrates (Frisch et al., 2007).
so the damage to temperate marshes is far less than that Many species of passerine birds are found on
in the Arctic. In northern Europe, overwintering water- saltmarshes, and a particular feature of North America is
fowl help to maintain low grass vegetation on saltmarshes. the number of sparrow taxa in saltmarshes which are of
Whether impacts of increased breeding populations on conservation concern (Greenberg et al., 2006). In Britain,
Eurasian Arctic marshes are as extensive as those in North a large proportion of the world population of twite
America is not reported in readily accessible literature. Carduelis flavirostris overwinter on saltmarshes, where
The impact of geese is a very striking example of con- they feed on seeds of a number of species (Norris,
sumer pressure controlling ecosystem structure. 2000). In Australia, one of the world’s rarest birds, the
Shorebirds (which include wading birds) also include orange-bellied parrot Neophema chrysogaster, overwin-
many migratory species. The migratory paths of some spe- ters on saltmarshes in the southeast mainland, feeding on
cies extend between hemispheres. Wading birds are the seeds of samphires.
smaller than waterfowl but are the long-distance flight Prior to forest clearance and the development of agri-
record holders. Satellite telemetry has revealed that culture, saltmarshes were one of the few open habitats in
bar-tailed godwits (Limosa lapponica) are capable of temperate regions and may have been important refuges
making nonstop flights of over 10,000 km from breeding for large herbivorous mammals (Levin et al., 2002). Today
territory in northern Alaska to New Zealand (Gill et al., on many marshes, the original large grazers have been
2005; Battley et al., 2012). Shorebirds mostly feed on replaced by livestock, but various species of deer utilize
mudflats or in shallow pools on marshes (Figure 7), but marshes, and, in Australia, saltmarshes are locally grazed
saltmarshes provide high-tide roosts. The availability of by large kangaroos and wallabies.
 SALTMARSHES 527

Small mammals, such as rabbits and hares, may be

obvious on some marshes. Other species such as voles
can be abundant but remain well hidden. North American
marshes are notable as habitat for a number of small rodent
taxa with limited geographic distributions which are now
regarded as threatened (Greenberg et al., 2006).
One group of mammals frequently ignored in assess-
ments of saltmarshes is bats. Spencer et al. (2009) have
documented the utilization of saltmarshes by
Microchiroptera feeding on the abundance of flying
insects. As many bat species are of conservation concern,
this finding suggests that investigation of bats on
saltmarshes should be undertaken more widely.

Invasive species
Invasion of ecosystems by introduced species is one of the
factors seen as a major threat to biodiversity in most of the
Saltmarshes, Figure 8 Spartina maritima at Knysna lagoon,
world’s environments (see Introduced Species, Invasive South Africa, with a Grey Heron Ardea cinerea.
Species). In the marine environment, estuaries are recog-
nized as being particularly exposed to invasion (Levin
and Crooks, 2011), and shipping is the major vector for a smaller number of perennial species which undoubtedly
introductions, either through ballast (historically from have major impacts. The two most important examples
stone ballast, more recently in water ballast) or as fouling are the genus Spartina and Phragmites australis.
organisms on ships’ hulls. An issue with detecting intro- Spartina is a genus of halophytic grasses which is most
ductions is lack of reliable baseline data for many regions diverse in the Americas. One species, S. maritima, is
and for many taxonomic groups. For example, it will be apparently native in the Old World, occurring in both
difficult to determine whether many microalgae or micro- Europe and Africa (Figure 8). Spartina species are now
organisms in estuaries are indigenous or introductions. found in temperate and subtropical saltmarshes worldwide
Increasing population numbers of a species might be sug- as a result of numerous documented deliberate introduc-
gestive of recent arrival, but recent arrival does not prove tions and natural spread from points of introduction
introduction. Natural processes of dispersal and establish- (Strong and Ayres, 2009). The introduced Spartina
ment did not end with the advent of humans. An increas- spp. can occur lower on the shore than other species, so
ing population could have been triggered in indigenous one of the reasons for introduction was to stabilize mud-
species by changes in the environmental conditions. Mod- flats. A consequence has been a loss of intertidal mudflats,
ern molecular genetic techniques may assist in both an important habitat in their own right.
detecting and determining the status of introductions, but One of the features of Spartina is the potential for inter-
their application requires adequate and appropriate sam- specific hybridization. This is a chance event, with only
pling. There has been controversy over whether the gas- a low probability of occurrence. However, successful
tropod Littorina littorea on the eastern American hybridization between the American Spartina alterniflora
seaboard was an introduction from Europe or whether and the indigenous S. maritima occurred in Southampton
unique American haplotypes indicate native status. The Water on the English Channel coast in the nineteenth cen-
apparently unique American haplotypes are probably tury, initially producing the sterile S. x townsendii which
a consequence of under sampling, and a number of lines subsequently gave rise to the fertile S. anglica (Strong
of evidence make a very strong case for introduction and Ayres, 2009). S. anglica is now the low marsh domi-
(Chapman et al., 2008). nant on much of the North European coast and also occurs
In the case of vascular plants, the very wide, unquestion- widely elsewhere as a result of deliberate introductions
ably native, distribution of many species in saltmarsh and (Ranwell, 1967). In the 1970s, the US Army Corps of
mangroves might suggest that species had reached the Engineers introduced the eastern American
limits of distribution through natural means, with little S. alterniflora into California. Hybridization with the
scope for further introduction. The deliberate introduction West Coast native S. foliosa occurred; the hybrid is rapidly
and subsequent establishment of mangroves in Hawaii spreading and through direct competition and continued
and California disprove the hypothesis (Sauer, 1988). In pollen flow threatens the survival of S. foliosa (Strong
the case of saltmarshes, large numbers of introduced plants and Ayres, 2009).
have been recorded. Many of these are annual species, The common reed Phragmites australis has one of the
recorded from upper saltmarshes, particularly in Mediterra- widest natural distributions of any vascular plant, occur-
nean climate regions. Whether these species have major ring in all continents except Antarctica. A European strain
biological impacts is uncertain. There is, however, of P. australis was introduced into North America in the
528 SALTMARSHES

late nineteenth century and during the twentieth century extensively in Australasia. This gives rise to questions for
spread widely, including into saltmarshes (Meyerson natural resource managers. Both saltmarsh and mangroves
et al., 2009). Although native P. australis had occurred are recognized as being of high conservation value.
in upper saltmarshes in America, the European form is Should action be taken to limit or prevent mangrove inva-
more salt tolerant (Vasquez et al., 2005), and this has been sion, or should the loss of saltmarsh be allowed to
a factor in its spread. It now forms extensive near mono- continue?
cultures at many localities. A range of adverse conse-
quences of this spread has been identified; however, it
also offers potential benefits particularly through accumu- Humans and saltmarshes
lation of peat, raising the ground surface and conferring There is a long history of human utilization of saltmarsh
protection against sea-level rise (Meyerson et al., 2009). resources (Adam, 1990; Gedan et al., 2009). Hunter gath-
Hybridization between native and introduced forms of ering was (and, locally, is) widely practiced by indigenous
Phragmites has been reported (Meyerson et al., 2010), peoples on saltmarshes around the world.
further complicating issues. In northern Europe, many early agricultural settlements
Although there are strong similarities between the from the late prehistoric onwards were close to, or even
floras of inland and coastal saline habitats, particularly at on, saltmarshes. In the Netherlands, marshes supported
generic level, there are differences. The genus Tamarix is grazing by livestock, were utilized for haymaking and
native to Asia and Africa, in arid and inland saline habi- for the collection of a variety of plant resources and even
tats. A number of species of Tamarix were introduced into for cropping (Bakker, 1989; Meier, 2004; Knottnerus,
the United States in the nineteenth century (Di Tomaso, 2005; Gedan et al., 2009). Similar practices were intro-
1998) where they were used as windbreaks, ornamentals, duced during the colonial era into the Americas and
and for erosion control. Subsequently, they have come to Australia.
be regarded as amongst America’s worst weeds. More The agricultural use of saltmarshes has declined in
recently, they have been reported invading coastal many parts of the world, but it is still locally important
saltmarshes in California (Whitcraft et al., 2007). Tamarix (Figure 9). Harvesting plants such as Salicornia for human
spp. are shrubs to low trees, but they lack modified aerial consumption has undergone a revival with the current
root systems, so that even though they may convert interest in using wild-collected produce.
saltmarsh to woodland, the new community formed would Grazing has considerable impact on vegetation, both
not be regarded as mangrove. because of direct effects on particular species and
In regions where saltmarsh and mangroves coexist, through trampling and compaction of the soil. The
expansion of mangroves into saltmarsh is occurring in effect of grazing will depend upon the nature of the
several parts of the world (Saintilan et al., 2009), but most grazing animal – sheep, cattle, and horses have different

Saltmarshes, Figure 9 Sheep grazing on a saltmarsh in the Leven estuary, northwest England. Saltmarshes in this region have a long
history of intensive grazing at high stocking rates (Gray, 1972). Saltmarsh lamb currently attracts a premium price at gourmet
butchers’ shops.
 SALTMARSHES 529

impacts – the timing of grazing, and the stocking rate. High eutrophication from general catchment runoff,
stocking rates can result in a very short turf with low vascu- stormwater inputs, and greater pressures for recreational
lar plant diversity, but less intense grazing can result in use. As the world population grows and becomes more
taller vegetation and greater diversity. In Britain, the most affluent, there is increasing need for greater development
diverse plant communities (Adam, 1981) are found at the of ports and construction of airports, oil and gas termi-
upper levels of sites with moderate grazing pressure. nals, and other industrial facilities. Ports, of necessity,
In northern Europe, species such as Atriplex must have waterfront access, while for many other forms
portulacoides and Limonium spp. are grazing sensitive, of development, availability of level land created by
and on heavily grazed marshes are either absent or marsh infill close to cities is seen as a major economic
restricted to microsites inaccessible to livestock, such as benefit. Construction of major airports on fill is viewed
creek sides. Grazing results in permanent changes to veg- as a means of securing locations where noise impacts
etation. If livestock is removed, the vegetation does not on residential suburbs can be minimized. The importance
revert to the community that would have been expected of saltmarsh habitat loss from major infrastructure devel-
on marshes which had never been grazed. In general, opment is recognized, but the economic and social bene-
when livestock grazing ceases the trend is for the develop- fits (at least in the short-term) of development prevail
ment of tall, rank very species poor grassland communities over environmental concerns.
(Bakker, 1989; Adam, 1990; Lambert, 2000). Development pressures are localized in their impacts,
The effects of livestock grazing are not only reflected in even if cumulatively the effects are large, but the growing
structure and composition of vegetation, but also by the human population, increasing living standards, and devel-
fauna, in utilization by birds, with herbivorous waterfowl opment will result in continuing release of greenhouse
favoring shorter grass swards over taller vegetation, and gases and consequent climate disruption. One of the con-
the composition of invertebrate communities. Spider and sequences of global warming will be an increase in the
beetle communities have been shown to be affected by volume of the oceans due to thermal expansion and, in
grazing; abundance of some spaces is increased by graz- the longer term, the melting of ice caps and glaciers.
ing, but under intense grazing, spider species richness A potential consequence of sea-level rise will be “drown-
declines (Petillon et al., 2007). ing” of saltmarsh and “coastal squeeze,” where natural
Saltmarshes on the east coast of the United States were topography or artificial constraints prevent compensation
heavily exploited as grazing land and haymaking from for loss of the seaward parts of marshes by retreat land-
early colonial times until relatively recently (Gedan ward. On a site-by-site basis, what is important will be rel-
et al., 2009). However, the legacy of centuries of human ative change in sea level; the eustatic rise may be
modification is not as obvious as it is in Europe. Why this compensated for by sedimentation or isostatic or tectonic
is so is not clear, but perhaps the structural and floristic change in land level. In some circumstances, the increase
simplicity of Spartina marshes limited the changes in atmospheric CO2 may lead to increased plant produc-
resulting both from agricultural use and its subsequent tion and greater incorporation of organic material into
decline. Ewanchuk and Bertness (2004) suggest that forb saltmarsh soils (Langley et al., 2009) (see Sea-Level
panne habitat in upper marshes in New England was Change and Coastal Wetlands).
reduced in extent by the effects of ditching which for three The role of saltmarsh in sequestering carbon in soil is of
centuries was used to facilitate livestock grazing, global significance (Chmura et al., 2003, Chmura, 2009).
haymaking, and mosquito control. The importance of “blue carbon” in ameliorating the
With the decline in direct exploitation of saltmarshes, effects of greenhouse gas emissions is likely to be of
they are increasingly valued for their ecosystems services, increasing importance in the future.
conservation, and for aesthetic experiences. Increased temperature may affect plant distribution
and evapotranspiration and hence soil salinity. Changes
in other aspects of climate such as rainfall patterns or
Threats the incidence of major storms will also have impacts on
Despite increasing recognition of the values of saltmarsh, saltmarshes, some local and some more widespread.
the ecosystem is under increasing threat worldwide. The The full range of climate change impacts is difficult to
range of threats facing saltmarshes has been identified in predict except in general terms (Ross and Adam, 2013),
a number of publications (Kennish, 2001; Adam, 2002; but it is clear that there will be changes in both the phys-
Adam et al., 2008; Silliman et al., 2009b) and need not ical and biotic components (Semeniuk, 2013). Increase
be recounted here (see Anthropogenic Impacts). in CO2 will affect the relative competitiveness of
The majority of the world’s human population is now plants, with C3 species likely to have an advantage over
urban, and many of the world’s largest cities are on or C4 (Mayor and Hicks, 2009) species. Hence, changes
close to estuaries. Even if environmental regulations are in the floristic composition of saltmarshes in temperate
in place, there is a continuing increase in environmental and subtropical marshes can be anticipated, with
degradation caused by accidental spills of chemicals concomitant changes in fauna. Spartina spp. are cur-
in the catchment, the accumulation of pollutants rently the most widespread C4 species on saltmarshes
from industrial discharges and domestic sources, (see Climate Change).
530 SALTMARSHES

Values Habitats Directive recognizes a range of saltmarshes as

As part of the evaluation of biodiversity, there has been requiring designation as Special Areas of Conservation).
recognition of the importance of biodiversity in the provi- Conservation of saltmarshes will not be achieved sim-
sion of ecosystem services, processes, and functions. The ply by designating sites as reserves. Given the range of
importance of saltmarshes for the maintenance of fisheries threats saltmarshes face, there will need to be active man-
has been recognized for more than half a century; other agement. Management regimes for saltmarshes need to be
services, although long recognized anecdotally by local integrated into wider coastal zone management plans
residents, had only recently been scientifically assessed. (Shepard et al., 2011), and given the importance of
Barbier et al. (2011) have reviewed the services provided saltmarshes for migratory birds (and in the case of water-
by saltmarsh and attempted an economic evaluation. Eco- fowl the adverse impacts they may have on saltmarshes),
nomic data are available from a limited number of there needs to be an international component to manage-
saltmarsh types and sites. While the data demonstrate high ment plans. Species of fish, for which saltmarsh is nursery
values in the instances studied, extrapolation to habitat, may distribute widely as adults, so national and
saltmarshes as a whole requires caution. international management of marine resources will also
Recent major storms and recognition that the incidents need to consider saltmarsh. Management of sediment sup-
and intensity of storms are likely to increase in some local- ply and hydrology requires saltmarsh management to be
ities as a result of climate change that has promoted inter- integrated into catchment management.
est in the role of saltmarshes in absorbing wave energy and Over the past half century, there has been increasing
protecting the hinterland (Costanza et al., 2008; Koch attention paid to rehabilitation, restoration, and recreation
et al., 2009). Shepard et al. (2011) have reviewed the role of saltmarshes (Zedler, 2001; Roman and Burdick, 2012).
of saltmarshes in protecting the coastline against storms. Substantial sums have been expended on this work, which
Vegetation cover plays a major role in attenuation of wave has frequently been mandated by planning and environ-
impact, while belowground biomass is important in limit- mental laws. Considerable progress has been made in the
ing erosion. Yang et al. (2012) demonstrated that Spartina field, but there is still debate as to how success of pro-
alterniflora, an introduced species in China, was more grams should be measured and whether or not it is
effective in attenuating wave energy because of its height achieved (Ambrose, 2000; Mossman et al. 2012a;
than the shorter native species. Whether, given the poten- Mossman et al. 2012b).
tially adverse impacts of S. alterniflora on Chinese
saltmarshes (An et al., 2007), continued planting of Summary
S. alterniflora is justified is a policy question for man- Saltmarsh is the natural fringing vegetation of many estu-
agers. Tall S. alterniflora is the dominant species in the aries, most particularly in temperate and higher latitudes,
Gulf and Atlantic coasts of the United States and may per- but saltmarsh also occurs in conjunction with mangroves
form a greater role in wave attenuation than the short veg- in tropical and subtropical regions. The development of
etation of most marshes elsewhere. The magnitude of saltmarsh involves dynamic interactions and feedbacks
attenuation is influenced by storm intensity and water between the biota and the physical environment.
depth, but it is clear saltmarsh can protect the coastline. Saltmarshes provide habitat for a rich biota. The ecosys-
The old concept of “wetlands as wasteland” is dead and tem services provided by saltmarshes include sustaining
buried. The importance of ecosystem services provided by fisheries, provision of habitat migratory birds, provision
wetlands is now firmly established. More intangible, but of shoreline protection, and sequestration of carbon.
no less important, is the aesthetic value of saltmarsh, long Anthropogenic pressures on estuarine resources and the
recognized by landscape painters and increasingly so by effects of increased carbon dioxide and climate disruption
a much wider public. will pose threats to the long-term sustainability of many
saltmarshes unless coordinated international action can
be undertaken.
Conservation and management
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lotype of the common reed Phragmites australis (Poaceae). a major ingredient in concrete (often 25 % by volume;
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Reclamation, 39, 3–20. and for making glass and ceramics.
Whitcraft, C. R., Talley, P. M., Crooks, J. A., Boland, J., and Gaskin, In some cases the harvested sand is processed for
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Elsevier. mixed in with these grains. The sands can be mined to
Woodell, S. R. J., 1974. Anthill vegetation in a Norfolk saltmarsh. remove the heavy minerals of interest in such a case.
Oecologia(Berlin), 16, 221–225. Sand is still mined from beaches, but this practice is
Yang, S. L., Shi, B. W., Bouma, T. J., Ysebaert, T., and Lou, X. X.,
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169–182. undesirable. Environmental issues such as the effects of
Yapp, R. H., Johns, D., and Jones, O. T., 1917. The salt marshes of enhanced turbidity or exposure of contaminants that arise
the Dovey estuary. Part II. The salt marshes (by R. H. Yapp and from mining activities should also be considered before
D. Johns). Journal of Ecology, 5, 65–103. undertaking sand mining (e.g., Meador and Layher, 1998).
Zedler, J. B. (ed.), 2001. Handbook for Restoring Tidal Wetlands.
Boca Raton: CRC Press. Since sand is found, and used, worldwide, sand mining
also takes place on a global scale. Different types of sands
are utilized for different purposes (concrete, foundry
Cross-references sands, etc.). Most of the marine sand mining occurring
on the US Atlantic and Gulf coasts is for beach nourish-
Anthropogenic Impacts ment purposes. It has been estimated that the volume exca-
Climate Change
Coastal Wetlands vated for this purpose in the twentieth century is on the
Estuarine Habitat Restoration order of 650 million cubic meters (Finkl and Hobbs,
Introduced Species 2009). This is larger than the volume of Lake Erie.
Invasive Species
Mangroves Techniques
Salt Marsh Accretion
Sea-Level Change and Coastal Wetlands A wide variety of techniques can be employed for sand
Shorebirds mining. Marine sand mining is usually accomplished with
Species Zonation the same types of equipment used for dredging projects,
536 SAND RIDGE

including clamshell buckets, backhoes, dustpan, and Definition

cutterhead suction dredges (see Dredging). The hydraulic Sand ridges are elongate coastal or shelf sand bodies that
approaches require dewatering of the sand-water slurry form bathymetric highs on the seafloor (Duane et al., 1972).
but can achieve higher rates and facilitate transport by
allowing pumping through long pipelines to desired dis-
charge points. Terrestrial deposits can likewise be exca- Description
vated by a wide variety of techniques, but most are Sand ridges are asymmetrical features composed of
classified as surface mining, digging from the top of the unconsolidated fine to coarse sand and gravel. They are
deposit downward, continually excavating the exposed typically 5-120 km long and 0.5-8 km wide, with heights
surface of the deposit. of more than 20 % of the water depth and slopes of less
than 1 . They occur on continental shelves in a wide range
Summary of water depths, where sufficient sand exists and currents
Sand mining is a major, global industry and will remain are strong enough to transport sand-sized sediment. They
important for many years to come. It has been utilized his- also usually occur in groups that are shore parallel, shore
torically to acquire building and industrial supplies, but oblique, or shore normal, with spacing between members
new applications arise over time, such as excavation of of 250 times the water depth, along topographically
tar sands for their energy value and the mining of frac irregular transgressive margins (Snedden and Dalrymple,
sands for use in hydraulic fracking operations to extract 1999). Based on the relative progression of ridge
subterranean hydrocarbons. Likewise, new environmental reworking and migration (Snedden and Dalrymple,
issues to be mitigated are likely to continue to arise. Ter- 1999), sand ridges belong to one of three classes. Class
restrial sand mining in particular tends to lead to protests I ridges are young ridges that maintain their original
from citizens concerned with land use and environmental nucleus and are at or near their point of origination. Class
issues, because it often results in highly visible, permanent II ridges are partially evolved ridges that have maintained
changes to the landscape. The western Great Lakes region some of their original nucleus and have migrated
in the United States (Wisconsin, Minnesota), for example, a distance less than their original width. Class III ridges
is the source of a large amount of sand being used for are fully evolved forms that have maintained none of their
fracking operations and is dealing with public protests original nucleus. They have migrated a distance greater
over the mining of these materials. than or equal to their original width.
Several hypotheses and models have been developed
that address the origin of sand ridges; these are largely
Bibliography based on their distribution and orientation. They include
Finkl, C. W., and Hobbs, C. H., III, 2009. Mining sand on the con- (1) the drowning of static, pre-transgressive features such
tinental shelf of the Atlantic and Gulf Coasts of the U.S. Marine as barrier islands, beach ridges, and interfluves (e.g.,
Georesources and Geotechnology, 27, 230–253. Veatch and Smith, 1939); (2) the formation of post-
Kosmatka, S. H., and Wilson, M. L., 2011. Design and Control of Con-
crete Mixtures, 15th edn. Skokie, IL: Portland Cement Association. transgressive features in equilibrium with the hydraulic
Meador, M. R., and Layher, A. O., 1998. Instream sand and gravel regime (e.g., Uchupi, 1968) or formed during storms
mining: environmental issues and regulatory processes in the (e.g., Duane et al., 1972; Swift and Field, 1981); and
United States. Fisheries, 23(11), 6–13. (3) the reworking of modern features associated with an
actively transgressing shoreline (e.g., Swift and Field,
1981; Robinson and McBride, 2008).
Cross-references
Anthropogenic Impacts
Dredge and Fill
Dredging Bibliography
Mass Physical Sediment Properties Duane, D. B., Field, M. E., Meisburger, E. P., Swift, D. J. P., and
Williams, S. J., 1972. Linear shoals on the Atlantic inner conti-
nental shelf, Florida to Long Island. In Swift, D. J. P., Duane,
D. B., and Pilkey, O. H. (eds.), Shelf Sediment Transport: Process
and Pattern. Stroudsburg: Dowden, Hutchinson, and Ross,
SAND RIDGE pp. 447–498.
Robinson, M. M., and McBride, R. A., 2008. Anatomy of
Marci M. Robinson a shoreface sand ridge revisited using foraminifera: False Cape
Shoals, Virginia/North Carolina inner shelf. Continental Shelf
US Geological Survey, Eastern Geology and Paleoclimate Research, 28, 2428–2441.
Science Center, Reston, VA, USA Snedden, J. W., and Dalrymple, R. W., 1999. Modern shelf sand
ridges: from historical perspective to a unified hydrodynamic
Synonyms and evolutionary model. In Bergman, K. M., and Snedden,
J. W. (eds.), Isolated Shallow Marine Sand Bodies: Sequence
Linear shoal; Sand shoal; Shoreface ridge; Tidal current Stratigraphic Analysis and Sedimentologic Interpretation.
ridge Tulsa, OK: SEPM, Special Publication 64, pp. 13–28.
 SANDBANKS 537

Swift, D. J. P., and Field, M., 1981. Evolution of a classic sand The location of sandbank formation has been used for
ridge field, Maryland sector, North America inner shelf. their classification. For example, sandbanks may be clas-
Sedimentology, 28, 462–482. sified as open shelf linear sand ridges (Type 1), wide estu-
Uchupi, E., 1968. Atlantic Continental Shelf and Slope of the United
States: Physiography. Washington, DC: US Government ary sand ridges (Type 2A), ebb and flood deltas (Type
Printing Office, 529-C, p. 30. 2Bi), shore attached ridges (Type 2Bii), banner banks
Veatch, A. C., and Smith, P. A., 1939. Atlantic Submarine Valleys of (Type 3A), and en-echelon ridges (Type 3B).
the United States and the Congo Submarine Valley. Washington, Sandbanks play a key role in natural coastal defenses
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Wales), navigation, and as resources for sediment aggre-
gates. Additionally, sandbanks are prime locations for off-
Cross-references shore wind farm deployments in coastal regions. They are
Bar also used as nesting grounds by sea lions and as feeding
Barrier Island and resting grounds by coastal birds such as seagulls and
Barrier Spits
Coastal Barriers oyster catchers.
Coastal Landforms There are many excellent examples of sandbanks.
Sedimentary Structures Two of these in the UK include the Great Yarmouth sand-
banks and the Isle of Portland sandbanks. The Great Yar-
mouth sandbanks may have been formed when the sea
transgressed from the south through the Dover Strait at
SANDBANKS the end of the Holocene period. Some deep channels
between the sandbanks were also formed due to strong
tidal currents. Two of the major sandbanks of the Isle
Vanesa Magar of Portland (West Shoal and Adamant Shoal) may have
Oceanology Division/Department of Physical been formed when the Isle of Portland was isolated from
Oceanography, CICESE, Ensenada, Mexico the mainland by a navigable strait. After sedimentation
caused the closure of the strait, these two sandbanks
Synonyms were no longer actively maintained and subsequently
Bank; Hummock; Ridge; Sands; Shoal migrated towards the two largest present-day headland
sandbanks of the region (Portland Bank and
Definition Shambles Bank).
Sandbanks are bedforms consisting of cohesive or
non-cohesive sediments in estuaries and continental shelf
areas that can be exposed at low tide. They are generally Bibliography
formed by sediment transport by tidal currents and waves Brown, J. M., and Davies, A. G., 2009. Methods for medium-term
as well as changes of tidal prism due to sedimentation in prediction of the net sediment transport by waves and currents
estuaries. in complex coastal regions. Continental Shelf Research, 29,
1502–1514.
Chaumillon, E., Bertin, X., Falchetto, H., Allard, J., Weber, N.,
Description Walker, P., Pouvreau, N., and Woppelmann, G., 2008. Multi
Sandbanks have been the focus of an array of studies (e.g., time-scale evolution of a wide estuary linear sandbank, the
Dyer and Huntley, 1999; Christopherson, 2002; Longe de Boyard, on the French Atlantic coast. Marine Geology,
251, 209–223.
Chaumillon et al., 2008; Reeve et al., 2008; Brown and Christopherson, R. W., 2002. Geosystems – An Introduction to
Davies, 2009; Neill and Scourse, 2009). They mainly con- Physical Geography, 5th edn. New York: Prentice Hall.
sist of sand particles with sizes varying from very coarse Dyer, K. R., and Huntley, D. A., 1999. The origin, classification and
(2 mm–1 m diameter) to very fine (0.10–0.05 mm) but modeling of sand banks and ridges. Continental Shelf Research,
also can include silt (0.05–0.002 mm), clay 19, 1285–1330.
(<0.002 mm), and biological matter. Typical locations of Neill, S.P., and Scourse, J. D., 2009. The formation of headland/
sandbanks are ebb and flood deltas of tidal inlets within island sandbanks. Continental Shelf Research, 29, 2167–2177,
doi:10.1016/j.csr.2009.08.008#_blank.
and in the vicinity of estuary mouths and near coastal Reeve, D. E., Horrillo-Caraballo, J. M., and Magar, V., 2008. Statis-
headlands. tical analysis and forecasts of long-term sandbank evolution at
Sandbanks generally form from tidal flows through Great Yarmouth, UK. Estuarine, Coastal and Shelf Science,
straits, past headlands and islands, where the currents are 79, 387–399.
rapid. At the ends of these features, the currents decrease,
and sediment deposition occurs. Sandbanks develop from
sand convergence driven by both tidal currents and waves. Cross-references
In estuaries, sediment infilling causes the decrease in the Deltas
tidal prism on a century and millenia time scale, leading Mass Physical Sediment Properties
to sandbank formation. Sediment Transport
538 SANDFLAT

SANDFLAT SEABIRDS

Jorge Manuel López-Calderón1, Alf Meling2 and Joanna Burger

Rafael Riosmena-Rodríguez1 Department of Cell Biology and Neurosciences, Ecology,
1
Programa de Investigación en Botãnica Marina, Evolution and Natural Resources, Department of Marine
Departamento Académico de Biología Marina, and Coastal Sciences, Environmental and Occupational
Universidad Autónoma de Baja California Sur, La Paz, Health Sciences Institute, Rutgers University, Piscataway,
Baja California Sur, Mexico NJ, USA
2
Universidad Autónoma de Sonora, Hermosillo, Sonora,
Mexico Synonyms
Estuarine birds; Marine birds; Pelagic birds
Synonyms
Mudflat; Tidal flat Definitions
Seabirds. Birds that live primarily on the oceans of the
Definition world and breed on oceanic islands, although it includes
A sandflat is an extension of unconsolidated sediment some members of seabird families that breed inland.
located preferentially in the lower intertidal zone. It is an Colonial. Several to hundreds of birds nesting in close
unstable area characterized by the constant resuspension proximity.
of sediment by tidal flood and ebb currents. Tidal forces
predominate over other hydrodynamic processes, forming Introduction
well-sorted sand deposits.
Seabirds are at home on land, in the air, and in water, mak-
Hydrodynamics ing them unique among vertebrates. They often switch
between these three environments on a daily basis, which
The amount of mud found in the sediment of a sandflat requires unique physiology and morphological adapta-
is determined by the bed slope, the amount of fine sedi- tions. They have adapted to all ecosystems on earth, from
ments derived from land, and the strength of the current. the Arctic to Antarctica. Seabirds can be defined as those
The sandflat-mudflat threshold is determined by the afore- that live, forage, and breed in marine environments,
mentioned factors. Finer muddy sediments accumulate including bays, estuaries, wetlands, coastal islands, oce-
during neap tides, while coarser sandy sediments accumu- anic islands, and the open ocean. They mainly forage at
late during spring tides. Hydrodynamic conditions of sea, either nearshore or offshore, and can spend days,
sandflats preclude development of submerged aquatic weeks, and even years at sea without coming to land. Once
vegetation; however, there are important resident commu- albatrosses leave their natal colony, they may wander the
nities of burrowing invertebrates (worms, bivalves, crusta- oceans for 5–10 years before coming to land to breed.
ceans) that are fundamentally important for local trophic While the ocean may seem uniform, seas vary with
networks and the recycling of nutrients. Sandflats can be seasonal cycles, El Nino events, and stochastically.
found adjacent to other types of habitats, such as The distribution and foraging behavior of seabirds are
saltmarshes located in the upper intertidal zone (Perillo, interconnected with oceanographic influences. For exam-
1996; Schwartz, 2005; Perillo et al., 2009). ple, 50 species of seabirds are regular residents of the trop-
ical Pacific Ocean, where specific currents, distinct water
Bibliography masses, El Nino events, and the presence of marine mam-
mals all affect seabird distribution (Balance et al., 2006).
Perillo, G. M. E. (ed.), 1996. Geomorphology and Sedimentology of
Estuaries. Amsterdam: Elsevier. Many seabirds forage with predatory fish or marine mam-
Perillo, G. M. E., Wolanski, E., Cahoon, D. R., and Brinson, M. M. mals because they force prey fish to the surface, making
(eds.), 2009. Coastal Wetlands: An Integrated Ecosystem them available to foraging seabirds.
Approach. Amsterdam: Elsevier.
Schwartz, M. L. (ed.), 2005. Encyclopedia of Coastal Science. Relevant life history
Dordrecht: Springer.
Seabirds are long-lived (20–60 years), delay breeding
(some albatrosses do not breed until they are 10 years
Cross-references old), lay small clutch sizes (most lay only one egg), have
extended parental care (weeks to 6 months or more), and
Sandbanks
Sediment Grain Size range in size from a 0.24 kg Snow Petrel (Pagodroma
Sediment Transport nivea) to a 16 kg male King Penguin (Aptenodytes
Tidal Flat patagonicus). Thus, seabirds have low clutch sizes and
Tidal Flat Salinity Gradient long-parental care, producing enough young to replace
 SEABIRDS 539

Seabirds, Figure 1 Great and Snowy Egrets nesting in Iva low bushes in Barnegat Bay, New Jersey.

themselves during their lifetimes. Even within seabirds, warning, colony defense, and information about food
life history strategies differ. Gulls and terns breed when sources. In addition, any bird in the colony has less of
they are 3–5 years old, lay two to three eggs, have incuba- a chance that it will be targeted by a predator because the
tion periods of a few weeks, produce a young or two predator has many birds to choose from (predator
a year, and live only 20 years, whereas albatrosses do swamping). If a solitary nesting seabird is located by
not breed until they are 8–10 years old, lay one egg, do a predator, it may be killed (Burger, 1982; Coulson, 2001).
not raise a young every year, and may live up to 60 years. The second decision, where to locate a colony, usually
David Lack (1968) first proposed that the life history strat- results in nesting in places that are inaccessible to mam-
egy of seabirds evolved because of energy limitations that malian predators, such as oceanic or offshore islands,
derive from the difficulty of feeding over the open ocean cliffs, or trees. Once selected, colony sites usually remain
where prey is unpredictable and irregular (Schreiber and stable unless they are destroyed by predators or people or
Burger, 2001a) (Figure 1). become unusable because of habitat loss, high tides and
Seabirds often nest in colonies of a few to several flooding, disease, or other physical disruption. The third
thousand pairs in places that are free of predators, such choice is where to defend a breeding territory and nest.
as offshore islands. Some seabirds have altricial young Usually males select a territory and court a female. Both
(must be feed from their parent’s bill, naked when born, members of the pair then participate in territory defense,
e.g., Pelicans), while others have precocial chicks incubation, and chick rearing.
(covered with down, able to walk immediately upon Since birds incubate their eggs, they are place based
hatching, e.g., gulls). Both types require long periods of during the incubation period and must select a safe place
food provisioning and predator protection while they are free from predators that will also provide protection from
in the nest and for weeks or months thereafter (Schreiber inclement weather and stressful temperatures. Remote
and Burger, 2001b, c). sensing and geographical information systems (GIS)
allow seabird biologists to assess habitat availability and
Habitat selection quality, monitor populations and colony use, and develop
Breeding habitat conservation plans (Gottschalk et al., 2005).
Seabirds nest on land and forage over water in estuaries,
bays, along coasts, or in offshore waters hundreds of kilo- Foraging
meters from coasts (Table 1). Nesting seabirds have three Seabirds mainly forage by swimming underwater to pur-
habitat selection decisions: (1) whether to nest in a colony sue prey, sitting on the water and picking up prey, dip-
or solitarily, (2) what general breeding site to choose, and ping prey from water while flying above, and plunge
(3) what specific nest site to select. Most seabirds nest in diving for prey from well above the water (Table 1). In
groups called colonies, and numbers range from dozens addition, some gulls frequent garbage dumps, pick up
in terns (e.g., Common Tern, Sterna hirundo) to thousands food from lawns or gardens, and scavenge dead fish or
of pairs (Sooty Tern, Onychoprion fuscatus). The advan- other prey along the shore. Species, such as gulls, that
tage of nesting solitarily is that cryptic birds can nest in have very diverse foraging methods generally nest along
vegetation and leave the nest quietly, thereby avoiding coasts, have larger clutch sizes, raise more young, and
predators, while the disadvantage is that there are no col- have shorter lifespans than pelagic species that forage
ony mates to provide early warning, help defend nests, over the open ocean. While it might seem that fish and
or provide information on food sources. The advantage other prey are plentiful in oceans, prey is not evenly dis-
of nesting in colonies is that colony mates provide early tributed and may be unavailable because birds cannot
540 SEABIRDS

Seabirds, Table 1 Orders of marine birds (After Burger and Schrieber, 2001a; Burger and Schrieber, 2001b; Brooke, 2001; Shealer, 2001,
Unpubl. data). The number of species is shown in parenthesis after the family name (authors disagree on exact numbers)

Order Family Types of birds Foraging Nesting

Sphenisciformes Spheniscidae (17) Penguins Underwater by pursuit Ground or burrows; offshore or

diving coastal islands, some on
Antarctic islands
Procellariiformes Diomedeidae (21) Albatrosses Surface dip or surface Ground, oceanic islands
seize
Procellariidae (79) Gadfly petrels, Hover dip or surface dip Islands, burrows
shearwaters, and fulmars and pattering
Pelecanidae (4) Diving petrels Diving Burrows
Hydrobatidae (21) Storm petrels Dipping, pattering, and Islands or high cliffs; crevices,
surface dip burrows
Pelecaniformes Phaethontidae (3) Tropic birds Surface seizing Crevices, rock burrows
Pelecanidae (7) Pelicans Surface seizing Plunge Ground, trees
diving
Frigatidae (5) Frigate birds Piracy, dipping Trees
Sulidae (10) Gannets and boobies Plunge diving Ground, trees
Subfamily (40) Cormorants, anhingas, and Pursuit diving Ground, trees pilings
Phalacrocoracinae darters underwater
Charadriiformes Stercorariidae (7) Skuas and jaegers Dipping, piracy Ground
Subfamily Larinae (50) Gulls Dipping, surface seizing, Ground, floating nests in
and piracy marshes, and trees/shrubs
Subfamily Sterninae (45) Terns Plunge diving, dipping, Ground, shrubs
and piracy
Rhynchopidae (3) Skimmers Skimming Ground
Alcidae (23) Auks Pursuit diving Crevices, burrows, and ground
underwater

Note: Rhynchopidae are not usually considered seabirds because they may forage in estuaries, but never use the open ocean

swim that deep, plunge dive that far below the surface, or they are vulnerable to invasive mammals such as feral cats
dip down far enough to catch fish. (Felis catus), rats (Rattus sp.), and mice (Mus spp.) that
prey on eggs, young, or even adults. On many islands,
Predators, competitors, and invasive species such as the United Kingdom Overseas Territories, cats
and rats have caused near extinction of several seabirds,
In addition to food availability (both types and quantity), resulting in critically endangered species (Hilton and
seabird survival and reproductive success are affected by Cuthbert, 2010). Examining 94 papers that demonstrated
predators and competitors. There are no mammalian pred- effects of rats on nesting seabirds, Jones et al. (2007)
ators on offshore or oceanic islands where most seabirds showed that 74 seabird species in ten families were
nest, and there are few avian predators so far from land. affected. Storm Petrels and small, burrow-nesting seabirds
Other seabirds nest on cliffs or rocky ledges where mam- were most affected, and gulls and terns were least affected.
malian predators have no access, and spaces are too small Removing rats and cats from islands with nesting seabirds
for avian predators to land. Seabirds that nest along coasts should be a high conservation priority (Jones et al., 2007).
either nest on cliffs, islands within bays, or in trees where
predators have little access. Their main competitors are
other seabirds of the same or a different species that nest Migration and overwintering
within these colonies. In some cases, space itself is lim- Seabirds have some of the longest migration routes of any
ited, restricting the number of pairs that can breed. Stiff birds; but since they mainly migrate over oceans, their pat-
competition among Northern Gannets (Morus bassanus) terns were a mystery until very recently. With advances in
for nest sites on rock outcroppings results in adults radiotelemetry (shore distance), satellite transmitters
remaining several weeks or months after the young fledge, (on large birds), and geolocators (for smaller birds), data
just to protect their territory (Mowbray, 2002). on specific migration routes can be combined to form
Invasive species provide a unique threat because they migration patterns for species. Most seabirds spend the
did not evolve with seabirds, and they have fewer adapta- winter at sea, never or seldom coming to land.
tions for coping with them. Invasive mammalian predators
pose the greatest threat to seabirds worldwide (Nettleship Threats to seabirds
et al., 1994). Seabirds nesting on oceanic islands evolved Because seabirds inhabit so many habitats, in all ecosys-
without predators and usually nest on the ground, where tems, they are exposed to many threats, including habitat
 SEABIRDS 541

loss, development, human activities and disturbance, and populations can be affected, either acutely or by
disease, toxic chemicals, plastics, invasive species, and chronic exposure. The chemicals that have been shown
fisheries. Habitat loss is the primary factor affecting sea- to affect seabirds include lead, mercury and other metals,
birds that nest along coasts because these regions are oil, polychlorinated biphenyls (PCB), dichlorodiphenyl-
highly developed, putting pressure on the cliffs, islands, trichloroethane (DDT), and other organochlorine com-
and bare ground where seabirds nest. In the long term, pounds (Burger and Gochfeld, 2001). Oil spills can
climate change may pose a great risk by affecting cause large-scale mortality (Kingston, 2002). DDT is
fisheries bycatch, spatial ecology, food sources, and prey the classic case of toxic effects. Populations of several
availability (Gremillet and Boulinier, 2009; Barbraud fish-eating birds (e.g., Brown Pelican, Pelecanus
et al., 2012). occidentalis; White Pelican, P. erythrorhynchos; and
Seabirds are exposed to a range of human activities that Gannets) collapsed in the 1960s because DDT interfered
disrupt their breeding, foraging, migrating, and with calcium and eggshell development (Blue et al.,
overwintering. Disruptions include direct disturbance, 1974). When adults sat to incubate, their thin-shelled eggs
such as people entering breeding colonies, scaring incu- broke. Plastics are synthetic organic polymers, and their
bating adults from nests, destroying eggs, bringing in increase over the last two decades resulted in large quan-
predators (e.g., dogs), and disrupting foraging with boats, tities of plastics entering the oceans. Seabirds are mainly
Jet Skis, or other water-related activities. Indirect effects affected by entanglement, exposure to PCBs from inges-
include increases in native predators because of availabil- tion, and compromises to the digestive tract because of
ity of food brought by humans (e.g., raccoons). Effects on ingestion of large quantities (Derraik, 2002). Many spe-
nesting and foraging seabirds can occur as a result of sci- cies of seabirds are endangered (Nettleship et al., 1994;
entific investigators or others that visit colonies or monitor Schreiber and Burger, 2001a).
seabird behavior. A meta-analysis of the effect of transmit-
ters on avian behavior found significant effects – birds Summary
with devices had greater energy expenditures and some
Seabirds, including penguins, albatrosses, petrels,
failed to nest (Barron et al., 2010). However, usually
boobies, pelicans, gulls, and terns, spend most of their life
devices are used on only a few individuals, and informa-
foraging at sea and breed on coastal or oceanic islands far
tion gained from these is greater than the cost to these
removed from mammalian predators. They are long-lived
individuals and any potential costs of alternative methods
and have delayed breeding, low clutch sizes, long incuba-
of learning about movements and migration patterns
tion periods, and extended parental care. Most nest in col-
(Burger et al., 2012).
onies that are either monospecific or contain many
Fisheries are a special case of human activities; fisher-
different species. Nesting in colonies allows for early
ies have a positive and a negative effect. Positive effects
warning, groups defense, predator swamping that reduces
include provision of food from offal, around factor ships,
the chances of any individual being eaten, and information
or near processing plants. Negative effects include mortal-
transfer about food sources. The threats to seabirds include
ity from capture in fishing lines or equipment or being
habitat loss and degradation, overfishing, invasive preda-
entangled and drowning (called bycatch, Moore et al.,
tors, toxics, plastics, and climate change. The biggest
2009) or because of competition with fisheries for fish.
threat to nesting seabirds is nonnative mammalian preda-
Prey depletion can be a problem during breeding when
tors, while fisheries provide the biggest threat to foraging
adults are limited to foraging distance around their nests.
seabirds (both mortality and competition for prey). Many
Seabirds and fisheries have interacted for centuries with
species of seabirds are endangered.
little effect until the rapid enhancement of fishing capabil-
ities and overexploitation of fish stocks that happened in
the last two centuries. However, the advent of large factory Bibliography
ships and deployment of longlines that extend for kilome- Balance, L. T., Pitman, R. L., and Fiedler, P. C., 2006. Oceano-
ters increased overfishing of fish stocks and massive graphic influences on seabirds and cetaceans of the eastern trop-
increases in captures in nets, especially albatrosses and ical Pacific: a review. Progress in Oceanography, 69, 360–390.
petrels (Montevecchi, 2001). Barbraud, C., Rolland, V., Jenouvrier, S., Nevous, M., Delord, K.,
Human activities are responsible for most of the toxic and Weimerskirch, H., 2012. Effects of climate change and fish-
eries bycatch on Southern Ocean seabirds: a review. Marine
chemicals in the environment, and where toxic elements Ecology Progress Series, 454, 285–302.
are present in soil or water (e.g., mercury in seawater), Barron, D. G., Brawn, J. D., and Weatherhead, P. J., 2010. Meta-
organisms have adapted to natural levels. Pollution in analysis of transmitter effects on avian behavior and ecology.
the ocean comes from dumping of wastes, dredging, run- Methods in Ecology, 1, 180–187.
off from towns and rivers, and point source pollution from Blue, L. J., Neely, B. S., Jr., Belisle, A. E., and Prouty, R. M., 1974.
factories, urbanization, suburbanization, and coastal busi- Organochlorine residues in Brown Pelicans: relation to repro-
ductive success. Environmental Pollution, 7, 81–91.
nesses and activities. Toxics and plastics threaten seabirds Burger, J., 1982. An overview of factors affecting reproductive
because they spend so much time in water where they are success in colonial birds. Colonial Waterbirds, 5, 58–123.
exposed externally, by inhalation and by ingestion of food Burger, J., and Gochfeld, M., 2001. Effects of chemicals and pollu-
and water (Burger and Gochfeld, 2001). Both individuals tion on seabirds. In Schreiber, E. A., and Burger, J. (eds.),
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Press, pp. 485–526. SEAGRASS PRODUCTION MODELS
Burger, J., Niles, L. J., Porter, R. R., Dey, A. D., Koch, S., and Gor-
don, C., 2012. Migration and over-wintering of Red Knots
(Calidris canutus rufa) along the Atlantic Coast of the United Jessie C. Jarvis
States. The Condor, 114, 302–313. Centre for Tropical Water and Aquatic Ecosystem
Coulson, J. C., 2001. Colonial breeding in seabirds. In Schreiber, Research (TropWATER), James Cook University, Cairns,
E. A., and Burger, J. (eds.), Biology of Marine Birds. Boca QLD, Australia
Raton: Mew Uprl MU’ CRC Press, pp. 87–114.
Brooke, M., 2001. Seabird systematics and distribution: a review of
current knowledge. In Schreiber, E. A., and Burger, J. (eds.), Synonyms
Biology of Marine Birds. Boca Raton: Mew Uprl MU’ CRC Ecological models
Press, pp. 57–87.
Derraik, J. G. B., 2002. The pollution of the marine environment
by plastic debris: a review. Marine Pollution Bulletin, 44,
Definition
842–852. Seagrass production models are conceptual or mathemati-
Gottschalk, T. K., Huettmann, F., and Ehlers, M., 2005. Thirty years cal simplifications (or abstractions) of the physiological
of analyzing and modeling avian habitat relationships using sat- processes associated with plant metabolism that results
ellite imagery data: a review. International Journal of Remote in plant growth or loss.
Sensing, 26, 2631–2656.
Gremillet, D., and Boulinier, T., 2009. Spatial ecology and conser-
vation of seabirds facing global change: a review. Marine Ecol- Introduction
ogy Progress Series, 391, 121–137. Seagrasses, submersed marine angiosperms, are important
Hilton, G. M., and Cuthbert, R. J., 2010. The catastrophic impact of components of global shallow coastal and estuarine eco-
invasive mammalian predators on birds of the UK overseas terri-
tories: a review and synthesis. Ibis, 152, 443–458. systems (Green and Short, 2003). Seagrass communities
Jones, H. P., Tershy, B. R., Zavaleta, E. S., Croll, D. A., Keitt, B. S., provide habitat, protection, and nursery functions for eco-
Finkelstein, M. E., and Howald, G. R., 2007. Severity of the nomically valuable fishery species (Heck et al., 2008),
effects of invasive rats on seabirds: a global review. Conserva- serve as indicators of and modify local water quality
tion Biology, 22, 16–26. conditions (Dennison et al., 1993; Moore, 2004), and link
Kingston, P. F., 2002. Long-term environmental impact of oil spills. nutrient and carbon cycles between the water column and
Spill Science and Technology Bulletin, 7, 53–61.
Lack, D. 1968. Ecological adaptations for breeding in birds.
sediment (Fourqurean et al., 2012). Due to their impor-
London: Chapman & Hall. tance within shallow coastal ecosystems, information
Montevecchi, W. A., 2001. Interactions between fisheries and sea- concerning the growth, resilience, and stability of seagrass
birds. In Schreiber, E. A., and Burger, J. (eds.), Biology of beds is necessary for effective coastal management.
Marine Birds. Boca Raton: Mew Uprl MU’ CRC Press, Ecological models are conceptual or mathematical
pp. 527–559. simplifications (or abstractions) of a real system (see
Moore, J. E., Wallace, B. P., Lewison, R. L., Zydelis, R., Cox, T. M., Ecological Modeling). Production models are a type of
and Crowder, L. B., 2009. A review of marine mammal, sea tur-
tle, and seabird bycatch in USA fisheries and the role of policy in ecological model that mathematically describes the phys-
shaping management. Marine Policy, 33, 435–451. iological processes associated with plant metabolism that
Mowbray, T. B., 2002. Northern Gannet (Morus bassanus). In results in plant growth or loss (Best et al., 2001). These
Poole, A. (ed), Birds of North America on Line. http://bna. models are useful tools in the quantitative analysis of com-
birds.cornell.edu/bna/species/693 plex ecosystems, such as seagrass meadows, and aid in the
Nettleship, D. N., Burger, J., and Gochfeld, M. (eds.), 1994. Threats development of hypotheses of feedback mechanisms that
to Seabirds on Islands. Cambridge: International Council for
Bird Preservation. impact plant growth and predict how plants may respond
Schreiber, E. A., and Burger, J. (eds.), 2001a. Biology of Marine to changes in water quality or management practices that
Birds. Boca Raton: Mew Uprl MU’ CRC Press. cannot be quantified using field and laboratory data alone
Schreiber, E. A., and Burger, J., 2001b. Seabirds in the marine envi- (Carr et al., 1997).
ronment. In Schreiber, E. A., and Burger, J. (eds.), Biology of Primarily seagrass production models have been used
Marine Birds. Boca Raton: Mew Uprl MU’ CRC Press, for scientific research focusing on quantifying physiolog-
pp. 1–16.
Schreiber, E. A., and Burger, J., 2001c. Table of seabird species and
ical responses to environmental conditions (Short, 1980;
life history characteristics. In Schreiber, E. A., and Burger, Wetzel and Neckles, 1986; Bach, 1993). More recently
J. (eds.), Biology of Marine Birds. Boca Raton: Mew Uprl the scope of production models has shifted to investigate
MU’ CRC Press, pp. 657–686. broader topics, such as altered trophic interactions
Shealer, D. A., 2001. Foraging behavior and food of seabirds. (Zaldívar et al., 2009; Baeta et al., 2011), and has been
In Schreiber, E. A., and Burger, J. (eds.), Biology of Marine coupled to hydrodynamic (Manca et al., 2012) and/or
Birds. Boca Raton: Mew Uprl MU’ CRC Press, pp. 137–178. spatially explicit models (Coffaro et al., 1997; Giusti
2001.
et al., 2010) to analyze the ecological roles and responses
of seagrass populations across larger scales. With increas-
Cross-references ing availability of long-term monitoring data sets and the
Shorebirds continued decline of seagrasses globally (Orth et al., 2006;
 SEAGRASS PRODUCTION MODELS 543

Waycott et al., 2009), the role of seagrass production Photosynthetic rates, described in the literature using
models has shifted to more of a synthesis-, forecast-, and species-specific photosynthesis-irradiance or PI curves,
management-driven focus (JØrgensen, 1994). increase linearly with light up to a saturating level past
which photosynthesis no longer increases (Lee et al.,
2007). In several seagrass production models, the relation-
Framework of seagrass productivity models
ship between light and photosynthesis in seagrass models
Mechanistic seagrass productivity models follow the flow is often defined by the Michaelis-Menten function or the
of carbon, the product of primary production, through hyperbolic tangent function (Vermaat and Hootsman,
a modeled environment and track the flow as it is modified 1994; Zimmerman et al., 1994; Madden and Kemp,
by the defined model parameters (Short, 1980). Most 1996). Both functions assume that photo-inhibition does
mechanistic seagrass production models follow a mass- not occur in seagrass beds and reduce Pmax either with
balance approach to quantify the change in a seagrass a light half-saturation constant (Michaelis-Menten) or by
population over time as a function of the rate of biomass a light-saturation threshold (hyperbolic tangent equation)
production from gross photosynthesis and loss due to to account for the impact of low light conditions on
respiration, mortality, and herbivory (Madden and Kemp, photosynthesis.
1996; Cerco and Moore, 2001). Mass-balance equations
are set up for all major, or state, variables and are collec-
Temperature
tively known as governing equations. Other examples of
common state variables found in seagrass production Temperature impacts seagrass production by controlling
models include epiphytes, phytoplankton, macroalgae, the rate of chemical reactions within the plant (Lee et al.,
zooplankton, and various fish species (da Silva and 2007). For most biological processes, there is an optimal
Asmus, 2001; Biber et al., 2004; Baeta et al., 2011). temperature range at which the process is the most effi-
cient (Thornton and Lessem, 1978). As temperatures
depart from the optimum range, the rates of processes will
Modeling environmental factors that influence change, impacting overall production (Lee et al., 2007).
seagrass production The relationship between temperature and physiological
Primary production is the result of gross photosynthesis, processes is species, location, and seasonally specific
which is the rate at which organic carbon and oxygen are (Orth and Moore, 1986; Thom, 1990). Therefore no single
produced through the conversion of light energy into chem- algorithm works for all species, and the selection of the
ical energy (Marker and Westlake, 1980). Gross equation used to describe the impact of temperature on
photosynthesis (P) is often modeled as a function at a - seagrass productivity should be based on a wide range of
species-specific optimal photosynthetic rate (Pmax) under temperatures (Carr et al., 1997).
optimal or known fixed environmental conditions
(Vermaat and Hootsman, 1994; Madden and Kemp, 1996; Nitrogen and phosphorus
Cerco and Moore, 2001). Parameters that modify the rate Many seagrass production models also consider the
of processes in seagrass production models should have impact of nutrients on gross photosynthesis as an impor-
a measureable impact on seagrass ecosystems, contain eco- tant growth-limiting factor (Zimmerman et al., 1987;
logically relevant equation coefficients, and avoid empirical Bocci et al., 1997; Lee et al., 2007). Seagrasses are capable
equations as much as possible (Best et al., 2001). While the of obtaining nutrients from both the water column and the
abiotic factors that influence Pmax vary, the availability of sediment, and uptake of nutrients by both roots and shoots
light, water temperature, availability of nutrients, and initial should be included when possible (Lee et al., 2007). Tra-
plant biomass are often included in most seagrass produc- ditionally the Michaelis-Menten equation has been used
tion equations (Carr et al., 1997). to model seagrass production response to nutrient limita-
tions. The equation uses an estimate of the half-saturation
Light constant that corresponds with the nutrient concentration
The primary factor impacting photosynthesis in coastal where productivity is one-half Pmax (Madden and Kemp,
seagrass ecosystems is light availability (Ralph et al., 1996; Cerco and Moore, 2001). Half-saturation constants
2007). However, not all light that reaches the surface of will also vary between the roots/rhizomes and leaves,
the water within a seagrass bed is available for photosyn- and all values are species specific.
thesis (Dunton and Tomasko, 1994; Zimmerman et al.,
1994). For most seagrass production models, the availabil- Modeling environmental factors that influence
ity of light at the leaf surface depends on (1) the total seagrass biomass loss
amount of available light, (2) the amount of light reflected Loss of seagrass biomass can be attributed to factors such
at the surface of the water, (3) water column light attenua- as physical disturbance, microbial decay, herbivory, high
tion, (4) the amount of dissolved and suspended particles rates of respiration, and mortality (Short, 1980; Madden
in the water column, and (5) light reduction by epiphytes and Kemp, 1996; Cerco and Moore, 2001). While the
on the leaf surface (Madden and Kemp, 1996). impacts of environmental factors such as temperature,
544 SEAGRASS PRODUCTION MODELS

dissolved oxygen, and nutrients on respiration have been needs to be taken to remember that predictions are limited
investigated, the mechanistic relationships underlying based on the model’s assumption of known interactions
these impacts are not well known (Marsh et al., 1986; (Best et al., 2001).
Hemminga, 1998; Clavier et al., 2011). Therefore, many
seagrass models use empirical relationships or set loss
rates equal to a constant proportion of plant biomass to Bibliography
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a significant role in seagrass bed resilience and expansion mass and nutrient dynamics in eelgrass (Zostera marina L.):
(Kendrick et al., 1999; Yang et al., 2013) and recovery applications to the lagoon of Venice (Italy) and Øresund
from large-scale declines (Plus et al., 2003; Jarvis and (Denmark). Ecological Modelling, 102, 67–80.
Borum, J., Pedersen, O., Greve, T. M., Frankovich, T. A., Zieman,
Moore, 2010); therefore, key components of the bed J. C., Fourqurean, J. W., and Madden, C. J., 2005. The potential
recovery and expansion dynamics may be missing from role of plant oxygen and sulphide dynamics in die-off events of
seagrass production models when sexual reproduction is the tropical seagrass, Thalassia testudinum. Journal of Ecology,
excluded. In particular, information on seedling physiol- 93, 148–158.
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Future production models should also broaden their growth. Aquatic Botany, 59, 195–215.
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Morse, 2000). Sediment characteristics including organic Labrosse, P., Diagne, A., and Hily, C., 2011. Aerial and under-
content, redox conditions, grain size, nutrient concentra- water carbon metabolism of a Zostera noltii seagrass bed in the
tions, and sulfide levels have all been shown to impact Banc d’Arguin, Mauritania. Aquatic Botany, 95, 24–30.
Coffaro, G., Bocci, M., and Bendoricchio, G., 1997. Application of
seagrass growth and survival (Goodman et al., 1995; structural dynamic approach to estimate space variability of pri-
Borum et al., 2005; Wicks et al., 2009). Therefore, to mary producers in shallow marine water. Ecological Modelling,
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impacts of the sediment environment on the mechanisms da Silva, E. T., and Asmus, M. L., 2001. A dynamic simulation
that drive production and loss within these systems need model of the widgeon grass Ruppia maritima and its epiphytes
to be accurately defined (Eldridge and Morse, 2000). in the estuary of the Patos Lagoon, RS, Brazil. Ecological
Modelling, 137, 161–179.
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and highlight areas where future research is needed (Best lagoon. Marine Ecology Progress Series, 107, 281–293.
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information vital to well-informed management of coastal sediment-seagrass interactions. Marine Chemistry, 70, 89–103.
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shallow regions of the Lower Chesapeake Bay. Journal of
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Orth, R. J., and Moore, K. A., 1986. Seasonal and year-to-year SEA-LEVEL CHANGE AND COASTAL WETLANDS
variations in the growth of Zostera marina L. (eelgrass) in the
Lower Chesapeake Bay. Aquatic Botany, 24, 335–341. Paula Pratolongo
Orth, R. J., Carruthers, T. J. B., Dennison, W. C., Duarte, C. M., Instituto Argentino de Oceanografía, CONICET-
Fourqurean, J. W., Heck, K. L., Jr., Hughes, A. R., Kendrick,
G. A., Kenworthy, W. J., Olyarnik, S., Short, F. T., Waycott, Universidad Nacional del Sur, Bahía Blanca, Argentina
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mortality. Aquatic Botany, 77, 149–164. level
546 SEA-LEVEL CHANGE AND COASTAL WETLANDS

Definitions level during the Holocene have been largely studied along
Coastal wetlands are wetlands within the zone of hydro- the eastern coast of North America, as well as marshes in
logic influence of sea level. northern Europe. The process of wetlands landward migra-
The perimarine zone is an area where freshwater tion under a rising sea level was early described by Dutch
nontidal wetlands persist under the control of sea level. geologists. Hageman (1969) termed the area where fresh-
water wetlands persist under the control of relative sea level
Introduction as the perimarine zone, and he studied the evolution of
freshwater swamps in the western Rhine/Meuse delta, in
Sea-level-controlled wetlands comprise a wide variety of response to the rise in sea level during the Holocene. There
environments from salt marshes to freshwater marshes are examples of sedimentary records (Waller, 1994; Kirby,
and swamps, fens, or barren salt flats, in a continuum of 2001) showing that peat-forming perimarine wetlands
increasing elevation from a shoreline to the upland. In accumulated deep layers of organic matter between about
the case of the intertidal zone, the primary abiotic control 6,000 and 2,000 years BP, and palynological analysis of
on wetland structure and function is a combination of these peat deposits showed sequences of salt marshes, reed
tidal inundation frequency, depth, and duration, known swamps, fens, and woodland carr communities developing
as hydroperiod (Brinson, 1993). Wetland environments under a rising sea level, which maintained a near-surface
below the highest astronomical tide experience direct tidal water table (Waller et al., 1999). A similar model of trans-
inundation, with decreasing frequency and duration as gression was described for the Virginia coast (Virginia,
a function of increasing elevation within the tidal frame. USA), a typical coastal barrier ecosystems extending along
For coastal wetlands landward, the water table is linked the seaward margin of the Delmarva Peninsula (Oertel
to the sea-level influence, which is an important control et al., 1989). In this system, a sustained sea-level rise during
on the groundwater position that provides the waterlogged the Holocene set up a similar sequence of state changes
conditions necessary for their development (Hageman, in wetlands along the mainland edge (Brinson et al.,
1969). The result of the interaction between hydrodynam- 1995). As transgression occurs, upland forest is replaced
ics and elevation is a shore-parallel zonation of plants, with high marsh, high marsh with low marsh, low marsh
where each zone tends to move both vertically and hori- with mudflats, and mudflats with open water (Christian
zontally in response to changing sea level and associated et al., 2000).
stressors (Hayden et al., 1995). In contrast to these well-studied examples of continuous
rising in relative sea level, little is known about wetland
Changes in relative sea level response in coastal environments that developed under dif-
While the globally averaged sea level has been rising from ferent conditions after the last glacial age. Some examples
the last glacial maximum to the present, the relative height from the Gulf of Bothnia describe a downward migration
of the sea with respect to land (relative sea level) can vary of plant zones in response to the continuous land uplift
from place to place due to local tectonic and hydrographic (Vartiainen, 1988; Ecke and Rydin, 2000) and the seaward
effects . As the mass of the continental ice melted, a huge expansion of pioneer plant communities (Zobel and Kont,
weight was released from continental shelves, which rose 1992). A more complex dynamics characterizes wetland
by isostatic rebound of the land. In those areas where the environments in the Atlantic coasts of southern South
ice load was the greatest and the largest rebound occurred, America, where the relative sea level reached
the land rose faster than the sea, the relative sea level a transgressive maximum during the Holocene (Cavallotto
decreased, the coast prograded, and new land emerged et al., 2004; Violante and Parker, 2004). In these systems,
over the last 10,000 years (e.g., Fennoscandia, Finland, the late Holocene marine regression resulted in wide
Labrador). In other areas, the coast initially receded from low-lying coastal landforms inherited from the former estu-
a rising sea until the relative sea level reached a transgres- arine dynamics. These coastal environments are commonly
sive maximum, after which the coast prograded, as the rel- occupied by perimarine wetlands, which undergo increas-
ative sea level decreased to its present elevation (e.g., the ing inundation under the current rising trends in relative
east coast of South America, Western Australia, and East sea level. In the Bahía Blanca Estuary, Argentina, a rising
China). Where the relative sea level fell rapidly, new land sea level is a major cause of wetlands loss in elevated Holo-
constantly emerged, the coastal wetlands continuum cene surfaces (Pratolongo et al., 2013), but the accelerated
migrated seaward, and the Holocene estuarine environ- erosion of soft sediments is also the main source of
ments became part of the terrestrial landscape. Where suspended solids to the tidal sediment budget, allowing
sea level rose, the Holocene estuaries were drowned and deposition and seaward expansion of low salt marshes
new wetlands formed landward. (Pratolongo et al., 2010).
A change in relative sea level produces an alteration in
the ecological state of wetlands, and the different plant asso-
ciations within the coastal wetlands continuum are expected Accelerated sea-level rise
to migrate in response to different hydrologic conditions. A major concern related to climate change is the recently
Coastal wetlands developing under a rising relative sea accelerated sea-level rise associated with the melting of
 SEA-LEVEL CHANGE AND COASTAL WETLANDS 547

sea ice, land ice, and thermal expansion of the ocean mangroves throughout the world, set against the land as
(Webb III et al., 1993; Wigley and Raper, 1993). There a fringe parallel to the shore, that seem capable of
has been considerable discussion as to how coastal wet- responding to sea-level rise by moving inland, but there
lands will develop in the future under climate-enhanced are also some exceptions. Wetlands growing on islands
sea-level rise (Reed, 1990; Simas et al., 2001). Early stud- within estuaries have no land to migrate (Kearney and
ies (Titus, 1987; Boorman et al., 1998) predicted the large- Stevenson, 1991; Wray et al., 1995). Similarly, the migra-
scale loss of coastal wetlands as a consequence of tion of wetlands inland may also be prevented in places
sea-level rise exceeding sediment supply. However, there where the landward slope is too steep or where people
is some evidence to suggest that, at some locations, have built hard barriers landward of the wetlands. In these
the geomorphic response of salt marshes is not sediment cases where transgression stalls, low sediment supply
limited. Many temperate salt marshes built from results in an eroding seaward margin, and
allochthonous sediment show a significant excess of verti- wetland communities may disappear by erosion over time
cal sediment accretion relative to sea-level rise (French, (Brinson et al., 1995).
2006; Stupples and Plater, 2007). In the Mississippi delta,
accretion rates greater than 10 mm year
1 have been mea- Conclusions
sured where there is sufficient sediment input from the Coastal wetlands have naturally evolved in response to
river (Conner and Day Jr., 1991; Cahoon et al., 1995; global changes. Numerous studies show the resilience of
Day et al., 2000), and mangroves in many estuaries in coastal wetlands to natural disturbances. However,
northern Australia tolerated sea-level rise of 8–10 mm changes in sea level, coupled with anthropogenic changes
per year in the early Holocene (Woodroffe, 1995). These on sediment loads, species introduction, nutrient enrich-
accretion rates are higher than most projections and sug- ment, and other human alterations are likely to have
gest that coastal wetlands can persist at a given location, a disproportionate impact on these systems.
in spite of high rates of sea-level rise, if there is sufficient
mineral and organic soil formation.
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10.1016/j.ecss.2013.07.016. Salt Marsh Accretion
 SECONDARY DUNE 549

The Secchi disk is named after its inventor, Italian

SECCHI DISK astronomer Angelo Secchi (1818–1878) (Cialdi and
Secchi, 1965).
Melanie D. Harrison
Water Quality Specialist, National Oceanic and
Atmospheric Administration, Santa Rosa, CA, USA Bibliography
Cialdi, M., and Secchi, P. A., 1865. Sur la Transparence de
la Mer. Comptes Rendu de l’Acadamie des Sciences, 61,
Definition 100–104.
Secchi disk is defined as a simple, standard instrument Hutchinson, G. E., 1957. A Treatise on Limnology. New York:
used to measure water clarity in ponds, lakes, reservoirs, Wiley. Geography, Physics, and Chemistry, Vol. 1.
estuaries, and oceans (Hutchinson, 1957). Whipple, G. C., 1899. The Microscopy of Drinking Water.
New York: Wiley.

Description
It is an 8-in. (20 cm) diameter circular plate, evenly Cross-references
divided by black and white quadrants attached to a PVC Water Clarity
pipe, dowel rod rope, or chain (Figure 1). The line con- Water Quality
tains marked measurements in inch or centimeter intervals
on the rod, pipe, rope, or chain with permanent ink, paint,
or clamps. The plate is lowered into the water of a lake or
other water body, and the depth (Secchi depth) at which it SECONDARY DUNE
is no longer visible from the surface is recorded. The rule
of thumb is that light can penetrate to a depth of 1.7 times Patrick A. Hesp and Graziela Miot da Silva
the Secchi disk depth. School of the Environment, Flinders University,
Most disks used in freshwater bodies have alternating Bedford Park, SA, Australia
black and white quadrants, while disks used in marine
environments are usually all-white. Early disks of the Synonyms
nineteenth century were all white; however, Whipple Back dune; Grey dune; Rear dune
(1899) modified the original disk and “Whipple’s” disk
became the standard in freshwater environments. The
Definition
all-black disk, developed and used in New Zealand, is
used in shallow rivers and streams, because the black disk Secondary dunes are also sometimes referred to as “rear
requires shallow water depths to measure water clarity. dunes” and “back dunes” (http://www.islandbeachnj.org/
Nature/physical/dunes.html) and “grey dunes” (http://peo-
ple.uncw.edu/hosier/BIE/bieclschd/present/dneslkmorph.
LIGHT PENETRATION LIGHT PENETRATION
with low Algae count with high Algae count htm). According to Davies (1980), the terms “primary
SECCHI
dune” and “secondary dune” are generic terms with
DISK relatively specific meanings. He defined “primary dunes”
as dunes derived primarily from the beach, and he identi-
fied two types, namely, free dunes with vegetation
unimportant (e.g., transverse ridges) and impeded dunes
with vegetation important (e.g., foredunes). Davies’
(1980, p. 157) “secondary dunes” are dunes derived
from erosion of impeded primary dunes, and he
documented two types, namely, (1) transgressive dunes
(e.g., blowouts, parabolics, transgressive sheets) and
(2) remnant dunes which are eroded remnants of vegetated
primary dunes.
Other authors have defined secondary dunes in far less
rigorous ways than did Davies (1980):
The dune closest to the ocean is the primary dune, or
foredune, followed by what are called secondary dunes or
back dunes. (http://www.islandbeachnj.org/Nature/physical/
dunes.html)
Secchi Disk, Figure 1 Example of an alternating black and white A secondary dune is “created by modification of the primary
Secchi disk commonly used in freshwater and marine water dunes or by transfers of sand inland from the position of the
bodies to measure water clarity. primary dunes.” (Psuty, 2008, p. 16)
550 SEDIMENT BUDGETS

Secondary dunes (or rear dunes) are located farther inland Definition
(from the foredune or primary dune) and are not often
directly exposed to marine influences. (http://www.crd.bc. A sediment budget is an accounting of sediment volumes
ca/watersheds/ecosystems/coastalsanddunes.htm) entering and exiting a particular region of an estuary or
coast on the temporal scale of interest.
As Davies (1980) noted, his scheme is not entirely sat-
isfactory. He stated that transgressive dunes are derived
from the erosion of primary vegetated dunes, but later in Introduction
their evolution may receive sand directly from the beach. Analysis of sediment budget is regularly applied in coastal
We now know that transgressive dunefields and dune and estuarine sciences and management studies to aid in
sheets may be initiated from erosion of foredunes or para- understanding sediment sources, sinks, and transport path-
bolic dunefields, but may be equally initiated from the ways in a selected region of an estuary or coast within
backshore without any vegetated dunes ever being present a predefined period of time. Depending on the temporal
(Hesp and Thom, 1990; Hesp and Walker, 2013). and spatial scales of interest, different hierarchies of
Transgressive dunes are therefore also “free primary knowledge complexity may be involved in accounting of
dunes.” It is common to now include Davies’ (1980) “free sediment fluxes, sources, and sinks from different pro-
primary dunes” (e.g., transverse ridges, barchans, etc.) cesses that give rise to additions and subtractions within
under a broader classification of transgressive dunefields, the system (Slaymaker, 1997). For example, a coastal
so his class of free primary dunes completely overlaps engineer may be interested in the physical processes
with secondary transgressive dunes and sheets. (e.g., waves, currents) inducing sediment volume change
It could be argued that the terms “primary dunes” and at an inlet or a channel in a period of years or decades
“secondary dunes” should now be abandoned given we but may neglect the biological and chemical compositions
understand much more about coastal dune evolution, of the sediment and their interactions with the environ-
dynamics, and coastal dunefield classification (Hesp, ment. A marine biologist or chemist may be interested in
2002, 2011; Hesp and Walker, 2012). the organic matter carried by mud that enters or exits
a bay but probably has no interest in the volume change
of gravels. A comprehensive study on the geomorpholog-
Bibliography ical history of a coastal system or a large lake, however,
Davies, J. L., 1980. Geographical Variation in Coastal Develop- has to take into account all possible sources and sinks that
ment. London: Longman. contribute to the sediment budget. Thus, to construct the
Hesp, P. A., 2002. Foredunes and blowouts: initiation, geomorphol-
ogy and dynamics. Geomorphology, 48, 245–268.
sediment budget for a specific estuary or coastal system
Hesp, P. A., 2011. Dune coasts. In Wolanski, E., and McLusky, D. S. on a temporal scale, possible sources, sinks, and transport
(eds.), Treatise on Estuarine and Coastal Science. Waltham: processes must be identified prior and their temporal and
Academic Press, Vol. 3, pp. 193–221. spatial variations have to be integrated into analysis. The
Hesp, P. A., and Walker, I. J., 2013. Aeolian environments: coastal sediment budget of a coastal system or an estuary is usu-
dunes. In Shroder, J., Lancaster, N., Sherman, D. J., and ally constructed through the following procedures.
Baas, A. C. W. (eds.), Treatise on Geomorphology. San Diego:
Academic Press. Aeolian Geomorphology, Vol.
11, pp. 109–133. Procedures
Psuty, N. P., 2008. The coastal foredune: a morphological basis for 1. Development of a “conceptual sediment budget” for
regional coastal dune development. In Martinez, M. L., and the research area is recommended in the planning stage
Psuty, N. P. (eds.), Coastal Dunes Ecology and Conservation.
Berlin: Springer. Ecological Studies, Vol. 171. prior to making detailed calculations (Dolan et al.,
1987). The conceptual budget serves as a qualitative
model giving a regional perspective of possible
Cross-references sources, sinks, and transport processes, containing the
Back Dune effects of specific morphological units (e.g., shoals,
Foredune inlets, cliffs, and anthropogenic structures). The con-
ceptual model can be constructed in part by referring
to existing sediment budgets developed for other sites
with similar environmental settings (Rosati, 2005),
SEDIMENT BUDGETS and incorporating additional possible sediment sinks,
sources, and transport pathways in the research area.
An example of a conceptual sediment budget
Wenyan Zhang constructed based on existing data for a coastal system
MARUM - Center for Marine Environmental Sciences, is shown in Figure 1c.
University of Bremen, Bremen, Bremen, Germany 2. Collecting available datasets for the research area that
are commonly used in sediment budget analysis. These
Synonyms may include (1) digital elevation models (DEM) cover-
Sedimentary budget ing both the terrestrial and subaqueous parts of the area
 SEDIMENT BUDGETS 551

Sediment Budgets, Figure 1 (a) DEM and major morphological units in a river-lagoon-barrier system at the southern Baltic Sea,
(b) cell division and sediment transport pathways in the system, and (c) constructed sediment budget of the system based on Emeis
et al. (2002) and Zhang et al. (2013).
552 SEDIMENT BUDGETS

in different periods, including seabed mapping and Figure 1b shows possible sediment transport pathways
profile measurements; (2) aerial photographs of the for an estuary and its adjacent coast.
research area in different periods, which are especially 5. Calculating the volume of sediment transfer in each
helpful for indicating morphological changes and transport pathway. A general equation for the sediment
major transport pathways of the area; (3) records and volume change (DV) of a cell is expressed by:
data of anthropogenic activities in the area and related X X
change of sediment properties and volumes; (4) records DV ¼ Qsource
Qsink þ I
O ð1Þ
and data of natural hazards (e.g., storms, floods) occur-
ring in the area within the time span of interest; where Qsource and Qsink represent the amounts of natural
(5) meteorological conditions of the area (e.g., wind sources (i.e., input) and sinks (i.e., output) transferred at
and wave monitoring data) and possible climate the cell boundary, respectively. I and O are the amounts
change impacts (e.g., sea level change data); of artificial sediment input (e.g., beach nourishment) and
(6) tectonic movement map of the area; (7) map of sed- output (e.g., dredging) transferred at the cell boundary,
iment grain-size distribution on the seabed; (8) mea- respectively. The quantity of sediment volume transfer
surements of suspended sediment concentration and on each transport pathway should be evaluated based
discharge in representative periods (e.g., flood and on the acquired data. For example, comparison among
ebb tides, dry and wet seasons); and (9) estimates of aerial photographs in different periods gives the detailed
waves, currents, and sediment transport aided by information on the rates of coastline change. Field mea-
numerical models. surements on currents, waves, and sediment concentra-
3. Dividing the research area into a series of cells. Each tions help to quantify the typical rates of sediment
cell acts as a control unit with clearly defined bound- transport caused by natural processes. Analysis of
aries. Sediment budget is cell-dependent, and calcula- extreme events provides information on the frequency
tion of sediment volume change is performed only at and magnitude of consequent sediment transport. Com-
the boundaries of each cell. Cells can be defined by parison among DEMs, coastal profiles, or aerial photo-
geological or morphological controls, available data graphs provides information on calibration and
resolution, coastal structures, and knowledge of the site validation of the estimated quantities in (1).
(Rosati, 2005). Depending on the specific sources, 6. Estimation of uncertainty in the calculated sediment
sinks, and transport processes in the research area, cells budget. One should consider that uncertainty always
with different sizes can be assigned. Sub-cells can also exists in the calculated sediment budget. Uncertainty
be defined within a cell to better measure the sediment of a sediment budget mainly comes from two sources:
budget of a region with varying rates of accretion and (1) measurement error and (2) true uncertainty due to
erosion. A well-known example is given by Bowen temporal and spatial variability of the transport pro-
and Inman (1966) who introduced the concept of litto- cesses (Rosati, 2005). Measurement error contains the
ral cells. A littoral cell is usually a zone parallel to the error made during measurement processes (e.g.,
shoreline, bounded by the foot of the foredune or cliff improper positioning of the instruments) and the error
in the landward direction and depth-closure point in in the measured signals (e.g., noises). Net sediment
the shoreward direction. Changes of the sediment vol- flux across an estuary mouth or an inlet is difficult to
ume in a littoral cell directly induce changes in the measure, particularly when the sediment flux on the
coastline, and ideally they are defined to minimize ebb and flood tides are high. In estuaries, sources and
longshore sediment exchange with adjacent cells, for sinks can be expressed as rates of exchange in a tidal
example, setting the lateral boundary of a littoral cell cycle. However, many of the exchanges are not at
at the nodal points where the net longshore transport steady rates, for example, sediment exchange in sto-
rate is zero or defining a pocket beach bounded by chastic extreme events (e.g., storms, floods) can be
rocky headlands, which is able to conserve its majority several orders of magnitude higher than in normal con-
of sediment, as a littoral cell. An example of cell divi- ditions. Another source of uncertainty comes from
sion of an estuary and its adjacent coast is shown in numerical models which are applied. For example, lit-
Figure 1a, b. toral drift is normally estimated on the basis of standard
4. Defining sediment transport pathways at the cell equations (e.g., the Coastal Engineering Research Cen-
boundaries. Sediment transport pathways specify the ter (CERC) (USACE, 1984) and Kamphuis
sediment transfer direction and corresponding pro- (Kamphuis, 1991) methods) or more complicated
cesses between adjacent cells. The transport pathways models (e.g., Zhang et al., 2013). However, different
can be estimated through knowledge of the processes models may yield quite different results on the rate of
(natural and anthropogenic) occurring at the site. longshore sediment transport even though they
Interpretation of aerial photographs, trend analysis of are based on the same boundary inputs. Thus, the
sediment grain-size distribution, and particle tracking estimated littoral drift is often biased and should be cal-
also aid to define sediment transport pathways. ibrated by field measurements. Impacts of unknowns
 SEDIMENT COMPACTION 553

(e.g., sediment porosity) and subordinate processes Cross-references

that are not taken into account in calculating the sedi- Sediment Grain Size
ment budget also contribute to the uncertainty.

The degree to which one process affects another and the SEDIMENT COMPACTION
contribution of a transport process to the sediment budget
both depend on the magnitude and frequency of recur- Nils-Axel Mörner
rence of these processes at a site (Dietrich et al., 1982). Paleogeophysics and Geodynamics, Saltsjöbaden, Sweden
A feasible way to better evaluate the uncertainty is to
develop a scheme which presents the probability distribu- Synonyms
tions of all possible sources and sinks in the research area.
Sediment dewatering; Sediment lithification
Summary Definition
Sediment budget analysis is useful in estuarine and coastal Clay, silt, gyttja, and peat are sediments that contain sub-
studies. A comprehensive understanding of the sediment stantial amounts of water at the time of deposition. As
source-to-sink transport and possible transport pathways the sediments continue to accumulate through time, they
within a system can be gained through a detailed budget undergo dewatering which leads to compaction.
analysis. Due to the complexities of sediment dynamics,
much progress is still needed to reduce the uncertainty Description
(e.g., improving the quality of field measurements and
knowledge of sediment transport by multi-scale pro- The transition from particles in suspension to their accumu-
cesses) for a better quantification of sediment budgets in lation in bottom sediments implies the loss of water so that
coastal and estuarine environments. the particles hold together by cohesion of friction. The pro-
cess of dewatering continues in the upper part of the sedi-
ment column until some sort of equilibrium is reached
Bibliography (Mörner, 2010; Brian et al., 2012). This is the case with
Bowen, A. J., and California. Inman, D.L., 1966. Budget of littoral clay, silt, and gyttja (organic matter). Peat is susceptible to
sand in the vicinity of Point Arguello, U.S. Army Coastal compaction (e.g., Jelgersma, 1961). Loading generates sed-
Engineering Research Center, Technical Memorandum iment compaction (e.g., when heavy harbor construction
No. 19, 56p. rests on soft sediments). Many tide gauges are located on
Dietrich, W.E., Dunne, T., Humphrey, N.F., and Reid, L.M., 1982.
Construction of sediment budgets for drainage basins. In such construction, which leads to site-specific subsidence
Sediment Budgets and Routing in Forested Drainage Basins. (e.g., Mörner, 2010). Dewatering and compaction are parts
U.S. Forest Service Gen. Technical Report, PNW-141, pp. 5–23. of the process of lithification. Artificial water withdrawal
Dolan, T. J., Castens, P. G., Sonu, C. J., and Egense, A. K., 1987. may generate substantial sediment compaction (e.g., in
Review of sediment budget methodology: oceanside littoral cell, the Bangkok region and in the Nigita area in Japan). Com-
California. In Proceedings, Coastal Sediments ’87. Reston, VA: paction is a serious problem in the reconstruction of
ASCE, pp. 1289–1304.
Emeis, K., Christiansen, C., Edelvang, K., Graf, G., Jahmlich, S., sea-level changes adding a factor of local to site-specific
Kozuch, J., Laima, M., Leipe, T., Loeffler, A., Lund-Hansen, subsidence which may be hard to define.
L. C., Miltner, A., Pazdro, K., Pempkowiak, J., Shimmield, G.,
Shimmield, T., Smith, J., Voss, M., and Witt, G., 2002. Material Bibliography
transport from the near shore to the basinal environment in the
southern Baltic Sea: II. Synthesis of data on origin and properties Brian, M. J., Long, A. J., Woodroff, S. A., Petley, D. N., Milledge,
of material. Journal of Marine Systems, 35(3–4), 151–168. D. G., and Parnell, A. C., 2012. Modelling the effects of sedi-
Kamphuis, J. W., 1991. Alongshore sediment transport rate. Journal ment compaction on salt marsh reconstructions of recent
of Waterway, Port, Coastal, and Ocean Engineering, 117(6), sea-level rise. Earth and Planetary Science Letters, 345(48),
624–641. 180–193.
Rosati, J. D., 2005. Concepts in sediment budgets. Journal of Jelgersma, S., 1961. Holocene sea-level changes in the Netherlands.
Coastal Research, 21(2), 307–322. Mededelingen van de Geologische Stichting Serie C, VI(7),
Slaymaker, O., 1997. A pluralist, problem-focussed geomorphology. 1–101.
In Stoddart, D. R. (ed.), Process and Form in Geomorphology. Mörner, N.-A., 2010. Some problems in the reconstruction of mean
London: Routledge, pp. 328–339. sea level and its changes with time. Quaternary International,
U.S. Army Corps of Engineers, 1984. Shore Protection Manual, 221, 3–8.
4th edn. Washington: U.S. Corps of Engineers. Department of
the Army.
Zhang, W. Y., Deng, J., Harff, J., Schneider, R., and Dudzinska- Cross-references
Nowak, J., 2013. A coupled modeling scheme for longshore sed- Estuarine Sediment Composition
iment transport of wave-dominated coasts – a case study from Sediment Erosion
the southern Baltic Sea. Coastal Engineering, 72, 39–55. Sediment Grain Size
554 SEDIMENT EROSION

Erosion did not stop but continues vertically to the bottom

SEDIMENT EROSION sediments, with rates from 0.1 to 0.5 m/year in front of the
seawalls (Frihy et al., 2008; El-Gamal, 2012).
Ayman A. Elgamal The actual erosion rate within an area may vary within
Marine Geology Department, Coastal Research Institute, estuarine systems and over time, depending upon individ-
National Water Research Center, Alexandria, Egypt ual site conditions and the frequency of storms or other
causes of erosion (Rogers and Skrabal, 2011). Breaking
Definition waves only several centimeters in height have the energy
Sediment erosion is the process of detachment and trans- to move sand and other sediments both offshore into
port of sediments by water, wind, ice, and gravity deeper water and alongshore. Once coastal sediments are
(Kumar and Ramachandra, 2003). in motion, they are often redistributed based on grain size
and density (Rogers and Skrabal, 2011). High-density
Description materials tend to concentrate in areas of beach erosion,
Estuarine erosion takes place by the wearing away of whereas minerals of lower density and coarse size are
shoreline and bottom sediments (Rogers and Skrabal, selectively entrained by waves and currents and preferen-
2011). On an estuarine beach, sediment erosion occurs pri- tially transported to zones of beach accretion where they
marily by wave action, tidal currents, littoral currents, and are deposited (Frihy et al., 1995; El-Gamal and Saleh,
deflation (CERC, 1984). 2012).
Erosion is a natural geological process, and the rate of El-Gamal (2012) summarized different indicators for
shoreline and bottom sediment erosion will vary from recognizing where erosion and accretion take place in
place to place (Rogers and Skrabal, 2011). It can be signif- beach profiles. Rogers and Skrabal (2011) grouped the
icantly modified by human activities (Striebig, 1999). The types of structures and methods for managing or control-
severe erosion of the Nile River Rosetta Estuary, Egypt ling estuarine basin erosion from the shoreline to bottom
(Figure 1), after establishment of Aswan High Dam, is areas. These include:
a good example of erosion due to anthropogenic activities • Land management (advance planning of building loca-
(Fanos, 1995; Stanley and Warne, 1998). Coastal defense tions and other development activities)
structures, particularly breakwaters, were constructed • Vegetation (e.g., marsh plants)
along the two sides of the Rosetta Promontory on land • Beach fill or nourishment (addition of sand to a beach to
(Fanos et al., 1995). These seawalls were 5.3 m above compensate for expected or realized losses)
mean sea level and 8 m deep. Mediterranean water • Shoreline hardening (e.g., bulkheads and seawalls)
has been in contact with these seawalls since 1995. • Sand traps (e.g., groins and breakwaters)

1900
Bibliography
1941
1900 CERC, 1984. Shore Protection Manual, vol II. Coastal Engineering
1964 Research Center (CERC), Department of the Army Water Ways
Experiment Station, Army Corps of Engineers, Washington, DC.
Old
1971 El-Gamal, A. A., 2012. New approach for erosion and accretion
1941 coasts discrimination. Journal of Coastal Research, 28(2),
389–398.
1964 El-Gamal, A. A., and Saleh, I. H., 2012. Radiological and mineral-
1982 ogical investigation of accretion and erosion coastal sediments in
Nile Delta region, Egypt. Journal of Oceanography and Marine
1971
Science (JOMS), 3(3), 41–55.
1988 Fanos, A. M., 1995. The impact of human activities on the erosion
Existing 1990 and accretion of the Nile delta coast. Journal of Coastal
1995 Research, 11(3), 821–833.
Fanos, A. M., Naffaa, G. M., Gewilli, M. Y., and Ali, M. M., 1995.
Erosion of Rosetta Promontory, the Nile Delta, Egypt. In Inter-
1982 Protection line national Conference on Coastal and Port Engineering in Devel-
Protection line
oping Countries, September 1995, Rio de Janeiro, Brazil.
1988 Frihy, O. E., Lotfy, M. F., and Komar, P. D., 1995. Spatial variations
0.0 1000 2000 3000 m in heavy minerals and patterns of sediment sorting along the Nile
1995
Delta. Egypt. Sedimentary Geology, 97, 33 p.
2005 Frihy, O. E., Shereet, S. M., and El Banna, M. M., 2008. Pattern of
2005 beach erosion and scour depth along the Rosetta Promontory and
their effect on the existing protection works, Nile Delta, Egypt.
Journal of Coastal Research, 24(4), 857–866.
Kumar, R., and Ramachandra, T. V., 2003. Water soil and sediment
Sediment Erosion, Figure 1 Erosion of the Nile River Rosetta investigation to explore status of aquatic ecosystem. In Presenta-
Estuary after construction of the Aswan High Dam (Fanos, 1995). tion at National Seminar on River Conservation and
 SEDIMENT GRAIN SIZE 555

Management, January 2–4, St. Thomas’ College Thrissur, Ker- five specific parameters. The graphic mean size is an arith-
ala, Limnological Association of Kerala. metic average of a series of diameter values. The median
Rogers, S., and Skrabal, T. E., 2011. Managing erosion on estuarine diameter is the 50th percentile diameter of a cumulative fre-
shorelines. The Sound Front Series.
Stanley, D. J., and Warne, A. G., 1998. Nile delta in its destruction quency curve drawn on arithmetic probability paper. Stan-
phase. Journal of Coastal Research, 14, 794–825. dard deviation is expressed as measures of dispersion
Striebig, B. A., 1999. Sustainable residential design and construc- (sorting) of sediments, and it is the square root of the arith-
tion. The Pennsylvania State University, Information Technol- metic average of the squares of all the deviations from the
ogy Services (ITS), https://www.courses.psu.edu/c_e/ mean size value of a series of observations. Skewness mea-
c_e433_bas124/erosion.html sures the asymmetry of the grain size distribution. Grain
size distribution is skewed when the mean deviates from
Cross-references the median. Skewness of the sediments for symmetrical
Sediment Transport
grain size distribution is zero. Skewness becomes negative
when the grain size is skewed toward smaller phi value, and
it is positive when skewed toward higher phi value. Kurto-
sis is the condition of peakedness or flatness of the graphic
representation of a statistical distribution.
SEDIMENT GRAIN SIZE
Expression of sediment grain size
Gautam Kumar Das Estuarine waters transport a wide range of sediments vary-
Department of Chemical Engineering, Jadavpur ing in size from 2 mm (0.002 mm) to more than 4 mm, but
University, Kolkata, West Bengal, India finer sizes dominate most estuaries. A few estuaries trans-
port sand (>62 mm), gravel, and larger sediments.
Synonyms Sediment grain size is measured in metric units as
Grain size analysis; Grain size distribution; Sediment par- propounded by Wentworth (1932). It was expressed as
ticles size; Texture phi (f) by Krumbein (1938), since the logarithmic diame-
ter has more significance in a discussion of the statistical
Definition relations of sediments. Sediment grain size in phi (f) is
Texture refers to the general physical appearance of the expressed as the negative logarithm to the base 2 of the
sediment. sediment particle diameter in mm. Thus sediment grain
Grain size is the average size of the grains in a sediment size is expressed as follows:
sample. It is also known as the particle size.
f ¼
log2 e
Sand consists of grains of particle size ranging from
0.0625 to 2 mm (0.002–0.08 in.). It pertains to particles where f is the sediment grain size and Ɛ is the negative
that lie between silt and granules on the Wentworth scale numerical value of the diameter. Ɛ is equal to 2, 1, ½, ¼,
of grain size. Sand size class ranges from
1.0 to 4.0 (phi). etc., whereas f is equal to
1, 0, +1, +2, etc. Thus f
Silt consists of grains of particle size ranging from increases with decreasing diameter.
0.008 to 0.0625 mm (0.0003–0.002 in.). It is intermediate Sand is the particle size of 0.0625–2 mm
in size between sand and clay. Silt size class ranges from (0.002–0.08 in.). It pertains to particles that lie between
4.0 to 8.0 (phi). silt and granules on the Wentworth scale of grain size.
Clay consists of grains of particle size between silt and Sand size class ranges from
1.0 to 4.0 (phi). Silt is
colloid. These include any of the various hydrous alumi- a particle size of 0.008–0.0625 mm (0.0003–0.002 in.).
num silicate minerals that are plastic, are expansive, and It is intermediate in size between sand and clay. Silt size
have ion-exchange capacities. Clay size class ranges from class ranges from 4.0 and 8.0 (phi). Clay is a particle size
8 (phi) and onwards. between silt and colloid. Any of the various hydrous alu-
minum silicate minerals are plastic, are expansive, and
Introduction have ion-exchange capacities. Clay size class ranges from
Sediment is made up of loose particles of sand, silt, and 8 (phi) to higher.
clay. Particle size refers to the diameter of individual
grains of sediment. It is a fundamental descriptive measure Sampling of estuarine sediments
of sediments from any environment. Grain size analysis of Utmost care is needed in sampling estuarine sediments
estuarine sediments is required to study the trends in sur- because the grain size analyses are sensitive to the manner
face processes related to dynamic conditions of transporta- in which the original samples are collected, handled, and
tion and deposition. preserved. Introduction of any foreign particle into the
sample through improper care, cleaning of equipment, or
Measures of the grain size distribution processing can alter the texture. Estuarine landforms such
The nature of grain size distribution in sediments of estua- as point bars, river mouth bars, tidal shoals, major tidal
rine or any environment can be described on the basis of inlets, and upstream and downstream of the rivers are ideal
556 SEDIMENT GRAIN SIZE

sites for the collection of samples. Data regarding tide, drawn on log-probability plots. It helps in the interpretation
current, waves, depth, turbidity, etc. are also collected dur- of separate populations of estuarine sediments. Three differ-
ing sampling of estuarine sediment for size analysis. An ent methods of plotting are considered for grain size distri-
instrumental tripod ALICE fitted with various sensors is bution, including grain size with frequency percent,
used to collect data regarding the above physical parame- cumulative frequency percent, and the log-probability
ters. Numerical models are used for data interpretation. cumulative frequency percent. Log-probability cumulative
frequency curves are the most accepted methods used by
Statistical analysis of grain size sedimentologists in assessing depositional environments
of estuaries. In each log-probability curve, there are at least
The texture of muddy sediments in estuaries is examined
four control points, i.e., four separate lognormal
by mechanical analysis following the sieving-cum-
populations, where each population is truncated and the for-
pipetting method. Sand and gravel fractions are deter-
mer one joined with the latter one to make a single grain
mined by sieve analysis (Krumbein and Pettijohn, 1938)
size distribution. Each lognormal population is composed
using sieves of different mesh sizes marked as ASTM
of different mean and standard deviation values.
(American Society for Testing and Materials). The statisti-
cal size parameters are calculated using the formula of
Folk and Ward (1957) from the cumulative curves drawn Grain size characteristics of different estuarine
on arithmetic probability paper. Subsequently, rapid sedi- landforms
ment analyzers (RSA) propounded by Zeigler et al. (1960) There is an interrelationship between grain size character-
and Schlee (1966) and electro-resistance multichannel istics and the depositional pattern in a tide-dominated
particle-size analyzers (EMPSA) are introduced for auto- estuarine environment. Interpretation of the grain size fre-
mated analysis and calculation of statistical parameters quency curve is based upon the pattern of curves and split-
of sediments. Contemporaneous with them, Kane and ting of each curve into segments separated by the marked
Hubert (1962) and Schlee and Webster (1967) developed breaks and inflection. Sediments from different geomor-
Formula Translation (FORTRAN) programs for textural phological areas such as point bar, mid-channel bar, swash
analysis of sediment particle parameters. Gradually, Algo- bar, river bank, and areas of other morpho-ecological
rithmic Language (ALGOL) by Jones and Simpkin interests may be considered for grain size analysis.
(1970), Beginner’s All-Purpose Symbolic Instruction
Code (BASIC) by Sawyer (1977), and handheld calcula- Texture of mudflat sediments
tors by Benson (1981) were programmed for statistical The graphic mean size of surficial and subsurface samples
grain size computations. About the same times, many lies within the silt fraction with moderately well to poor
workers (Muerdter et al., 1981; Poppe et al., 1985) and sorting. Sediments of mudflat samples show a positive
organization (Coulter Electronics Inc., 1989) introduced skewness and reflect infiltration of suspended clay from
hardware and software packages for electro-resistance tidal standstill through the pore spaces of the dominating
multichannel particle-size analysis. Introduction of silt and subordinate sand populations. Sediments of the
computer-driven, integrated particle-size analysis instru- mudflats traversing the creek bottom, however, exhibit
ments fitted with settling tubes (Zeigler et al., 1964; Rigler negative skewness. This is because of the mixing of
et al., 1981) automated and modernized sediment grain a greater proportion of sand fraction with the dominant silt
size analysis. The settling tube, also called rapid sediment population. The removal of clay with flowing creek water
analyzer design based on using the pressure differential leaves the creek bottom with more sands compared to
between two columns of water that have a common head, other places of the mudflat, and this leads to a negative
provides for efficient analysis of sand-sized material by skewness of the distribution patterns. Cumulative curves
setting the grains where results are relayed to a personal drawn from the mudflat sediments reveal close similarity
computer associated with data acquisition software drivers in pattern. The same is true for the creek bottom sediments
(Syvitski et al., 1991). A computer program called when considered separately. The prominent breaks in the
GRADISTAT (Blott and Pye, 2001) has been written for cumulative curves reflect changes in the mode of transport
the rapid statistical analysis of size data from any standard of suspended particles.
measuring technique. The program runs with a Microsoft
Excel package. It is very useful and produces a range of Texture of sand flats and silt flats
graphical representations, including frequency curves The sediments of sand flats consist of well-sorted 95 %
and plots. fine to very fine sands in comparison to silt flat sediments
having 10 % sand, 95 % silt, and 5 % clay. Cumulative
Cumulative curves curves for the sand flat sediments are very similar in pat-
Cumulative curves plotted on arithmetic probability paper tern and differ much from the silt flat sediments.
represent grain size distributions of different subpopula-
tions which have a lognormal distribution depicting differ- Texture of tidal shoal sediments
ent modes of transportation of sediments (Visher, 1969). Tidal shoals, in general, show greater accumulation of
Sediment grain size is determined from grain size curves mud in the upstream portion and sand in the downstream
 SEDIMENT GRAIN SIZE 557

stretches. This characteristic depositional behavior of Texture of mangrove swamp sediments

sediments favors tidal accumulation rather than its for- Tidal flats are mostly siliciclastic. The sediments are gener-
mation from unidirectional flow from the upper to the ally silty to sandy. All the cumulative curves are
lower stretches. The downstream accumulation of sand non-lognormal. The silty sediments are extremely zigzag
suggests the influence of flood flow, whereas the and reflect deposition out of ebb-flood cycles. The statisti-
upstream mud is the result of a standstill during the high cal size parameters show that the sediments are mainly well
and low tides. sorted to moderately sorted, with graphic mean size belong-
The sediments are generally very well-sorted to ing to the silt fraction. Most sediment samples show a slight
medium-sorted fine silt in nature. Samples are slightly tendency of negative skewness. The cumulative curves
negative to slightly positively skewed (SK1) and are largely show high peakedness, with KG values often greater
platykurtic in nature. Inflections generally at two to than 1.0. The cumulative curves for the sandy sediments
three truncation points are a statistical fact for all exhibit three distinct subpopulations (i.e., rolling, saltation,
the samples. All the cumulative curves are mostly and suspension subpopulations). The cumulative curves for
nonlinear, showing close resemblance in pattern with the silty sediments, however, show more intricate zigzag
each other. patterns reflecting ebb-flood cycles.

Texture of point bar sediments Texture of dune sands

Texturally, the sediments are generally composed of 95 % The grain size of dune sands generally shows lognormal
silt, with subequal proportions of fine sand and clay distribution with very fine sand and well-sorted sediments.
forming the remaining 5 %. Sediments are mostly well The graphic mean size ranges between 2.5 and 3.5 phi.
sorted to moderately sorted, with graphic mean size The graphic standard deviation indicates well-sorted finer
belonging mostly to the silt fraction. Most of the sedi- sand particles. The sorting value reflects reworked inter-
ments show a slight tendency toward negative skewness. tidal zone sediments through wind transportation cycles.
The cumulative curves mainly exhibit higher peakedness, Dune sediments exhibit a slightly positive skewness
with KG values often close to 1.0. because of admixture of very small quantity of finer
Sandy sediments of the point bars show a completely suspended particles to the lognormal saltation population.
different pattern from that of sandy silty sediments. The Kurtosis of sediments is very close to 1.0 indicating log-
cumulative curves for these sediments show a nonlinear normal distribution with mesokurtic characteristics.
pattern and are perfectly comparable to that of the sandy
point bar sediments. These sediments display two major Textural sensitiveness
inflections at 2.25 phi and 3.25 phi, respectively. These There is rhythmicity in the nature of deposition, which
inflections divide the curves into three subpopulations as perhaps indicates the depositional pulses for the ebb and
rolling, siltation, and suspension, respectively. The salta- flood flows through rivers. The inflection points in the
tion population constitutes about 75 % of the materials, curves between successive ebb-flood cycles are marked
the rest being deposited by either rolling or suspension. at 2.5–3.0 phi, 3.5 phi, 5–6 phi, and 8 phi sizes, respec-
Silty sediments having range from 4 to 9 phi show exactly tively. Such rhythmicity in the nature of depositional
the same pattern where the three subpopulations can be behavior of tidal sediments is supposed to be highly pro-
well recognized. In these curves, the inflections take place cess responsive.
at 6–7 phi, respectively. The central saltation population Sandy sediments (ranging from 1.5 to 4 phi) from the
constitutes about 75 %. These sediments do reflect their point bars and mid-channel bars of estuaries display
deposition from tractive movements of water in the estua- a completely different pattern from that of sandy silt sedi-
rine flow condition. ments. The cumulative curves for these sediments exhibit
a nonlinear pattern and are comparable to that of the other
point bar sediments. These sediments have two major
Texture of marsh sediments inflections, one at 2.25 phi and the other at 3.0–3.5 phi.
The marsh sediments are composed of 90 % silt with These inflections divide the curves into three subpopula-
subequal proportions of fine sands and clays forming the tions as rolling, saltation, and suspension, respectively.
rest. The marsh sediments are mostly moderately well The saltation population generally constitutes about
sorted to moderately sorted, with the graphic mean size 75 % of the total material, the rest being deposited by
belonging to the silt fraction. Most samples exhibit either rolling or suspension. Silty sediments having
a slight tendency of negative skewness. The negative a range from 4 to 9 phi from similar areas also show
skewness in the marshy region of the estuaries perhaps exactly the same pattern where the three abovementioned
indicates trapping of larger bed-load particles by the marsh subpopulations can be well recognized. In these curves,
vegetation. The cumulative curves primarily show a high the inflections take place at 6–7 phi, respectively. The cen-
peakedness with greater KG values. The statistical size tral saltation population constitutes about 70 %. These
parameters of the sediment samples are very much analo- sediments reflect their deposition from tractive move-
gous to lagoon or distal shelf sediments. ments of water in a unidirectional flow condition.
558 SEDIMENT RESUSPENSION

Summary Schlee, J., 1966. A modified woods hole rapid sediment analyzer.
Journal Sedimentary Petrology, 30, 403–413.
Sediment grain size is considered one of the most impor- Schlee, J., and Webster, J., 1967. A computer program for grain-size
tant tools for the interpretation of depositional environ- data. Sedimentology, 8, 45–54.
ments in estuaries (Das, 2009). Sorting indicates the Syvitski, J. P. M., Asprey, K. W., and Clattenburg, D. A., 1991. Prin-
process of modification of the sediments, whereas graphic ciples, design and calibration of settling tubes. In Syvitski,
mean size reflects the environment of sediment accumula- J. P. M. (ed.), Principles, Methods and Applications of Particle
tion. Rigorous flow transports the sediments in the deposi- Size Analysis. New York: Cambridge University Press, pp. 3–21.
Visher, G. S., 1969. Grain size distributions and depositional pro-
tional environment causing poor sorting of sediment cesses. Journal of Sedimentary Petrology, 39(3), 1074–1106.
particles. Sediments are skewed in selective transporta- Wentworth, C. K., 1932. The mechanical composition of sediments
tion, and a particular sediment population is characterized in graphic form. University of Iowa Studies in Natural History,
by inclusive graphic kurtosis (Davis, 1983). 14(3), 127.
Grain size of estuarine sediments reflects the nature of Zeigler, J. M., Whitney, G. G., Jr., and Hayes, C. R., 1960. Woods
source sediments and their hydrodynamic condition of hole rapid sediment analyzer. Journal Sedimentary Petrology,
deposition. Generally erosion dominates along the sea- 30, 490–495.
Zeigler, J. M., Hayes, C. R., and Webb, D. C., 1964. Direct readout
ward reach of the estuary, with high wave energy and of sediment analyses by settling tube for computer processing.
deposition predominating in the landward reaches of rela- Science, 145, 51.
tively quieter environment. Thus finer muddy sediments
are deposited on the estuarine banks and flanks of the
mid-channel bars and point bars, with low depositional Cross-references
energy. Sediment Sorting
Sediment Transport

Bibliography
Benson, D. J., 1981. Textural analyses with Texas instruments
59 programmable calculator. Journal of Sedimentary Petrology, SEDIMENT RESUSPENSION
51(2), 61–62.
Blott, S. J., and Pye, K., 2001. GRADISTAT: a grain size distribu-
tion and statistics package for the analysis of unconsolidated sed- Tian-Jian Hsu
iments. Earth Surface Process – Landforms, 26, 1237–1248. Civil & Environmental Engineering, University of
Coulter Electronics Inc., 1989. Coulter Multisizer AccuComp Color Delaware, Newark, DE, USA
Software: Reference Manual. Florida: Hialeah.
Das, G. K., 2009. Grain size analysis of some beach sands from the Definition
Indian coasts. Geographical Review of India, 71(1), 10–18.
Davis, R. A. D., 1983. Depositional Systems. Englewood Cliffs, NJ: Sediment resuspension is the suspension and redistribu-
Prentice-Hall. tion of previously deposited sediment particles in the
Folk, R. L., and Ward, W., 1957. Brazos river bar-A study in the sig- water column due to hydrodynamic forcing.
nificance of grain size parameters. Journal of Sedimentary Sediment suspension is the mobilization and entrain-
Petrology, 27, 3–26. ment of sediment particles from the bed due to hydrody-
Friedman, G. M., 1967. Dynamic processes and statistical parame-
ters compared for size frequency distribution of beach and river namic forcing.
sands. Journal of Sedimentary Petrology, 37, 327–354.
Jones, S. B., and Simpkin, P., 1970. A computer program for the Introduction
calculation of hydrometer size analyses. Marine Geology, 9, Sediment resuspension plays a critical role in estuarine
M23–M29. sediment budgets. Sediment fluxes in an estuary can be
Kane, W. T., and Hubert, J. F., 1962. FORTRAN program for the
calculation of grain-size textural parameters on the IBM 1620 contributed from both marine and terrestrial origins.
computer. Sedimentology, 2, 87–90. Resuspension of marine sediments, mainly sand, by near-
Krumbein, W. C., and Pettijohn, F. J., 1938. Manual of Sedimentary shore waves, currents, and wave-induced currents, shapes
Petrography. New York: D. Appleton – Century. coastal landforms (Dean and Dalrymple, 2002). Often,
Moiola, R. J., and Weiser, D., 1968. Textural parameters: an evalu- marine sand can also enter (or reenter) an estuary facili-
ation. Journal of Sedimentary Petrology, 38, 45. tated by tidal asymmetry and estuarine circulation
Muerdter, D. R., Dauphin, J. P., and Steele, G., 1981. An interactive
computerized system for grain size analysis of silt using electro- (MacCready and Geyer, 2010). A significant amount of
resistance. Journal of Sedimentary Petrology, 51, 647–650. terrestrial sediment is fine-grained, such as clay and silt.
Poppe, L. J., Eliason, A. H., and Fredericks, J. J., 1985. APSAS: Fine-grained sediments become cohesive in estuaries and
An Automated Particle-Size Analysis System. U.S. Geological form floc aggregates (also called mud) through floccula-
Survey Circular, vol 963. tion (Winterwerp and van Kesteren, 2004). Flocs are vehi-
Rigler, J. K., Collins, M. B., and Williams, S. J., 1981. A high – cles of organic carbon, nutrients, and pollutants, which
precision digital recording sedimentation tower for sands.
Journal Sedimentary Petrology, 51, 642–644. further leads to many ecological and geochemical out-
Sawyer, M.B., 1977. Computer Program for the Calculation of comes in the water column and benthic boundary layer
Grain-Size Statistics by the Method of Moments. U.S. Geological (Santschi et al., 2005). When terrestrial sediments are
Survey Open File Report 77–580. delivered to estuaries or river mouths, significant trapping
 SEDIMENT RESUSPENSION 559

and deposition occur due to diminishing flow intensity, over a muddy bed (Beardsley et al., 1995), where a thick
estuarine stratification, and flocculation (Wright, 1977; layer of mud up to several meters is present (Trowbridge
Geyer et al., 2004). Hence, sediment resuspension and Kineke, 1994). The existence of the lutocline also
becomes the key process to further deliver sediments off- implies significantly suppressed mixing of other solutes
shore before the journey of sediment source to sink can and hence effects on benthic boundary layer. Since the
be completed (Wright and Nittrouer, 1995). Although sed- presence of sediment can attenuate turbulence, there exists
iment resuspension is driven by the overlaying hydrody- a carrying capacity for both current-dominant and wave-
namics, resuspension also leads to a variety of seabed dominant sediment resuspension (Winterwerp, 2001;
characteristics, such as bedform and fluid mud, which in Ozdemir et al., 2011), and that for a given flow intensity
turn determines the bottom friction (dissipation) experi- and sediment characteristics (e.g., settling velocity), the
enced by the overlaying hydrodynamics. flow can only sustain a maximum amount of sediment
load. When the carrying capacity is exceeded (e.g., flow
Mechanisms of sediment resuspension intensity decreases when approaching slack water),
turbulence in the boundary layer is significantly
Sediment resuspension is generally driven by bottom
suppressed and catastrophic sediment settling occurs.
boundary layer flow, which is a layer of sharp transition
Also facilitated by hindered settling effect, a thick layer
of flow velocity from nearly zero at the bed to a large mag-
of concentrated sediments is accumulated near the bed,
nitude of overlaying hydrodynamics. Hence, the flow
called fluid mud, and experiences a slow consolidation
shear in a bottom boundary layer, defined here as the ver-
process (Mehta, 1991). The formation of fluid mud and
tical gradient of streamwise flow velocity, is significant,
a laminarized boundary layer give rise to a greatly
and it is the main ingredient of turbulence production
enhanced bulk flow viscosity near the bed, which
and dispersion of solutes (Pope, 2000). Consequently, sur-
appears to also cause a large surface wave dissipation rate
ficial bed sediments are mobilized and suspended in the
(Sahin et al., 2011).
bottom boundary layer. Boundary layer flow and sediment
In the intertidal environments, large sediment
resuspension can be driven by tidal currents, river out-
resuspension at the intratidal time scale is often observed
flows, surface waves, and even internal waves. Moreover,
at very shallow water depths near the land-water interface,
anthropogenic causes such as dredging and ship waves
called turbid tidal edge (e.g., Christie and Dyer, 1998;
can also cause sediment resuspension. Once sediments
Nowacki and Ogston, 2013). On the daily and fortnightly
are suspended in the bottom boundary layer, depending
time scale, a significant amount of sediment is exchanged
on the bed slope and the characteristics of suspension
between the tidal channel and adjacent flats. The net
(see next section), downslope gravitational force may also
exchange is highly dependent on seasonal variability due
play a critical role to transport sediment in the downslope
to vegetation. For example, observations at the intertidal
direction.
mudflats of Willapa Bay (USA) show that during the win-
ter period, flats are not vegetated and channels are filled
Resuspension processes with thick layers of muddy deposits delivered from the
Although turbulence is the key mechanism of sediment flats during ebb flow. On the contrary, with only limited
suspension, when a significant amount of fine sediments vegetation during the summer period, sediments appear
is suspended in the bottom boundary layer, flow turbu- to be trapped on the flats, and hence the channels are
lence can be attenuated through sediment-induced density observed to be free of mud (Boldt et al., 2013).
stratification. This is typically observed through the for-
mation of the “lutocline,” a sharp negative gradient of
suspended sediment concentration at some elevation Summary
above the bed. Such turbulence-sediment interaction leads New sensor technology has provided a wealth of insights
to several critical processes. In moderate sediment con- into the sediment-laden bottom boundary layer regarding
centration (or relatively intense overlaying flow), the fluid-sediment interactions and sediment properties that
lutocline separates the lower turbulent boundary layer are essential to the understanding and modeling of sedi-
from the upper nonturbulent layer. Hence, sediments are ment resuspension (Mikkelsen et al., 2004; Traykovski
mostly accumulated below the lutocline and may establish et al., 2007). In recent years, numerical simulations based
sufficiently large buoyancy anomaly to drive offshore- on two-phase flow principles are capable of resolving
directed gravity flow (Traykovski et al., 2000; Wright most of the three-dimensional turbulence-sediment inter-
and Friedrichs, 2006). Because flow turbulence eventually actions; they also reveal new insights on the mechanism
leads to energy dissipation through energy cascade (Pope, of sediment resuspension (Cantero et al., 2009; Ozdemir
2000), when turbulence is attenuated by the presence of et al., 2010). Several critical aspects of sediment
sediments, mean flow kinetic energy increases. This phe- resuspension warrant future studies. A robust flocculation
nomenon is called drag reduction. During AMASSEDS module needs to be incorporated into models for sediment
(A Multidisciplinary Amazon Shelf Sediment Study; resuspension. On the same note, appropriate parameteriza-
Nittrouer et al., 1991), a significant reduction of bottom tion of erosion flux for cohesive sediment bed capturing
drag coefficient is observed as tidal currents propagate consolidation and characteristic of aggregates is necessary
560 SEDIMENT SORTING

(Sanford, 2008; Winterwerp et al., 2012). Moreover, the colloids and micro- to macro-scale flocs in marine, freshwater
role of vegetation on sediment resuspension needs to the and engineered systems. In Droppo, I. G., Leppard, G. G., Liss,
better quantified (Nepf, 2012). S. N., and Milligan, T. G. (eds.), Flocculation in Natural and
Engineered Environmental Systems. Boca Raton: CRC Press,
pp. 191–209.
Traykovski, P., Geyer, W. R., Irish, J. D., and Lynch, J. F., 2000. The
Bibliography role of wave-induced fluid mud flows for cross-shelf transport
Beardsley, R. C., Candela, J., Limeburner, R., Geyer, W. R., Lentz, on the Eel River continental shelf. Continental Shelf Research,
S. J., Castro, B., Cacchione, D., and Carneiro, N., 1995. The M2 20, 2113–2140.
tide on the Amazon shelf. Journal of Geophysical Research, 100 Traykovski, P., Wiberg, P., and Geyer, W. R. 2007. “Observations
(C2), 2283–2319. and modeling of wave-supported sediment gravity flows on the
Boldt, K. V., Nittrouer, C. A., and Ogston, A. S., 2013. Seasonal Po prodelta and comparison to prior observations from the Eel
transfer and net accumulation of fine sediment on a muddy tidal shelf.” Continental Shelf Research, 27(3–4), 375–399.
flat: Willapa Bay, Washington. Continental Shelf Research, 60, Trowbridge, J. H., and Kineke, G. C., 1994. Structure and dynamics
S157–S172. of fluid mud on the Amazon continental shelf. Journal of Geo-
Cantero, M. I., Balachandar, S., and Parker, G., 2009. Direct numer- physical Research, 99(C1), 865–874.
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lent channel flow. Journal of Turbulence, 10, N27, doi:10.1080/ non-cohesive sediment. Journal of Geophysical Research, 106,
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Christie, M. C., and Dyer, K. R., 1998. Measurements of the turbid Winterwerp, J. C., and van Kesteren, W. G. M., 2004. Introduction
tidal edge over the Skeffling mudflats. In Black, K. S., Paterson, to the Physics of Cohesive Sediment in the Marine Environment.
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tidal Zone. London: Geological Society. Special Publication, Winterwerp, J. C., van Kesteren, W. G. M., van Prooijen, B., and
Vol. 139, pp. 45–55. Jacobs, W., 2012. A conceptual framework for shear flow–
Dean, R. G., and Dalrymple, R. A., 2002. Coastal Processes with induced erosion of soft cohesive sediment beds. Journal of Geo-
Engineering Applications. Cambridge: Cambridge University physical Research, 117, C10020, doi:10.1029/2012JC008072.
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Geyer, W. R., Hill, P. S., and Kineke, G. C., 2004. The transport, mouth: a synthesis. Geological Society of America Bulletin, 88,
transformation and dispersal of sediment by buoyant coastal 857–868.
flows. Continental Shelf Research, 24(7–8), 927–949. Wright, L. D., and Friedrichs, C. T., 2006. Gravity-driven sediment
MacCready, P., and Geyer, W. R., 2010. Advances in estuarine transport on continental shelves: a status report. Continental
physics. Annual Reviews of Marine Science, 2, 35–58, Shelf Research, 26, 2092–2107.
doi:10.1146/annurev-marine-120308-081015. Wright, L. D., and Nittrouer, C. A., 1995. Dispersal of river sedi-
Mehta, A. J., 1991. Understanding fluid mud in a dynamic environ- ments in coastal seas: six contrasting cases. Estuaries, 18,
ment. Geo-Marine Letters, 11, 113–118. 494–508.
Mikkelsen, O. A., Milligan, T. G., Hill, P. S., and Moffatt, D., 2004.
INSSECT – an instrumented platform for investigating floc
properties close to the seabed. Limnology and Oceanography: Cross-references
Methods, 2, 226–236. Sediment Budgets
Nepf, H. M., 2012. Flow and transport in regions with aquatic veg- Sediment Erosion
etation. Annual Review of Fluid Mechanics, 44, 123–142. Sediment Grain Size
Nittrouer, C. A., DeMaster, D. J., Figueiredo, A. G., and Rine, J. M., Sediment Transport
1991. AmasSeds: an interdisciplinary investigation of a complex Tidal Hydrodynamics
coastal environment. Oceanography, 4, 3–7. Wave-Driven Sediment Resuspension
Nowacki, D., and Ogston, A., 2013. Water and sediment transport of
channel-flat systems in a mesotidal mudflat: Willapa Bay, Wash-
ington. Continental Shelf Research, 60, S111–S124.
Ozdemir, C. E., Hsu, T.-J., and Balachandar, S., 2010. A numerical
investigation of fine particle laden flow in oscillatory channel: SEDIMENT SORTING
the role of particle-induced density stratification. Journal of Fluid
Mechanics, 665, 1–45, doi:10.1017/S0022112010003769.
Ozdemir, C. E., Hsu, T.-J., and Balachandar, S., 2011. A numerical Michel Michaelovitch de Mahiques
investigation of lutocline dynamics and saturation of fine sedi- Oceanographic Institute of the University of São Paulo,
ment in the oscillatory boundary layer. Journal of Geophysical Sao Paulo, Brazil
Research, 116, C09012, doi:10.1029/2011JC007185.
Pope, S. B., 2000. Turbulent Flows. Cambridge: Cambridge Uni- Definition
versity Press.
Sahin, C., Safak, I., Sheremet, A., and Mehta, A. J., 2011. Observa- Sediment sorting is the degree of dispersion of a grain-size
tions on cohesive bed reworking by waves: Atchafalaya Shelf, distribution around a central value (mean, median,
Louisiana. Journal of Geophysical Research, 117, C09025, or mode).
doi:10.1029/2011JC007821.
Sanford, L. P., 2008. Modeling a dynamically varying mixed sedi- Description
ment bed with erosion, deposition, bioturbation, consolidation,
and armoring. Computational Geosciences, 34, 1263–1283, Sorting can reflect both sediment source and/or transport,
doi:10.1016/j.cageo.2008.02.011. with aeolian-transported sediments being among the best
Santschi, P. H., Burd, A. B., Gaillard, J.-H., and Lazarides, A. A., sorted and glacial sediments being among the poorest
2005. Transport of materials and chemicals by nanoscale sorted. The measurement of the degree of sorting of
 SEDIMENT TOXICITY 561

a grain-size distribution can be given by any of the statis- the food chain (USEPA, 1994a). The concentration of
tical dispersion measurements; the standard deviation is contaminants in sediments can be several orders of magni-
the most common. The kurtosis (or “peakedness”) has also tude greater than in the overlying water, and therefore
been widely used by investigators as a sediment sorting measurements of water quality may differ greatly from
parameter, especially between 1960 and 1980. sediment quality (USEPA, 1994a). The bioavailability of
One of the most frequently used sorting parameters is a contaminant is sensitive to local environmental
the “inclusive graphic standard deviation” proposed by variables, including sediment geochemistry, pH, and
Folk and Ward (1957), in which the 68 % and 90 % of oxygen concentration.
the median value have been employed to define sorting Chemical, biological, and ecological methods of quan-
criteria: very well sorted, well sorted, moderately well tifying sediment quality have been developed; however,
sorted, moderately sorted, poorly sorted, very poorly each method has its shortcomings (Chapman, 1989). For
sorted, and extremely poorly sorted. example, assessing toxicity by measuring individual
After Folk and Ward’s seminal paper, several other chemicals in sediments may miss unmeasured chemical
works used grain-size parameters to delineate between compounds or may not account for changes in bioavail-
environments (Sahu, 1964). Another approach was the ability for different sediment types. In addition, the
use of grain-size parameters to determine the net transport additive effect of mixtures of chemicals further compli-
in beach, estuarine, and shelf environments. In all of the cates the prediction of sediment toxicity based on chemi-
papers published on this subject, sediment sorting was cal data. To determine whether contaminants in
shown to play a major role in the determination of the sediments are harmful to benthic organisms, the EPA has
direction of transport, since it is assumed that sorting is developed methods that measure the survival after
always better towards the direction of transport 10-day incubations of a freshwater amphipod (Hyalella
(McLaren and Bowles, 1985; Gao and Collins, 1992). azteca) or midge (Chironomus tentans) or an estuarine
or marine amphipod (Ampelisca abdita, Eohaustorius
Bibliography estuarius, Leptocheirus plumulosus, and Rhepoxynius
abronius) (USEPA, 1994a, b). Methods to determine sub-
Folk, R. L., and Ward, W. C., 1957. A study in the significance of
grain size parameter. Journal of Petrology, 37, 327–354. lethal effects, including effects on reproduction and
Gao, S., and Collins, M., 1992. Net sediment transport patterns growth, have also been developed (USEPA, 2000, 2001).
inferred from grain-size trends, based upon definition of trans- To effectively determine the magnitude and extent of sed-
port vectors. Sedimentary Geology, 81, 47–60. iment contamination, data from several different methods
McLaren, P. A., and Bowles, D., 1985. The effects of sediment must be integrated.
transport on grain-size distributions. Journal of Sedimentary
Petrology, 55, 457–470.
Sahu, R., 1964. Textural parameters: an evaluation of fluvial and Bibliography
shallow marine deposits. Journal of Sedimentary Petrology, 34, Chapman, P. M., 1989. Current approaches to developing sediment
513–520. quality criteria. Environmental Toxicology and Chemistry, 8,
589–599.
USEPA, 1994a. Methods for Measuring the Toxicity and
Cross-references Bioaccumulation of Sediment-Associated Contaminants with
Sediment Grain Size Freshwater Invertebrates. Duluth: Office of Research and
Development. EPA600/R-94/024.
USEPA, 1994b. Methods for Assessing the Toxicity of Sediment-
Associated Contaminants with Estuarine and Marine Amphi-
pods. Washington, DC: USEPA Office of Research and Devel-
SEDIMENT TOXICITY opment. EPA/600/R-94/025.
USEPA, 2000. Methods for Measuring the Toxicity and
Bioaccumulation of Sediment Associated Contaminants with
Steven Colbert Freshwater Invertebrates, 2nd edn. Duluth: USEPA Office of
Department of Marine Science, University of Hawai’i at Research and Development. EPA/600/R-99/064.
Hilo, Hilo, HI, USA USEPA, 2001. Method for Assessing the Chronic Toxicity of Marine
and Estuarine Sediment Associated Contaminants with the
Definition Amphipod, 1st edn. Washington, DC: USEPA Office of Research
and Development. Leptocheirus plumulosus. EPA/600/R-
Sediment toxicity is a measure of the negative impact of 01/020.
contaminated sediments on aquatic organisms. USEPA Office of Research and Development. EPA-600/R-94/024.
Duluth, Minnesota.
Background
Contaminated sediments can potentially be detrimental to Cross-references
aquatic organisms, both benthic and pelagic, and therefore Halogenated Hydrocarbons
have negative impacts across aquatic ecosystems. Marsh Sediment Toxicity
Contaminants may be directly toxic to aquatic life or Oil Pollution
can be a source of contaminants for bioaccumulation in Trace Metals in Estuaries
562 SEDIMENT TRANSPORT

The interaction between fluid and solid particles is

SEDIMENT TRANSPORT greatly influenced by the sediment characteristics. Sedi-
ments are commonly divided into cohesive and
Pedro J. M. Costa non-cohesive components. In cohesive sediments, the
Centro de Geologia, Faculdade de Ciencias, Universidade resistance to erosion depends on the strength of the cohe-
de Lisboa, Lisbon, Portugal sive bond between the particles. The problem of erosion
resistance of cohesive soils is a very complex one, and at
Definition present our understanding of the physics of it is still very
The simplest definition of sediment transport is the trans- incomplete. The non-cohesive soils generally consist of
port of granular particles by fluids. The main agents by larger discrete particles (e.g., sand, pebbles, cobbles, and
which sedimentary materials are moved include gravity boulders); the movement of these particles essentially
(gravity transport), river and stream flow, ice, wind, and depends on the physical properties of the individual parti-
estuarine and ocean currents. Running water and wind cles, such as their size, shape, and density. The most
are the most widespread transporting agents. In both cases, important physical property of a sediment particle is its
three mechanisms operate, although the particle size of the size which has a direct effect on its mobility.
transported material is very different, owing to the differ- There are dozens of sediment transport functions that
ences in density and viscosity of air and water. The three predict sediment transport based on sediment size, weight,
processes are rolling or traction, in which the particle fall velocity, water velocity, channel depth, channel width,
moves along a sedimentary bed but is too heavy to be slope, roughness, and water temperature. However, in
lifted from it; saltation; and suspension, in which particles many of these empirical solutions, some assumptions are
remain permanently above the bed, sustained there by the made. Moreover, in sediment transport, two important
turbulent flow of the air or water (U.S. Army Corps of concepts are settling rate and boundary shear stress. Set-
Engineers, 2003). tling rate describes the tendency for sediment particles to
fall out of suspension. Boundary layer shear stress
describes the tendency for a moving fluid to bring sedi-
Importance of sediment transport ment particles into suspension.
Sediment transport has been studied for centuries and
remains a challenging area of research for earth and marine
scientists. The general term of “sediment transport” includes Sediment entrainment or the threshold
a number of environmental processes that take place at for sediment motion
a wide range of spatial and temporal scales. The full under- Sediment entrainment is defined as the transition from
standing of sediment transport is fundamental to assessing repose to displacement. The incipient motion of sediment
a range of heterogeneous geological, engineering, and occurs when the stability of a particle is disturbed. Such
environmental processes. Over the recent years, the devel- instability can be attributed to the imbalance of forces
opment of sediment transport research was transformed (or force moments) caused by the forces exerted on the
from descriptions of simple empirical\phenomena to more particle in the flow. The threshold of particle motion is
complex numerical models in which the flow and the attained for a given ratio between driving and stabilizing
resulting sediment transport are detailed. forces. Simplifying, the driving forces on a sediment par-
The study of sediment transport processes includes the ticle resting on other particles on an originally plane hori-
movement of particles, rocks, and other earth materials by zontal bed are the tractive stress T  (horizontal) and the
various processes. Transport is driven by gravity effects lift force (caused by the Bernoulli effect). The horizontal
and by friction effects with the air or the fluid containing drag, created by the flow, consists of skin friction acting
the sediment. Sediment transport due to fluid motion on the surface of the grain and from drag due to
occurs in rivers, lakes, estuaries, seas, and other bodies a pressure difference on the upstream and downstream
of water due to currents and tides, in glaciers as they flow, sides of the grain because of flow separation.
and on terrestrial surfaces under the influence of wind. The motion of a fluid flowing across its bed tends to
Sediment transport due only to gravity can occur on slop- move the bed material downstream. A submerged grain
ing surfaces in general, including hill slopes, scarps, cliffs, on the surface is subjected to a weight force and the hydro-
and the continental shelf – continental slope boundary. dynamic forces. Below some critical hydraulic conditions,
Sediment transport is usually divided into three types: the hydrodynamic forces will be so small that particles
bed load, saltation, and suspension. Bed-load transport is submerged weight will move very rarely or not at all.
defined as the type of transport where sediment grains roll However, a slight increase in flow velocity above this
or slide along the bed. Saltation transport is defined as the hydraulic critical condition will initiate appreciable
type of transport where single grains jump over the bed at motion by some of the particles on the bed. This hydraulic
a length proportional to their diameter, periodically losing critical condition is termed the condition of initiation of
contact with the bed. Sediment is suspended when the flux motion and is computed in terms of either mean flow
is intense enough such that the sediment grains are velocity in the vertical or the critical bed shear stress
suspended over the bed. (also known as the tractive force or the drag force).
 SEDIMENT TRANSPORT 563

Sediment Transport, Figure 1 Critical entrainment probability and its relationship with shear stress and Reynolds number.

The entrainment of sediment has been investigated by particle Reynolds number (Figure 1). The Shields stress
many approaches. One approach is to determine the criti- conciliates settling rate and boundary layer shear stress
cal shear stress for incipient motion of sediment. The work in order to predict when a moving fluid will transport sed-
of Shields (1936) is the most well-known entrainment cri- iment. The Reynolds number (Re) predicts the extent of
terion. Quantification of the threshold shear stress is the turbulence in a fluid based on flow velocity (u), character-
basis for prediction of transport rate in many bed-load istic length (l) which represents flow geometry, fluid
equations (e.g., Meyer-Peter and Müller, 1948; Parker, density (r), and fluid viscosity (m). Turbulent flow has
1979). On the other hand, some researchers support the Re > 2,000, and laminar flow has Re < 500. Flow with
existence of a range of threshold values for initial sedi- Re between 500 and 2,000 is transitional.
ment movement and thus employ the probabilistic model In dimensionless terms, the condition for bed-load
as an alternative approach to sediment entrainment (e.g., motion is when bed shear stress (t0) exceeds a critical
Einstein, 1942; Grass, 1970; Gessler, 1971) and the pre- value (t0)c:
diction of bed transport (e.g., Einstein, 1950; Paintal,
1971). The field and laboratory observations also confirm t0 ¼ ðt0 Þc
the variability of critical shear stress that can be attributed The Shields parameter is the nondimensional number
to a number of random factors (e.g., temporal fluctuations used to calculate the initiation of motion of sediment in
of turbulent flow, heterogeneity of grain size, shape and a fluid flow:
density, bed-grain geometry, sediment availability, expo-
sure and sheltering effect, bed roughness, etc.). t0
t ¼
ðrs
rÞgd s
Bed-load transport where t* is dimensional shear stress, rs is the density of
When the bed shear stress exceeds a critical value, sedi- sediment, r is the density of fluid, g is acceleration due
ments are transported in the form of bed load and to gravity, and ds is a characteristic particle diameter of
suspended load. The sediment transport rate may be mea- the sediment.
sured by weight, mass, or volume. In practice, the sedi- Table 1 presents a summary of empirical and semiem-
ment transport rate is often expressed per unit width and pirical correlations of bed-load transport.
is measured either by mass or by volume. To make predic- Yallin (1963, 1972) developed a bed-load equation
tions about the conditions under which sediment will be incorporating reasoning similar to Einstein (1942, 1950),
transported, it is common to use Shields stress and the but with a number of refinements and additions.
564 SEDIMENT TRANSPORT

Sediment Transport, Table 1 Empirical and semiempirical correlations of bed-load transport (Chanson, 1999)

Reference (1) Formulation (2) Range (3) Remarks (4)

Boys (1879) qs ¼ lto(to
(to)c) l was called the

characteristic sediment
coefficient
l ¼ ðr0:54

rÞg Schoklitsch ð1914Þ Laboratory experiments
s
with uniform grains of
various kinds of sand and
porcelain
l / ds
3/4 Straub (1935) 0.125 < ds < 4 mm Based upon laboratory data
0
Schoklitsch qs ¼ l ð sin yÞk ðq
qc Þ 0.305 < ds < 7.02 mm Based upon laboratory
(1930) experiments
qc ¼ 1:944  10
2 d s ð sin yÞ
4=3 1:06 < s < 4:25
Shields (1936) s ¼ 10 sin y to
ðto Þc
q
q s rg ðs
1Þ d s   1:56 < d s < 2:47 mm
Einstein qs rðs
1Þgd s qs Laboratory experiments.
q ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 2:15exp
0:391 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi < 0:4
(1942) to ðs
1Þgd s 3 Weak sediment transport
ðs
1Þgd 3s formula for sand
1:25 < s < 4:25 mixtures. Note: ds 
0:315 < d s < 28:6 mm d35 to d45
2=3
Meyer-Peter ðrg ðm_ s Þ2 1.25 < s < 4.2 Laboratory experiments.
(1951), m_ 2=3 sin y
9:57ðrg ðs
1ÞÞ10=9 ¼ 0:462ðs
1Þ Uniform grain size
Meyer-
ds ds distribution
Peter and
Müller
(1948)
qs  3=2 Laboratory experiments.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 4to

0:188 Particle mixtures. Note:
ðs
1Þgd s 3 rð s
1Þgd s

  ds  d50
qs
Einstein Design chart pffiffiffiffiffiffiffiffiffiffiffiffiffi3ffi ¼ f rðs
1
qs
to
Þgd s
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi < 10 Laboratory experiments.
(1950) ðs
1Þgd s For sand mixtures. Note:
ðs
1Þgd 3s
ds  d35 to d45
1:25 < s < 4:25
0:315 < d s < 28:6 mm
m_ s ¼ 2500ð sin yÞ3=2 ðq
qc Þ qc ¼ 0:26ðs
1Þ5=3 d 40 ð sin yÞ
7=6
3=2
Schoklitsch Based upon laboratory
(1950) experiments and field
measurement (Danube
and Aare rivers)
qs  rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Nielsen qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ 12t0 to 1:25 < s < 4:22 Re-analysis of laboratory
(1992)
0:05 0:69 < d s < 28:7 mm data
ðs
1Þgd 3s rðs
1Þgd s rðs
1Þgd s

Note: m_ ¼ mass water flow rate per unit width; m_ s ¼ mass sediment flow rate per unit width; q ¼ volumetric water discharge; qs ¼ volumetric
sediment discharge per unit width; (to)c ¼ critical bed shear stress for initiation of bed load

Yang (1972, 1973) approached the total transport from the of the theory can be found in Einstein and Chien (1955),
energy expenditure point of view and related the transport Vanoni (1984), Hassanzadeh (1985, 1979), and many
rate to stream power. Shen and Hung (1971) derived others (Graf, 1971; Graf and Altinakar, 1998; Raudkivi,
a regression equation based on laboratory data for the 1976). Mei et al. (1994) also reported a study on the
sand-sized particles. Using the same concept, Ackers and hyperconcentrated fluid mud in rivers.
White (1973) defined sediment transport functions in Several researchers have proposed formulas to estimate
terms of three dimensionless groups, namely, size, mobil- the characteristics of the bed-load layer (Table 2).
ity, and transport rate of sediments. Their functions are
based on flume data carried out with flow depths up to
0.4 m. One of the most extensive field and laboratory stud- Settling velocity
ies of sediment transport is that by Van Rijn (1984). He has The settling velocity of sediment is one of the key
presented a method which enables the computation of the variables in the study of sediment transport. It hinges on
bed-load transport as the product of the saltation height, the type of flow (laminar, transitional, and turbulent) of
the particle velocity, and the bed-load concentration. the fluid that transports the particle. Stokes’ settling theory
The theoretical equation for the distribution of describes the velocity of a spherical particle settling
suspended sediment in turbulent flow has been given by through a fluid – depending on a balance between the drag
H. Rouse. Further useful information on the modification force and the gravitational force. At the settling velocity,
 SEDIMENT TRANSPORT 565

Sediment Transport, Table 2 Bed-load transport rate calculations (Chanson, 1999)

Reference (1) Bed-load layer characteristics (2) Remarks (3)

 qffiffiffiffiffiffiffi
Fernandez-Luque and van Beek (1976) Vs ðt Þc Laboratorydata
V ¼ 9:2 1
0:7 t

1:34  s  4:58
0:9  d s  3:3 mm
0:08  d  0:12 m
Nielsen (1992) C s ¼ 0:65 Simplified model
ds  
¼ 2:5 t
ðt Þc
ds
Vs
¼ 4:8
V  2 1=3  
C s ¼ 0:117 t
ðt  Þc
1
v t
Van Rijn (1984a, 1993) ds ðs
1Þg For ðt Þ < 2 and d s ¼ d 50
 c
  !0:7 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Based on laboratory data
ds ðs
1Þg 1=3 t 0:2  d s  2 mm
¼ 0:3 d s
1
ds v2 ðt Þc d > 0:1 m
  ! sffiffiffiffiffiffiffiffiffiffi Fr < 0:9
Vs ðs
1Þg 1=3 ðt Þc
¼ 9 þ 2:6log10 d s
8
V v2 t
 2
1=3   ds ¼ d50.
0:117 v t
Cs ¼
1 Based on laboratory data
ds ðs
1Þg ðt  Þc 0:2  d s  2 mm
 1=3 !0:7 rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ds ðs
1Þg t d > 0:1 m
¼ 0:3 d s
1 Fr < 0:9
ds v2 ðt Þc
Vs
¼7
V

Notes: V* ¼ shear velocity; (t*)c ¼ critical Shields parameter for initiation of bed load

the drag force (Fd) on the sphere is balanced by the excess The fall or settling velocity of a particle is assumed to
of the gravitational force (Fg): be a steady-state motion. It is also a function of size, shape,
  density, and viscosity of fluid. In addition, it depends on
F d ¼ Cdrf Ap w2s =2 the extent of the fluid in which it falls, on the number of
particles falling, and on the level of turbulence intensity.
  Often the estimation of settling velocity of sediment has
F g ¼ r p
r f ws g been done by applying predictive formulas developed by
assuming the grains to be spheres. It is well known that
Introducing R, the submerged specific gravity: the shape of natural sediment particles departs from
rp
rf a sphere. This departure will have some consequences,
R¼ one being that the settling velocity will be lower than that
rp
of a sphere with the nominal diameter. Due to the practical
Introducing the drag coefficient defined in terms of the implications of this difference, several formulas have been
Reynolds number (Re): proposed to calculate the settling velocity of natural
nonspherical grains (e.g., Graf, 1971; Zanke, 1977;
ws D Hallermeier, 1981; Dietrich, 1982; van Rijn, 1984;
Re ¼
v Swamee and Ojha, 1991; Julien, 1995; Cheng, 1997;
Soulsby, 1997; Ahrens, 2000). Also, it is the empirical
Producing an equation for the (Stokes) settling velocity work of Bagnold (1941) that particularly focuses on aeo-
(ws): lian sediment transport.
The deviation of a particle’s shape from a sphere is gen-
RgD2
ws ¼ erally quantified by a shape factor. The most commonly
18v used is Corey’s shape factor, which is given by
where Cd is the drag coefficient, rf is the density of the c
fluid, rp is the density of the particle, Ap is the area of csf ¼ pffiffiffiffiffi
ab
the particle, ws is the particle velocity, g is the gravitational
acceleration, R is the radius of the spherical object, and v is where a, b, and c are the longest, intermediate, and
kinematic viscosity. shortest axes of the particle.
566 SEDIMENT TRANSPORT

Sediment Transport, Figure 2 The Hjulstrom diagram shows the relationship between the velocity of water flow and the transport
of loose grains. Once a grain has settled, it requires more energy to start it moving than a grain that is already in motion. The cohesive
properties of clay particles mean that fine-grained sediments require relatively high velocities to re-erode them once they are
deposited, especially once they are compacted.

The flows that are required to pick up grains of certain n ¼ kinematic viscosity of the fluid (1.0  10
6 kg m
1 s
1
sizes have been extensively studied empirically, and the for water at 20  C), and C1 and C2 are constants. For nat-
results are plotted in Hjulstrom diagrams (Figure 2). ural sand grains, Ferguson and Church (2004) recommend
A Hjulstrom diagram is an empirical measure of the min- C1 ¼ 18 and C2 ¼ 1.
imum velocity required for moving particles of different
sizes. The diagram shows grain entrainment on a plot of
log grain size versus log flow speed. Note that larger Sedimentary structures
grains require higher flows, in general. The water speed Sedimentary structures directly linked with sediment
that is required to transport a grain is called the critical transport are parallel bedding, ripples, dunes, sand waves,
velocity. and graded bedding. Structures form on the surface of
Furthermore, the equation of Ferguson and Church a bed when topography influences the strength of the flow
(2004) also expresses settling velocity (w in ms
1) as (and thus the strength of the Bernoulli effect). Erosion
a function of sediment size D in m: occurs where flow is strongest and directed into the bed.
Deposition occurs where flow is slower. Deposition ordi-
RgD2 narily creates laminae that are parallel to the depositional
w¼ surface. Small ripples have small laminae that dip down-
C1 þ √0:75 C2 Rg D3
stream because that is where deposition occurs. Flat beds
where R ¼ submerged specific gravity (1.65 for quartz have flat laminae. Large dunes have coarser laminae that
in water), g ¼ acceleration due to gravity (9.8 m s
2), dip downstream.
 SEDIMENT TRANSPORT 567

Summary Meyer-Peter, E., 1951. Transport des matieres solides en general et

problem speciaux. Bulletin Genie Civil d’Hydraulique Fluviale,
The purpose of this work is to briefly summarize the com- 5 (in French).
plex processes, mechanisms, and physics involved in sed- Meyer-Peter, E., and Müller, R., 1948. Formulas for bed-load trans-
iment transport. This contribution provides a short review port. In Proceedings of the 2nd Meeting of the International
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des Geschiebetriebs. Schweiz. Bauzeitung 67, Nr. 3.
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Paintal, A. S., 1971. Concept of critical shear stress in loose
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568 SEDIMENTARY STRUCTURES

SEDIMENTARY STRUCTURES

Gautam Kumar Das

Department of Chemical Engineering, Jadavpur
University, Kolkata, West Bengal, India

Synonyms
Bedding; Bedforms

Definition
Sedimentary structures. Primary or mechanical structures
formed by physical processes in the sedimentary
environment.
Bedforms. Primary sedimentary structures formed by
the interactions between turbulence of flow and sediment
grains.
Antidunes. Large-scale structures looking like plane
beds formed after destruction of bedforms in the upper
part of the higher flow regime.
Megaripples. Large-scale sedimentary structures Sedimentary Structures, Figure 1 Lingoid Ripples.
formed in the upper part of the lower flow regime.
Ripples. Small-scale sedimentary structures formed in
the lower part of the lower flow regime.
Bedding planes. Surface sedimentary structures.
Bedding. Layering characteristics of the sequence.

Introduction
Sedimentary structures are surficial or internal, mega-
scopic, three-dimensional features of sediments or sedi-
mentary rocks (Pettijohn and Potter, 1964). These
structures have been called mechanical or primary struc-
tures (Potter and Pettijohn, 1977) due to their formation
by physical processes. In the modern environment, flow
regimes at varying speeds and velocities produce different
sedimentary structures that are called bedforms. Sands are
deposited in a diverse suite of ripples, megaripples, sand
waves, rill marks, rhomboid marks, backwash ripples,
Sedimentary Structures, Figure 2 Rill Marks.
swash marks, and current crescents in the central basin
and marginal areas of an estuary (Elliot, 1983). Ripple
bedforms have a tendency to occur in the intertidal areas Bedforms characters
of mid-channel bars or point bars where tidal current The sand-dominated middle to lower stretch of the estuary
velocities are at a minimum, whereas megaripples and is significantly important where intertidal sand bodies
sand waves are confined to the depressed zones of tidal with numerous bedforms of different scales are exposed
sand bars. after each tide. The exposed portion of the estuarine bed
Small ripples are formed by an increase in flow veloc- and the intertidal mid-channel bars exhibit various
ity, and these migrate in the direction of flow. With bedforms of tidal origin. Inherent unsteadiness in flow
a continuous increase in flow velocity, the small ripples conditions, reversals of tidal currents, and bedform-flow
may enlarge and change slope giving rise to megaripples. interactions are the causal factors for the frequent changes
Plane beds and antidunes gradually form as a result of the in bedform architecture (Middleton, 1965; Conybeare and
destruction of megaripples at higher flow rates. Ripples Crook, 1968). Thus, bedforms, though not static, are per-
and megaripples are the most commonly observed manent features which often display a quasi-equilibrium
bedforms in the estuarine environment (Davis, 1983) form under effects of variable tidal dynamics. Smaller
(Figures 1, 2, and 3). bedforms quickly change their orientations, but large
 SEDIMENTARY STRUCTURES 569

convex-up profiles, and their crestal heights remain more

or less constant without showing any well-developed
scour pits in the troughs. The troughs contain small ripples
with crestal orientations perpendicular to the megaripple
crests. This is because of the generation of secondary flow
along the troughs of megaripples during falling water
levels (Das and Bhattacharya, 2000).

Undulatory megaripples
Undulatory megaripples possess long wavy or undulating
crests and are devoid of well-developed scour pits in front
of their slip faces. In this respect, they differ from lunate
megaripples of Reineck and Singh (1980) or type II
megaripples of Dalrymple et al. (1978) in which the crest
line is broken and megaripples possess distinct scour pits
in front. Both in-phase and out-of-phase arrangements of
undulations are present in a single megaripples train. The
crests have forward tonguelike projections and steep pro-
files in contrast to those of the straight-crested
megaripples. The megaripples surface is ornamented by
linguoid ripples, which are produced by emergence.
Allen’s (1968) category of megaripples corresponds to
this type of bedform.

Sedimentary Structures, Figure 3 Wave Ripples. Small-scale bedforms

Ripples
sand waves do not, in response to changing flood-ebb These small-scale bedforms occur under the direct influ-
conditions (cf. Dalrymple et al., 1978; Collinson and ence of both ebb and flood-tidal currents as well as under
Thomson, 2006). the influence of any one of these currents where they orig-
inate along troughs of megaripples. The chief controlling
Types of bedforms factors for their formation are the general slope of the
Three different scales of bedforms (i.e., small scale, inter- troughs, strength of tidal currents, and direction of wings.
mediate scale, and large scale) are recognizable by differ- The small-scale ripples are of various types as described
ent geomorphic units in mesotidal and macrotidal below.
estuaries. The small-scale bedforms include all varieties
of ripple marks. The intermediate-scale bedforms include Straight-crested ripples
two different types of megaripples, and the large-scale These ripples have more or less straight crests and crestal
bedforms include sand waves. trains of successive ripples which run parallel for a few
meters. The surface undulation is very gentle because of
Morphology of the bedforms low ripple heights.
Large-scale bedforms Linguoid ripples
Sand waves These ripples exhibit a crescentic pattern in plan. The
These features are the largest-scale bedforms observed on crests are arcuate to tonguelike and have forward closures.
the mid-channel bar surface. They are flood-oriented, These ripples produce appreciable surface undulations.
two-dimensional forms and appear mainly on the floor The lee slopes can be easily measured in the field.
of the flood-dominated portions of the bar surface. They
lack scour pits in their troughs and spurs on the slip faces. Wave ripples
Ripples superimposed on the sand waves have been par- These ripples are asymmetrical in plan and closely associ-
tially planed off by the preceding ebb flow. ated with straight-crested ripples. They typically have
steep lee slopes and very gentle stoss slopes. In addition,
The intermediate-scale bedforms they exhibit a positive linear correlation between their
Straight-crested megaripples length and height measurements.
The mid-channel bar surface is extensively sculptured by
trains of straight-crested megaripples. They are parallel Flat-topped ripples
to each other over considerable distances but exhibit These bedforms occur as superimposed ripples over
minor sinuosity. These megaripples have broadly larger bedforms and along troughs of megaripples.
570 SEDIMENTARY STRUCTURES

The flattening of the crests is evident for both linguoid and swash. In the estuarine areas, the swash marks document
asymmetric wave ripples. The troughs are very narrow the limit of the outer bank of the estuaries, and as
compared to the width of the crests. Because of the greater a result their alignment is at right angles to the shoreline
flattening of the ripple crests, the crestal lines often get alignment. Hence, the swash marks in an estuary occur
obliterated; the crestal width increases with almost at right angles to their counterparts on the sea
a corresponding decrease of trough width. The mechanism beach.
of formation of flat-topped ripples is attributed to scouring
by tidal currents. Rill marks
Rill marks are dendritic erosional structures on a sandy
Double-crested ripples swash platform made by a system of small rivulets origi-
These ripples generally contain double crestal trains with nating from the flow of a thin layer of water during
identical spacing of 5–6 mm. The ripples are typically a falling water stage. Rill marks are of various forms and
asymmetric with almost straight crests which often termi- dimensions, and their morphological variations are pri-
nate laterally against the linguoid ripples. These form as marily controlled by local topography, slope of sediment
a result of changes in water depth with changing tidal surface, grain size, and water flow. Rill marks are quite
level. The double-crested ripples are often supposed to abundant in sandy platforms of estuarine environments.
be diagnostic of intertidal flats (Reineck and Singh,
1980; Terwindt, 1988). Klein (1970) explained the mech- Partially conical rill marks
anism of formation of secondary currents over the primary These small bedforms appear in the form of partially
ones from estuarine environments. developed conical depressions whose walls are sculptured
by fine rills. The cones are about 15 cm across. Water
Ladder-back ripples drained from the conical rills unites to form larger rill
These are interference ripples in which two sets of ripples marks (70–80 cm long) with accumulation tongues down-
maintain an oblique to perpendicular relationship with slope. A sudden change in slope of the platform is marked
their crestal trains. Reduction of water depth particularly by a change in the morphological variety of rill marks.
during the ebb phase controls the size and orientation of
the current ripples. With the decline of water level, the size Bifurcating rill marks
of the current ripples decreases resulting in superimposi- These rill marks often exhibit downslope bifurcation and
tion of the small ripples over the larger sets. Many differ- a sinuous or meandering pattern. The bifurcation is often
ent configurations exist for the ladder-back type to quite overt with the last bifurcations opening in the down-
complex network of ripple trains. slope and downcurrent direction. These rill marks are con-
fined to a slope angle ranging from 2 to 3 and extend for
Backwash ripples a distance of 3–8 m on sandy platforms.
The backwash ripples occupy the highest topographic
areas and concentrate in the regions of maximum advance Branching rill marks
of wave swash in the swash platform of the estuarine envi- Branching rill marks are composed of small rill systems
ronments. These are gentle undulations parallel to river bundled together to form a broad channel. They have very
banks and formed away from the river channel margin. prominent bifurcations that yield a dendritic pattern. The
The bedform is prominent due to variations of color from finer rills unite together downslope and are often confined
the crests to the troughs of the ripples. They are sinuous in to an eroded broad channel whose walls stand 3–4 cm
plan and are generally asymmetrical in profile with lee high from the rill floor, which is also characterized
slope direction toward the river channel. Dark minerals, by coarser lag materials of fragmental shells and mud
mostly biotite, concentrate along the troughs of the rip- pellets. Branching rills occur on a slope angle ranging
ples, whereas light-colored quartzo-feldspathic minerals from 3 to 6 .
mark the ripple crests. Thus, instead of being marked by
their relief, these ripples are characterized by sinuous, Rhomboid marks
alternate light and dark color bands. Rhomboid marks are formed on swash platforms in estua-
rine regions. They are diamond-shaped structures with
Swash marks their long diagonals aligned at right angles to the longitu-
These are tiny, curved ridges or markings on a sandy dinal profile of a river. There are two different sets of
swash platform. The curved ridges with their convexity rhomboid marks which appear as superimposed
landward mark the maximum advance of wave swash. large-scale and small-scale reticulate patterns on the sandy
The ridges are generally of insignificant heights and surface. The smaller set ranges from 2 to 3 cm along their
exhibit strike-wise continuation for several meters longer diagonals and 0.8–1.2 cm along their shorter diag-
although with minor breaks at places. The ridges are gen- onals. The larger set has longer diagonals about 1 m and
erally comprised of very fine sand grains. Swash marks shorter diagonals about 45 cm. Rhomboid marks have
result from the lobate fronts of dying waves during back- positive relief of a few mm to less than 1 cm from the nor-
wash and mark the line of maximum advance of wave mal sediment surface. Rhomboid marks originate from
 SEDIMENTARY STRUCTURES 571

a relatively high-velocity condition of the wave backwash Flaser bedding

when the depth of the water sheet is less than 1.5 cm. They These structures are cross-stratified ripple bedding
appear on the sandy platform when the wave backwash containing thin streaks of mud in the crests and troughs
moves down leaving the area exposed. of the ripples. Most flaser laminations are the wavy flaser
type of Reineck and Wunderlich (1968) in which the mud
Current crescent flasers are concave upward when they occupy the crests of
These are crescent-shaped (U or V shaped) structures ripples and concave downward in the overlying ripple
which widen in the flow direction. They have been shown crests. The flaser-bedded unit continues laterally for
to form around dead gastropod or pelecypod shells with a length of 4–5 m. The mud flasers are often of several
their arms open downslope toward the river direction millimeters in thickness.
(Bhattacharya, 1993). The tapering end of the dead gastro-
pod shells always points downcurrent and in a downslope Conclusion
direction. Current crescents form around scattered tools of Sedimentary structures in estuarine environments have
permeable or impermeable bluff bodies like pellets, shell been utilized for interpreting the hydrodynamic conditions
fragments, mineral matters, lithic fragments, vegetation of their formation. These structures reflect the effects of
hummock, and decomposable organic matter in the aque- macrotidal regimes, moderate wave energy, and longshore
ous estuarine environment. currents (at river mouth bars). The various scales of tidal
Internal sedimentary structures cycles involving neap-spring and ebb-flood, together with
wave swash and backwash, impart some modifications in
These structures in the estuarine environment are only the bedforms. Internal manifestation of bedforms is the
seen in several trench sections cut through the direct consequence of surface features, and many of these
mid-channel bar and washover flat sediments during short are interpreted as features of the tidal domain. The mor-
periods of their surface exposure at falling water stages. phology of the bedforms of Hugli-Matla estuary in India,
Internal sections from dugout trenches of mid-channel with some exception, closely resembles those described
bars exhibit large-scale tabular cross-stratifications with for the Bay of Fundy, Canada, and Loughor estuary, UK.
dip directions controlled by the dominating flow. Some It is concluded that the perpendicular-to-shoreline
trench sections in the mid-channel bars clearly reveal alignment of bedforms like backwash ripples, swash
penecontemporaneous deformations of sandy laminae marks, rhomboid marks, current crescents, and internal
toward the crestal part of the arrested megaripples. The sedimentary structures of estuarine environments may be
deformations are marked by slightly disturbed layers with interpreted incorrectly as beach features in the rock record.
minor undulations or folding. The megaripples bedding Such a misinterpretation may lead to a 90 error in the
is followed upward by parallel laminations as internal mapping of a local palaeo-shoreline at the mouth of an
structures with distinct tidal bundles characterized by estuary.
alternations of sand-mud couplets and sets of horizontal
stratifications. In some occasions, convex-upward
reactivation surfaces are present within cross-bedded Bibliography
units. Allen, J. R. L., 1968. Current Ripples: Their Relation to Patterns of
Sedimentary units depicting a number of internal sedi- Water and Sediment Motion. Amsterdam: North Holland.
mentary structures have been recognized in washover flats Bhattacharya, A., 1993. Backwash and swash oriented current cres-
following sequences from bottom to top. cents: indicators of beach slope, current direction and environ-
ment. Sedimentary Geology, 84, 139–148.
Collinson, J. D., and Thomson, D. B., 2006. Sedimentary Struc-
Hummocky cross-stratification tures. London: George Allen.
These are low-angle (2–3 ) undulating cross- Conybeare, C. E. B., and Crook, K. A. W., 1968. Manual of Sedi-
stratifications. The lamina sets (15–20 cm thick) are both mentary Structures. Bureau of Mineral Resources, Geology
concave and convex upward with wavelength of 1–2 m and Geophysics, Canberra A.C.T. Australia, Bull. No 102.
and height of 6–15 cm. Texturally, the hummocky cross- Dalrymple, R. W., Knight, R. J., and Lambiase, J. J., 1978.
stratifications are comprised of sand-sized particles. Bedforms and their hydraulic stability relationships in a tidal
environment, Bay of Fundy, Canada. Nature, 275, 100–104.
Das, G. K., and Bhattacharya, A., 2000. Bedform morphodynamics
Planar tabular cross-stratification in the meso-macrotidal Thakuran River of Sunderbans, NE
The planar tabular cross-beds form an isolated set in India. Coastal Zone Management, S.D.M.C.E.T., Dharwad &
between the underlying hummocky cross-stratifications IGCP, 367, Special publication number, 2, pp. 101–106.
and the overlying sand-mud laminated units. The foreset Davis, R. A. D., 1983. Depositional Systems. Englewood Cliffs:
dips landward at an angle of 15–18 and therefore indi- Prentice-Hall.
cates their flood-tidal origin. The mud drapes and mud Elliot, T., 1983. Facies, sequences and sand-bodies of the principal
clastic depositional environments. In Parker, A., and Sellwood,
laminae punctuated within foreset laminae refer to slack- B. W. (eds.), Sediment Diagenesis. Dordrecht: D. Reidel Pub-
ening structure. The planar cross-beds are not laterally lishing Company, pp. 1–56.
persistent. They grade to flaser beds and farther away Elliot, I., and Gardiner, A. R., 1981. Ripple, megaripples and sand
(15–20 m) into parallel-laminated units. wave bedforms in the macrotidal Loughor Estuary, South Wales,
572 SEICHE

UK. International Association of Sedimentologists, 5, 51–64. Bibliography

Special Publication.
Chow, V. T., 1964. Handbook of Applied Hydrology. New York:
Klein, G. d V., 1970. Depositional and dispersal dynamics of inter-
McGraw-Hill.
tidal sand bars. Journal of Sedimentary Petrology, 40,
Darwin, G. H., 1898. The Tides and Kindred Phenomena in the
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Solar System. New York: Houghton, Mifflin and Company.
Middleton, G. V. (ed.), 1965. Primary Sedimentary Structures and
Tver, D. F., 1979. Ocean and Marine Dictionary. Ithaca: Cornell
Their Hydrodynamic Interpretation – A Symposium. Tulsa: Soci-
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Pettijohn, F. J., and Potter, P. E., 1964. Atlas and Glossary of Pri-
mary Sedimentary Structures. New York: Springer.
Potter, P. E., and Pettijohn, F. J., 1977. Paleocurrents and Basin
Analysis, 2nd edn. New York: Springer. SHANNON-WEAVER DIVERSITY INDEX
Reineck, H. E., and Singh, I. B., 1980. Depositional Sedimentary
Environments. New York: Springer.
Reineck, H. E., and Wunderlich, F., 1968. Classification and Origin Selene Ortiz-Burgos
of Flaser and Lenticular Bedding. Sedimentology. New York: Facultad de Estudios Superiores Zaragoza, Universidad
Wiley. Nacional Autónoma de México (UNAM), Delegación
Terwindt, J. H. J., 1988. Paleo-tidal construction of inshore tidal Iztapalapa, DF, México
depositional environments. In de Boer, P. L., van Gelder, A.,
and Nio, S. D. (eds.), Tide-Influenced Sedimentary Environ-
ments and Facies. Dordrecht: D. Reidel Publishing Company, Synonyms
pp. 233–264. Shannon index; Shannon information index; Shannon-
Wiener index

Definition
The Shannon-Weaver diversity index is based on commu-
SEICHE nication theory. The uncertainty is measured by the Shan-
non Function “H0 .” This term is the measure
Murat Aksel corresponding to the entropy concept defined by
Civil Engineering Department, Istanbul Kultur University Xn
Atakoy Campus, Istanbul, Turkey H0 ¼
ðpi ln pi Þ ð1Þ
n¼1
Synonyms 0
where H is the diversity index, pi is the proportion of each
Eagre; Sea swell
species in the sample, and ln pi is the natural logarithm of
this proportion (Shannon and Weaver, 1949; Spellerberg
Definition and Fedor, 2003; Magurran, 2004).
A seiche is a stationary wave oscillation that causes water
surface oscillations in any period and height under the Description
effects of different forces such as an earthquake, wind, The Shannon-Weaver diversity index is one widely used
barometric pressure, and tide. It was initially described index for comparing diversity between various habitats
by the Swiss hydrologist François-Alphonse Forel in (Clarke and Warwick, 2001). It assumes that individuals
1890, who made the first scientific observations in Lake are randomly sampled from an independent large popula-
Geneva, Switzerland (Darwin, 1898). tion, and all the species are represented in the sample
(Shannon and Weaver, 1949). The value of the Shannon-
Description Weaver diversity index usually ranges from 1.5 to 3.5
The period of a seiche varies from a few minutes to and only rarely exceeds 4.5.
an hour or more. Wind is the most common seiche produc- The principal objective of a diversity index is to obtain
ing force, causing water surface heaves against the a quantitative estimate of biological variability that can be
downwind shore. Windbreaks cause water surface used to compare biological entities in space or in time.
oscillation. The period of a seiche is calculated using This index takes into account two different aspects that
Merian’s formula: contribute to the concept of diversity in a community: spe-
cies richness and evenness.
1 2L
T¼ pffiffiffiffiffi ð1Þ
n gd Bibliography
Clarke, K. R., and Warwick, R. M., 2001. Changes in Marine Com-
where T is the period of the seiche, n is the number of munities: An Approach to Statistical Analysis and Interpretation,
the nodes of seiche, L is the mean length of the basin, 2nd edn. Plymouth: PRIMER-E.
d is the mean depth, and g is the acceleration of gravity Magurran, A., 2004. Measuring Biological Diversity. Oxford:
(Chow, 1964). Blackwell.
 SHELLFISH PRODUCTION 573

Shannon, C. E., and Weaver, W., 1949. The Mathematical Theory whereas sedimentologic concentrations may be produced
of Communication. Urbana: University of Illinois Press. by lateral migration of channels and by storm surges
Spellerberg, I. F., and Fedor, P. J., 2003. A tribute to Claude (shell pavements and spits). The more refined genetic
Shannon (1916–2001) and a plea for more rigorous use of
species richness, species diversity and the ‘Shannon – Wiener’ scheme proposed by Kidwell (1991) on the basis of their
Index. Global Ecology and Biogeography, 12, 177–179. depositional histories and stratigraphies organizes shell
beds into four broad categories: event, composite, hiatal,
and lag concentrations. In this manner, the interpretation
Cross-references yields a stratigraphic signature of the shell-rich sedimentary
Species Richness body to identify the final concentration process and
a “taphonomic characterization” of the fossil remains to
reconstruct their history before and during the concentra-
tion event(s). These four types of shell beds are not
SHELL BEDS discrete categories because they intergrade, and each one
may be present in the supra-, inter-, and subtidal environ-
Luca Ragaini ments characterizing the estuarine systems.
Department of Earth Sciences, University of Pisa, Pisa,
Italy Bibliography
Fursich, F. T., 1995. Shell concentrations. Eclogae Geologicae
Synonyms Helvetiae, 88(3), 643–655.
Coquinas; Lumachelles; Shell gravels Kidwell, S. M., 1991. The stratigraphy of shell concentrations. In
Allison, P. A., and Briggs, D. E. G. (eds.), Taphonomy. London:
Plenum Press, pp. 211–290.
Definition Kidwell, S. M., Fursich, T. T., and Aigner, T., 1986. Conceptual
Dense deposits of biologic hard parts more than 2 mm in framework for the analysis and classification of fossil concentra-
size in estuarine and other environments are generally tions. Palaios, 1(3), 228–238.
defined as shell concentrations (Kidwell, 1991). They
are also known as coquinas, lumachelles, or shell gravels, Cross-references
as well as the more familiar term “shell beds.”
Biogenic Sedimentary Structures
Biogenous Sediment
Description Sedimentary Structures
Although the term “shell beds” refers to a particular geo- Stratigraphy of Estuaries
metric arrangement of biogenic remains, the great variety
of shell beds reflects the diverse descriptive approaches
used to classify them. The scheme proposed by Kidwell
et al. (1986) is based on different field observations SHELLFISH PRODUCTION
of the shell deposits, such as their biofabric, geometry,
taxonomic composition and internal structure, which can Islay D. Marsden
be measured in the field by nonspecialists. This procedure School of Biological Sciences, University of Canterbury,
allows investigators to obtain a range of ecological, hydro- Christchurch, New Zealand
dynamic, and topographic data on the mode of the shell
bed formation (Fursich, 1995).
Definition
There are different classification schemes for shell beds.
In the basic approach proposed by Kidwell et al. (1986), Shellfish production involves evaluation of shellfish
shell beds may be plotted in six areas of a schematic ternary resources, often collated from annual surveys, listing the
diagram: biologic, sedimentologic, and diagenetic pro- wet weight and monetary value of capture fisheries and
cesses are the end-members, whereas three mixed areas aquaculture. In ecological terms, production quantifies
reflect combinations of these factors. Comparative analysis biological productivity, the amount of organic matter, or
may yield data on environmental indicators to characterize, its equivalent in dry matter, carbon, or energy which is
according to prevailing shell bed types, the ideal transect accumulated over time.
from marginal marine to fully marine depositional settings,
even if the same type of shell bed may appear in different Introduction
environments. In general terms, marginal marine environ- World fisheries and aquaculture production have
ments (e.g., estuaries) exhibit a diverse assortment of bio- increased over the past five decades at a rate of 3.2 %
genic and sedimentologic concentrations. In the intertidal per year to 148 million tons, worth $217.5 billion USA
and supratidal settings, for example, biogenic concentra- dollars, with most used for human consumption (FAO,
tions include channel-margin oyster bars, mussel clumps, 2012). Highest consumption has been in Asia, where
levels of deep-burrowing infaunal bivalves in life position, annual consumption reached 20.7 kg/capita. In China,
ray pits, bird’s nests, and hermit crab-generated beds, with an expanding economy and increased domestic
574 SHELLFISH PRODUCTION

income, the per capita consumption has reached 31.9 kg 1980s and 1990s, with Asia contributing 89 % of the
per annum. While the production of fisheries and aquacul- world aquaculture production in 2010. In Europe,
ture has varied considerably between geographical increased aquaculture production has been due to cage
regions, shellfish production in many areas has remained culture of salmon, with notable declines in bivalves from
steady. A declining global marine catch over the last few 61 % of the total in 1980 to 26.2 % in 2010. In North
years and the increased percentage of overexploited spe- America, bivalve production appears to be declining,
cies have led to both negative ecological effects and ongo- whereas in South America, Brazil, and Peru, there has
ing negative social and economic consequences. For many been strong growth. This increase in bivalve production
years, China has been the world’s leading shellfish and may be due to the formation of the non-fed aquaculture
fish exporter, but some countries, such as Vietnam, have systems, which avoid the problems associated with feed.
experienced rapid growth. Vietnam is now the fourth larg-
est exporter in the world. The largest importers are the Shellfish species in estuaries
USA and Japan. In many parts of the world, fisheries pro-
Broadly defined, shellfish are edible invertebrates usually
duction is small scale and susceptible to fluctuations. New
molluscs, crustaceans, or echinoderms. While mussels,
guidelines are being developed to promote good
oysters, scallops, clams, lobsters, and shrimps are well-
governance as well as inclusiveness, transparency, gender
known shellfish, less recognized groups include sea
equality, and respect and involvement of stakeholders.
urchins, sea cucumbers, abalone, and whelks. All have
World fisheries production statistics are divided into
a hard external covering, a shell or exoskeleton which pro-
wild capture and aquaculture, with further divisions into
tects them from extremes of environment and/or preda-
inland and marine production. Using these divisions, it is
tors. Shellfish species that live or are cultured in
often difficult to identify and single out the brackish water
estuaries are generally euryhaline or salt tolerant, the main
production in estuaries. Data for mussels, clams, cockles,
groups being mussels, oysters, clams, and shrimps. These
and scallops are included in the marine–water aquaculture
are further divided into capture species that are harvested
group, where in 2010 they comprised 75.5 % of the total
from natural populations and aquaculture species grown
catch, down from 84.6 % in 1980. After 1980, there was
in shallow embayments or brackish water ponds. Even
an increase in finfish culture which continued to increase
with the increasing expansion of aquaculture, wild
at a rate of seven times that of molluscs. Brackish water
populations remain important in many parts of the world.
aquaculture, however, has increased over the past 20 years,
Bivalves are a traditional food for many people.
and while only contributing 7.9 % of the world production
Collecting methods have changed over the years,
in terms of quantity, it corresponds to 12.8 % of the total
combining traditional methods with new technology.
value because of the high value of marine shrimps
The collection and aquaculture methods for bivalves are
cultured in brackish ponds. Brackish water crustaceans
summarized by Gosling (2003); wild shellfish are col-
dominate aquaculture production with bivalves making
lected by hand, dredges, and metal baskets, but in deeper
up only a small percentage (2.1 %) of the total (FAO data
water they are collected by diving or in small boats.
in 2010). The aquaculture statistics for 2010 are impres-
Mussel and clam production is often higher in estuaries
sive: 2.7 million tons of white leg shrimp; 4.8 million tons
than in the open sea, but this may be a consequence of
of clams, cockles, and arc shells; 4.5 million tons of oys-
increased food availability or food quality rather than
ters; and more than 1.5 million tons of mussels and scal-
a direct salinity effect (Seed and Suchanek, 1992). Produc-
lops. The question is – are these production rates
tion from a biological and/or ecological viewpoint
sustainable?
requires knowledge of the life history of species; this is
because it depends on population regulating mechanisms,
Catch and aquaculture trends including recruitment, growth, and reproduction.
Over the last 20 years, changes have occurred in catch
statistical trends for marine bivalve species. Clams and Capture species
cockles once formed more than half of the annual capture Oyster fisheries have a long history; for example, the flat
catch (Figure 6; FAO, 2012), but more recently they have oyster Ostrea edulis in Europe and the American oyster
been equalled by scallops, where the most productive fish- Crassostrea virginica in the USA are the species which
eries are in coastal offshore habitats rather than estuaries. contributed most to the commercial harvest. There have
In estuaries, the production of mussels and oysters appears been declines in these species over time (Gosling, 2003),
to have remained steady or even have declined over the and they now make up only a small percentage of the oys-
past 10 years; however, there appears to be increased ter landing from aquaculture (FAO data). Numerous spe-
scope for future expansion. There is, however, some cies of mussels are consumed worldwide; many belong
uncertainty with the FAO database; not all countries to the genera Mytilus and Perna. The extent of the wild
identify catches by species, and aquaculture numbers fishery is uncertain because, even where this is claimed,
may not be distinguished from capture fisheries. there may be enhancement by transferring seed from natu-
Global aquaculture production has increased since ral habitats onto culture beds for ongrowing. Countries
1990, although at a slower rate than was recorded in the that identify wild mussel catches include the Netherlands,
 SHELLFISH PRODUCTION 575

Germany, and Denmark in the Wadden Sea. For clams, and blood cockle Anadara granosa. Spat are mostly sup-
about 30 different species are collected worldwide, but plied from hatcheries and once they reach 10 mm in length
two species, the surf clam Spissula solidissima and the are seeded into substrate where mesh is laid over the bed to
ocean quahog Arctica islandica, make up a large propor- preclude crab and bird predation. The shellfish are usually
tion of the clam landings; these are characteristic of off- harvested when the shell length exceeds 20 mm, either by
shore benthic marine habitats rather than estuaries. Clam hand or mechanical harvester (Gosling, 2003).
species found in USA estuaries include Mercenaria Scallops grown in estuaries provide options for aqua-
mercenaria and Mya arenaria, and in Europe, one of the culture in many countries, through a combination of
dominant bivalves is the cockle, Cerastoderma edule, natural seeding, collectors, or hatchery-produced spat
found from mid-tide to low-tide level in sandy bottom and ongrowing on the seafloor on rafts or in nets
estuaries. Characterized by irregular recruitment, thought (Shumway and Parsons, 2006). Scallop production in
to be a result of weather conditions, successful spatfall estuaries over the last 10 years has been declining world-
occurs approximately once every 6 years. Scallops are wide with mortalities and population fluctuations, espe-
a high-value seafood, and natural populations have cially in short-lived species. The Atlantic bay scallop
fluctuated widely in estuarine regions as a result of Argopecten irradians and Yesso scallop Mizuhopecten
overharvesting, contaminants, and toxic algal blooms. At yessoensis are cultured in a number of countries, and there
present there appears to be low production values for is increased interest in polyculture with seaweeds.
scallops in estuaries, contrasting with the successful
production of offshore species.
Shrimp and crustacean production
The whiteleg or Pacific white shrimp Litopenaeus
Aquaculture vannamei is the most productive of the internationally
Aquaculture depends on a ready supply of seed which can introduced marine crustaceans. Native to Mexico, this
be collected from the field or from brood stock held in shrimp lives in the ocean down to 72 m, but juveniles
a hatchery. While the mussel industry depends mainly on occur in estuaries where temperatures exceed 20  C all
wild spat, clam and oyster aquaculture depends on year. Juveniles are captured in estuaries or cultured in
hatchery-produced seed. There may be problems with dis- ponds, where they reach 30–35 g after about 7 months.
eases in hatcheries, but this may be lessened once the There are issues associated with the expansion of the
shellfish are in the field. During the grow-out phase trays, shrimp industry – the removal of mangroves to
ropes, platforms, and rearing nets must be cleaned of foul- construct ponds, pollution of coastal waters by pond
ing organisms. Mussel culture systems include bottom effluent, salinization of groundwater, and agricultural land
culture, poles, and rafts and suspended culture systems (Roth et al., 2008). Brood stock are sea caught, and one
using buoys as undertaken in New Zealand and Chile. In eye is ablated, resulting in repeated maturation and
Spain, Mytilus galloprovincialis is cultured on wooden spawning. The hatcheries range from small backyard
rafts in the rias of Galicia, where temperatures range hatcheries to large environmentally controlled purpose-
between 10 C and 20  C and the salinity is about 34 psu built buildings. Using natural productivity of the ponds,
(Gosling, 2003). Mussels reach harvestable size of this species costs less to produce than the more carnivo-
8–10 cm in about 15 months; growth is slowest in summer rous Penaeus monodon where similar culture techniques
probably because of a restricted food supply due to strati- were used previously. In 2010 the catch was greater than
fication of the water column. Mussels grown in Europe are 2.8 million tons. The main producers in 2004 were China
usually depurated before sale. (700,000 t), Thailand (400,000 t), Indonesia (300,000 t),
Oyster culture has expanded worldwide, with the and Vietnam (50,000 t). The freshwater prawn
Pacific oyster Crassostrea gigas contributing most to the Macrobrachium is targeted for ongrowing in some coun-
4.6 million tons cultured annually. China, Japan, Korea, tries such as Bangladesh because prawns grown from wild
and France are major producers. Seed supply is normally larvae are considered to be of high quality. Juveniles along
from natural settlement onto artificial collectors, and the Gulf coast recruit to sea grass and mangrove estuaries
ongoing culture methods include bottom, rack, and hang- where they grow in the rich productive estuarine waters
ing rope culture. Bottom culture involves sticks, mesh providing a refuge from predation.
bags, or ground culture, and after a pre-growing period, Of the total catch from both wild capture and farms,
the spat are scraped off, graded, and put back onto the crabs comprise only a small proportion (20 %) of the
bottom for periods of up to 2 years (Gosling, 2003). annual crustacean catch, and although the catch statistics
For all types of culture, the growing time to harvest ranges are dominated by the Japanese blue crab, the blue crab
between 2 and 4 years depending on local growth Callinectes sapidus, the Dungeness crab Metacarcinus
conditions. magister, and the mud crab Scylla serrate each contributes
Clams total more than three million tons a year, includ- 20,000 t annually. Although these landings might appear
ing the Japanese carpet shell or Manila clam Ruditapes low, they are important because of their high monetary
philippinarum (which has been introduced to France, the value compared with bivalves. Chesapeake Bay in the
UK, and Ireland), hard clam Mercenaria mercenaria, USA once had a thriving industry for hard shell and soft
576 SHELLFISH PRODUCTION

shell blue crabs, but more recently the harvests have fluc- countries. In Japan for hundreds of years, the people of
tuated both here and in other regions. There have been Oki Island used loose stone piles to encourage the sea
a number of attempts to understand the relationships cucumbers to aggregate, aestivate, and protect juveniles
between production and environmental variables, river and young (Choo, 2008). In tropical and subtropical
discharges, wind, temperature, salinity, rainfall, and hurri- waters in Asia, the fisheries are multispecies, whereas in
cane events, but few patterns have emerged (Fogarty and temperate areas there may be a single species such as the
Lipcius, 2007). Modelling studies by Mistiaen et al. (2003), Apostichopus japonicus. Indonesia is the largest capture
however, have confirmed that productivity would be neg- fishery for holothuroids and, together with the Philippines,
atively impacted by poor water quality, suggesting that comprises about 47 % of the world landings, averaging
a decline in the oxygen content of the water to 4 mg l
1 2,572 t wet weight per annum between 2000 and 2005
could lead to a 48 % decline in the harvest with the same (Choo, 2008). It is uncertain what proportion of this catch
level of fishing effort. is collected from estuarine regions and compared with
Populations of the Dungeness crab S. magister, in the fish, crustaceans, and molluscs. These landings represent
Northeast Pacific, have large amplitudes with a 9–10-year only a small percentage (less than 0.25 %) of the total
cyclic periodicity. The adults breed offshore, but, like landings. Many boom-and-bust sea cucumber fisheries
some other decapod crustaceans, rely upon nursery have occurred since the 1950s as markets rapidly
grounds to produce the adult stocks which are captured expanded, and regional assessments now suggest that
in other locations. Larvae of the Dungeness crabs enter 81 % of the sea cucumber stocks have declined due to
estuaries where they grow faster than their cohorts, overfishing, and the average size reduced by as much as
occupying other nearshore habitats (Armstrong et al., a third. In addition the harvesters have moved from near-
2003). These juveniles, especially those from the large shore habitats to offshore habitats and into less valuable
estuary zones close to the ports, are important because species (Anderson et al., 2011). These fisheries are likely
mortality is low and abundances are high. There is there- to be difficult to manage because of incomplete knowl-
fore a high economic value of the estuarine zone in stabi- edge about their life cycle, aging, and reproductive cycles.
lizing coastal landings. Also, while some countries lack regulations (Indonesia
Found in most of the Indo-Pacific, the mud crab Scylla and the Philippines), other populations such as those in
serrata is a short-lived species, commonly collected for Japan and Alaska are well managed (Clark et al., 2009).
food and more recently cultured in some countries includ- In the last 20 years, there have been advances in farming
ing the Philippines. Associated with mangroves it survives practices for sea cucumbers, especially in China, Ecuador,
in low salinities down to 20 psu in estuaries. In subtropical Indonesia, Japan, Malaysia, and the Philippines. China is
and tropical Australia, S. magister is highly sought after, the largest producer of sea cucumbers; farming and ranching
but there are size restrictions and no female crabs are taken in the Liaoning and Shandong Provinces, landings have
from Moreton Bay, allowing females to reproduce. Over exceeded 10,000 t (dry weight) per annum. Pond culture is
the years the commercial catch has increased (113.3 t in the preferred method of farming, sometimes employing
2003), but there is a high recreational catch which exceeds unused shrimp ponds containing shelters to protect the
the commercial harvest. It is also suggested that the forma- animals (Chen, 2004). Sea ranching is a less expensive
tion of marine reserves provides the potential for allowing option for some species, such as Apostichopus japonicus
exploited species to recover from the effects of fishing the prickly sea cucumber, where the temperature, salinity
(Pillans et al., 2005). Culture techniques have been tried range (28–31 psu), and sufficient natural food are good.
using pens within mangrove areas in the Philippines and In a sea ranch experiment, Zhang and Liu (1998) report
sourced with juveniles from the natural habitat a mean output for this species of 273 kg (dry weight) per
(Trino and Rodriguez, 2002). This study found that mixed hectare, at a density of 12.9 individuals m
2.
sex monoculture in the mangroves was feasible and that
production was acceptable at stocking densities in the
range 0.5–1.5 crabs m
2. It was concluded that this aqua- Management of shellfish production
culture venture is possible without the need to remove For both wild populations and aquaculture, there are man-
mangroves to build aquaculture ponds. agement techniques aimed at increasing shellfish produc-
tion. For oysters on the Atlantic and Gulf coasts of North
America, the increased shelling of beds and the removal
Echinoderm (sea cucumber) production of gastropod and starfish resulted in a 45-fold increase in
Following worldwide trends for shellfish production, sea the production in Long Island Sound Connecticut in the
cucumbers, which belong to the class Holothuroidea, are 1990s (Dumbauld et al., 2009). Management is also
highly prized in Asia, where they are sold as trepan or required for the oyster industry to minimize the effects
beche-de-mer and have a variety of food and medicinal of shellfish disease and polluting effects of land runoff
uses. They occur in nearshore habitats, including muddy and urban development. Other improvements have been
shores close to estuaries, sea grass beds, and rocky and achieved through transplants, seeding, silt removal, and
coral reefs. Sea cucumber fishing is important to the liveli- reef construction to protect brood stock. Over the past
hoods of coastal communities, especially in developing 10 years, there has been increasing interest in developing
 SHELLFISH PRODUCTION 577

polyculture systems within controlled estuarine systems. Armstrong, D. A., Rooper, C., and Gunderson, D., 2003. Estuarine
In the Netherlands, mussels have been grown with production of juvenile Dungeness crab (Cancer magister)
polychaetes, while in Asia sea cucumbers Holothuria and contribution to the Oregon-Washington coastal fishery.
Estuaries, 26, 1174–1188.
scabra have been grown with seaweed and shrimp. Burkholder, J. M., and Shumway, S. E., 2011. Bivalve shellfish
In parts of the world where important shellfish species aquaculture and eutrophication. In Shumway, S. E. (ed.), Shell-
have declined, management regimes have been controlled fish Aquaculture and the Environment. Hoboken: Wiley,
since the 1960s by Fisheries Acts introduced by govern- pp. 155–215.
ments or local regulations. Thus, many countries have Chen, J., 2004. Present status and prospects of sea cucumber indus-
limits such as total allowable catch (TAC) as for cockles try in China. FAO Document Repository, Fisheries and Aquacul-
ture Department.
in the UK and permits which limit licenses, restrict fishing Choo, P. Z., 2008. Population status, fisheries and trade of sea
days or season length, and return small individuals to the cucumbers in Asia. In Toral-Granda, V., Lovatelli, A., and
sea. Other mechanisms to protect and/or enhance shellfish Vasconcellos, M. (eds.), Sea Cucumbers. A Global Review of
production are using restoration techniques to develop Fisheries and Trade. FAO Fisheries Technical Paper, Vol.
wetlands and other habitats or artificially seed juveniles 516, pp. 81–118.
into suitable habitat. In Florida, for example, ready-to-set Clark, J. E., Pritchett, M., and Hebert, K., 2009. Status of Sea
Cucumber Stocks in Southeast Alaska and Evaluation of the
pedi-veligers of the bay scallop are released into estuaries Stock Assessment Program. Anchorage: Alaska Fish and Game.
(Leverone et al., 2010). For successful restoration, not Fishery Data Series, Vol. 09–12.
only is there a need for ready supply of viable or healthy Dumbauld, B. R., Ruesink, J. L., and Rumrill, S. S., 2009. The eco-
spat, but the sites must be in areas where individuals are logical role of bivalve aquaculture in the estuarine environment:
likely to survive, grow, develop, and maintain sustainable a review with application to oyster and clam culture in West
populations. There is therefore a need to better understand Coast (USA) estuaries. Aquaculture, 290, 196–223.
the biology of individual target species and how they FAO, 2012. The State of the World Fisheries and Aquaculture.
Rome: Food and Agricultural Organization of the United
respond to environmental stressors. Nations, ISBN 978-92-5-107225-7, p. 22
Fogarty, M. J., and Lipcius, R. N., 2007. Population dynamics and
fisheries. In Kennedy, V. S., and Cronin, L. E. (eds.), The Blue
Summary Crab Callinectes sapidus. Maryland: Maryland Sea Grant Book,
Estuarine shellfish production is predicted to remain pp. 711–755.
steady or increase over the next 20 years despite wide- Gosling, E., 2003. Bivalve Molluscs Biology, Ecology and Culture.
spread belief that global change could fundamentally Oxford: Blackwell Science, Fishing News Books.
Leverone, J. R., Geiger, S. P., Stephenson, S. P., and Arnold, W. S.,
change the nature of shallow intertidal habitats (Allison 2010. Increase in bay scallop (Argopecten irradians)
et al., 2011). The threats to increased production include populations following releases of competent larvae in two west
increased incidence of pollution events, eutrophication, Florida estuaries. Journal of Shellfish Research, 29, 395–406.
and toxic algal blooms (Burkholder and Shumway, Mistiaen, J. M., Strand, I. V., and Lipton, D., 2003. Effects of envi-
2011). There are, however, other threats; for example, ronmental stress on blue crab (Callinectes sapidus) harvests in
invasive or transplanted species such as the Manila clam Chesapeake Bay tributaries. Estuaries, 26, 316–322.
Ruditapes philippinarum or oysters Crassostrea gigas Pillans, S., Pillans, R. D., Johnstone, R. W., Krafft, P. G., Haywood,
D. E., and Possingham, H. P., 2005. Effects of marine reserve
may replace native species. While these species can protection on the mud crab Scylla serrata in a sex-biased fishery
become important dietary components, there is currently in subtropical Australia. Marine Ecology Progress Series, 295,
little understanding about how such introductions could, 201–213.
in the long term, affect the functioning of estuarine sys- Roth, B. M., Rose, K. A., Rozas, L. P., and Minello, T. J., 2008. Rel-
tems. According to Dumbauld et al. (2009), unlike other ative influence of habitat fragmentation and inundation on
anthropogenic influences, aquaculture systems do not brown shrimp Farfantepenaeus aztecus production in northern
Gulf of Mexico salt marshes. Marine Ecology Progress Series,
degrade water quality. Thus, together with whole ecosys- 359, 185–202.
tem management, the increased use of modern tools, Seed, R., and Suchanek, T. H., 1992. Population and community
genetics, breeding, improved hatchery techniques, GIS, ecology of Mytilus. In Gosling, E. M. (ed.), The Mussel Mytilus:
and modelling, there will be ongoing developments in pro- Ecology Physiology, Genetics and Culture. Amsterdam:
duction techniques which should support millions of peo- Elsevier, pp. 87–169.
ple around the world who are employed in the shellfish Shumway, S. E., and Parsons, G. J. (eds.), 2006. Scallops: Biology,
Ecology and Aquaculture. Amsterdam: Elsevier.
industry. Trino, A. T., and Rodriguez, E. M., 2002. Pen culture of mud crab
Scylla serrata in tidal flats reforested with mangrove trees.
Aquaculture, 211, 125–134.
Bibliography Zhang, Q.L., and Liu, Y.H., 1998. The Techniques of Sea Cucumber
Allison, E. H., Badjeck, M. C., and Meinhold, K., 2011. The impli- Culture and its Enhancement. China: Ocean University
cations of global climate change for molluscan aquaculture. Publishing House.
In Shumway, S. E. (ed.), Shellfish Aquaculture and the Environ-
ment. Hoboken: Wiley, pp. 461–490.
Anderson, S. C., Fleming, J. M., Watson, R., and Lotze, H. K., 2011. Cross-references
Serial exploitation of global sea cucumber fisheries. Fish and Bivalve Aquaculture
Fisheries, 12, 317–339. Estuarine Habitat Restoration
578 SHORE PROTECTION

Maryland Department of Natural Resources, 1992;

SHORE PROTECTION Eurosion, 2004).
Shore protection methods along estuarine coasts are
C. Scott Hardaway, Jr. wide ranging but can be grouped into two basic types,
Virginia Institute of Marine Science, College of William & defensive and offensive. These are described below.
Mary, Gloucester Point, VA, USA
Defensive
Synonyms Perhaps the most common method of estuarine shoreline
Shore erosion control protection is to harden the shore with bulkheads, seawalls,
or revetments. The primary goal of hardening the shore is
Definitions to protect the coast from wave attack by creating a barrier
to the erosive forces, waves, and currents. According to
Shore protection along estuarine coasts generally occurs the NRC (2007), traditional hardening methods often uti-
when shorelines are actively eroding. Shoreline erosion, lize local materials such as stone, wood, and concrete
a natural process, is primarily a function of a rising sea and are built using techniques familiar to local marine
level and the impinging local wave climate. contractors and waterfront property owners. While wood
bulkheads are common in many areas, stone walls
Introduction (riprap revetments), constructed of local rock, have
Many of the processes that govern erosion (and accretion) become more widely used in areas where rock is available.
on the open ocean coasts also apply to estuarine coasts, but Bulkheads, typically made of wood, are actually
compared to the typically long linear nature of open retaining walls that consist of the elements shown in
coasts, estuarine coasts are more sheltered and exhibit Figure 1. The wood is usually treated for longevity in the
a more irregular configuration. Estuarine coasts often dis- estuarine environment. The vertical sheet piles are usually
play very distinct geomorphic compartments containing tongue and groove and are either driven or “jetted” into
a complex mix of resources that may vary from compart- place. Horizontal members called wales run along the face
ment to compartment. The relatively lower wave energy of the sheet piling in one or two levels. Tie rods or tie
conditions found along estuarine coasts create unique backs are positioned every 1.8–2.4 m and anchored land-
environments that foster habitats and ecological commu- ward to hold the pilings or wales as shown. The tie rods are
nities, such as marshes and mudflats, not typically found often placed in conjunction with outside pilings for stabil-
on open coasts (NRC, 2007). ity. For a typical bulkhead design, the sheet piles should be
in the substrate at least as much as the length above. Also,
Extent the structure should allow for groundwater and wave
overtopping to be released through the wall via weep holes
Just about every estuarine shoreline can experience ero- or other drainage feature.
sion, but typically when fetch exposures exceed about
Some walls are made of concrete and rely on
1.6 km, land loss by wave action becomes more common a foundation or footing for stability. They may be referred
(Hardaway and Byrne, 1999). Documented land loss from to as seawalls although this is generally afforded nomen-
shoreline erosion is found in numerous estuaries around
clature for open ocean coasts. The footing is built first,
the USA including Chesapeake Bay, Long Island Sound, and then the vertical wall is built upon it. These must be
Delaware Bay, Puget Sound, and Mobile and Galveston properly connected with re-bar or other connecting mem-
Bays. Land loss in Chesapeake Bay over the 100-year ber. Having them both poured in place at the same time is
period from 1850 to 1950 is estimated at over 190 km2
best. The wall is a gravity structure meant to withstand
(Singewald and Slaughter, 1949; U.S. Army Corps of wave action and hold the land side in place. As with bulk-
Engineers, 1973; Byrne and Anderson, 1978). The heads some type of drainage system to allow for ground-
response by land owners is to protect their property.
water and wave overtopping may be required.
Sloped stone walls are commonly called rock revet-
Types ments, although revetments can be made of other materials
Techniques used to address erosion along estuarine coasts such as concrete units, concrete mats, or gabions. Stone
may be placed into broad categories, such as those pro- revetments armor the front face of the eroding shoreline.
posed by Nordstrom (1992), Rogers and Skrabel (2001), They are commonly constructed with one or more layers
and Rogers (2005). Most guidelines and reports on shore of graded riprap (Figure 2). The eroding bank is often
protection employ the same basic concepts to discuss graded to provide a subgrade for the placement of filter
approaches such as structural or “hard” methods versus fabric, bedding stone, and then armor. Two layers are pre-
nonstructural or “soft” approaches (USACE, 1981, 1984; ferred, but one made on large stone can be used. Armor
New York Sea Grant, 1984; Virginia Marine Resources stone must be of sufficient size to withstand the design
Commission, 1989; Ward et al., 1989; Pile Buck, 1990; storm wave condition.
 SHORE PROTECTION 579

Shore Protection, Figure 1 Photo of a typical bulkhead in Chesapeake Bay, USA. Cross section and plan view of bulkhead
construction (Modified from Hardaway and Byrne, 1999).
580 SHORE PROTECTION

Shore Protection, Figure 2 Top: Photo of a typical revetment in Chesapeake Bay, USA, and bottom: cross section of elements
necessary for proper stone revetment design. Two layers of armor stone overlay a bedding stone layer with filter cloth between the
earth subgrade and bedding layer. Armor size is dependent on the design wave height which is determined from an analysis of wave
climate for each project site (Modified from Hardaway and Byrne, 1999).

Offensive coast, where it protects the sand fill from wave scour.
Offensive methods include those that extend beyond the Marsh plants are established in the sand fill with intertidal
eroding bank slope to intercept the impinging wave cli- species planted from about mean tide level adjacent to the
mate well before impacting the bank. This often involves back of the sill to about mean high water. The sand fill
creating marsh fringes and/or beaches, sometimes referred might go up a couple of meters against the eroding bank
to as living shorelines. Establishing a marsh fringe or to a certain level of protection. Bank grading from that
beach for shore protection either by planting the existing point landward increases the stability of the system. The
substrate or by beach nourishment is considered combination of rock, sand, plants and possibly bank grad-
a nonstructural or “soft” approach. Establishing marsh ing provides a coastal gradient for storm wave attenuation
fringes on their own is limited to very fetch-limited shore- and long-term sea-level rise (Hardaway et al., 2012).
lines of <1.6 km (Hardaway et al., 1985). Establishing a stable protective beach along estuarine
Marsh fringe establishment in higher fetch conditions shorelines can be done with groins or breakwaters with
(up to 8 km) usually requires the addition of sand and beach nourishment. Beach nourishment alone is usually
some type of sand containment structure. In the only placed on public beaches which seek to create recre-
Mid-Atlantic, the use of a sill system is widely used ational areas in addition to shore protection. When
(Figure 3). A sill system consists of sand fill to create sta- beach nourishment is placed without a structure, ongoing
ble beach and planting substrate. This is anchored by maintenance generally is necessary. Groins often are
a rock sill that runs along the nearshore parallel to the used to capture littoral sands to create a wide beach.
 SHORE PROTECTION 581

Shore Protection, Figure 3 Sand fill with stone sill and marsh plantings in Chesapeake Bay, USA. Top: after sand fill placement but
before planting; middle: after 4 years; and bottom: the cross section used for construction (From Hardaway et al., 2010).

Worldwide, groins are a commonly used shore protection along open ocean coasts, installations have become more
method. Usually made of wood or rock, the installation of widespread in estuarine setting, particularly Chesapeake
more than one is often referred to as a groin field. Groin Bay (Hardaway and Gunn, 2010, 2011).
length and spacing varies with site conditions. When there Detached breakwater systems (Figure 5) operate on the
is abundant sand moving alongshore, a groin field can be principal that by placing breakwaters offshore a certain
an effective shore protection method (Figure 4). However, distance that alongshore transport in their lee can continue
in sand-poor systems, downdrift impacts can be signifi- and thus have minimal impact to the downdrift shoreline.
cant. In fact, there is almost always an impact downdrift The degree of attachment (or detachment) is primarily
which often leads to a “domino” effect with those proper- a function of breakwater length vs. distance offshore. If
ties affected adding more groins. the breakwater unit is shorter than its distance offshore,
Offshore breakwaters have been used to provide a salient is likely to develop but if the breakwater unit is
a stable protective beach in many areas around the world. equal to or closer that its distance offshore, a tombolo will
Along estuarine shorelines, they are either detached or most likely develop (Chasten et al., 1993). Attached
attached breakwaters. Attached breakwaters are often breakwater systems (Figure 6) often have a series of
called headland breakwaters. Breakwaters can be used breakwater and pocket beaches and usually require sand
with or without sand nourishment. Though primarily used fill to complete the shore protection system. The sand
582 SHORE PROTECTION

Shore Protection, Figure 4 Examples of groin fields in Chesapeake Bay, USA, that have top: a sufficient sand supply to create
a protective beach and backshore and bottom: an insufficient supply of sand such that the groins act as littoral barriers and prevents
sand from reaching the downdrift shoreline (From Hardaway and Byrne, 1999).

can be both recreational and protective. Where possible, measuring several design parameters: fetch exposure,
the sand fill is sloped from the breakwaters to elevation storm surge frequency, shore orientation, nearshore
of the desired level of protection at the bank. This provides bathymetry, and bank height and composition. The project
a sloped area that can reduce the wave energy that impacts wave climate will be affected by these parameters, and the
the bank under high water conditions (Hardaway et al., power and frequency storm waves that impact the shore
2005). will determine basic components of a shore protection
structure such as structure height and rock size. Although
Design elements developed primarily for open ocean coasts, the Coastal
The design of shore protection along estuarine coasts often Engineering Manual (USACE, 2000) provides the theo-
is predicated on what has been done before and what is retical basis for physics of shore protection.
locally permissible. In the context of designing an The level of protection of a system describes the storm
engineered application, consideration of the hydrody- conditions against which a shore protection system would
namic forces, usually waves, acting against the shore maintain its integrity. In most US localities, the Federal
should be evaluated. Site assessment should include Emergency Management Agency (FEMA) has created
 SHORE PROTECTION 583

Shore Protection, Figure 5 Detached breakwaters that have a shore salient behind the structures. These structures are located in an
area with a strong sand transport system and are designed to allow the sediment to move through the system to downdrift shores.

Shore Protection, Figure 6 Attached headland breakwaters and sand fill create a recreational beach and ecological buffer in an area
with an insufficient sand supply. Without structures, any sand placed on the shoreline would be transported away from the site. In
order to maintain a sandy beach, the breakwaters are built close to shore so that the sand will maintain an attachment behind the
structure minimizing sand loss from the system.
584 SHORE PROTECTION

flood insurance rate maps and reports that provide the Hardaway, C. S., Jr., Milligan, D. A., Wilcox, C. A.,
return frequency of the 10, 50, 100, and 500 years events. Meneghini, L. M., Thomas, G. R., and Comer, T. R., 2005. The
These events relate to the annual chance of 10 %, 2 %, Chesapeake Bay Breakwater Database Project: Hurricane
Isabel Impacts to Four Breakwater Systems. Gloucester Point,
1 %, and 0.2 %, respectively; the mapped area will have VA: Virginia Institute of Marine Science, College of William &
a storm surge at that elevation. The elevations associated Mary.
with the 10 % or 2 % storm events generally are used for Hardaway, Jr., C. S., Milligan, D. A., Hobbs, C. H., Wilcox, C. A.,
design purposes. O’Brien, K. P., and Varnell, L., 2010. Mathews County Shoreline
According to the NRC (2007), the possibility exists that Management Plan. Virginia Institute of Marine Science, College
the level of protection will be exceeded by an event greater of William & Mary, Gloucester Point, Virginia
Hardaway, Jr., C. S., Milligan, D. A., and Duhring, K., 2012.
than the “design storm.” The level of protection employed Living shoreline design guidelines for shore protection in
will translate to the amount of risk or damage the property Virginia’s estuarine environments. Special Report in Applied
owner is willing to accept or incur and the amount Marine Science and Ocean Engineering No 421. Virginia Insti-
budgeted for installing protection. Larger projects with tute of Marine Science, College of William & Mary, Gloucester
more shore protection cost more. Some level of damage Point, VA
may be deemed acceptable depending on the size of the Maryland Department of Natural Resources, 1992. Shore Erosion
Control Guidelines for Waterfront Property Owners. Annapolis:
project and the value of the property to be protected. Maryland Department of Natural Resources, Water Resources
Administration.
Conclusion National Research Council (NRC), 2007. Mitigating Shore Erosion
along Sheltered Coasts. Washington, DC: The National Acade-
Ongoing development along the world’s estuaries has made mies Press.
shore protection necessary for erosion abatement and the New York Sea Grant, 1984. Analysis, Design and Construction of
protection of costly infrastructure. The decision to protect Coastal Structure. New York: Geotechnical Engineering Group,
estuarine coasts comes down to desired level of protection Cornell University, for New York Sea Grant Institute, New York
and costs. Shoreline protection using defensive systems or Sea Grant, Stony Brook.
Nordstrom, K. F., 1992. Estuarine Beaches. London: Elsevier.
hardening the coast is a proven commodity. Likewise the Pile Buck, 1990. Coastal Construction. Jupiter, FL: Pile Buck,
widespread use of offensive systems to create beaches and Incorporated.
marshes has shown their ability to provide shore protection, Rogers, S., 2005. Complexities in Evaluating the Impact of Estua-
if done properly, and should be considered from an estua- rine Erosion Management Alternatives. Washington, DC:
rine habitat perspective. Over the long term, the broader National Research Council. Presentation to the NRC Committee
coastal profile delivered by these systems may allow habitat on Mitigating Shore Erosion along Sheltered Coasts, Washing-
ton, DC 2005.
transition where tides continue to rise. Rogers, S., and Skrabel, T. E., 2001. Managing Erosion
on Estuarine Shorelines. Raleigh, NC: North Carolina Sea
Grant.
Bibliography Singewald, J. T., Jr., and Slaughter, T. H., 1949. Shore Erosion in
Byrne, R. J., and Anderson, G. L., 1978. Shoreline erosion in tide- Tidewater Maryland. Baltimore: Bulletin 6, Dept. of Geology,
water Virginia. Special Report in Applied Marine Science and Mines and Water Resources.
Ocean Engineering No. 111. Virginia Institute of Marine Sci- U.S. Army Corps of Engineers (USACE), 1973. Chesapeake Bay
ence, College of William and Mary, Gloucester Point, VA. existing conditions report. In The Bay Processes and Resources.
Chasten, M. A., Rosati, J. D., and McCormick, J. W., 1993. Engi- Baltimore District: U.S. Army Corps of Engineers.
neering design guidance for detached breakwaters as shore sta- U.S. Army Corps of Engineers (USACE), 1984. Shore Protection
bilization structures. Tech. Report CERC-93-19, U.S. Army Manual, 4th edn. Washington, DC: U.S. Army Corps of
Corps of Engineers, Waterways Experiment Station, MS Engineers.
Eurosion, 2004. Living With Coastal Erosion in Europe: Sediment U.S. Army Corps of Engineers (USACE), 1981. Low Cost Shore
and Space for Sustainability. Part IV-A Guide to Coastal Erosion Protection: A Guide for Engineers and Contractors. Monroe-
Management Practices in Europe: Lessons Learned. Hague, The ville: GAI Consultants.
Netherlands: Eurosion, Directorate General Environment Euro- U.S. Army Corps of Engineers (USACE), 2000. Coastal Engineer-
pean Commission. ing Manual. Vicksburg: U.S. Army Corps of Engineers, Coastal
Hardaway, Jr., C. S., and Byrne, R. J., 1999. Shoreline management Hydraulics Laboratory.
in Chesapeake Bay. Special Report in Applied Marine Science Virginia Marine Resource Commission, 1989. Shoreline Develop-
and Ocean Engineering No. 356. Virginia Institute of Marine ment BMP’s. Newport News: Virginia Marine Resource
Science, College of William & Mary, Gloucester Point, VA. Commission.
Hardaway, C. S., Jr., and Gunn, J. R., 2010. Design and perfor- Ward, L. G., Rosen, P. S., Neal, W. J., Pilkey, O. H., Jr., Pilkey,
mance of headland bays in Chesapeake Bay, USA. Coastal Engi- O. H., Sr., Anderson, G. L., and Howie, S. J., 1989. Living with
neering, 57, 203–212. Chesapeake Bay and Virginia’s Ocean Shores. Durham: Duke
Hardaway, C. S., Jr., and Gunn, J. R., 2011. A brief history of head- University Press.
land breakwaters for shore protection in Chesapeake Bay, USA.
Shore and Beach, 79(1), 26–34.
Hardaway, C. S., Jr., Thomas, G. R., Fowler, B. K., Hill, C. L., Frye, Cross-references
J. E., and Ibison, N. A., 1985. Results of the vegetative erosion Bulkheads
control project in the Virginia Chesapeake Bay System. In Webb, Coastal Erosion Control
F. J., Jr. (ed.), Proceedings Conference on Wetlands Restoration Headland Breakwater
and Creation. Tampap: Hillsborough Community College, p. 144. Revetments
 SHOREBIRDS 585

a month. They have low fecundity, delayed maturity, and

SHOREBIRDS high annual survival. In most species, the young are preco-
cial, meaning that they are covered with down upon hatch-
Joanna Burger ing, and once they dry off, they are able to move about and
Department of Cell Biology and Neurosciences, find their own food, although their parents still guard them
Ecology, Evolution and Natural Resources, Department of until they fledge. The length of the breeding cycle depends
Marine and Coastal Sciences, Environmental and somewhat upon where they breed. Species that breed in
Occupational Health Sciences Institute, Rutgers harsh Arctic environments must have a short period to
University, Piscataway, NJ, USA breed and usually make only one breeding attempt. Spe-
cies that breed in temperate regions have an extended
Synonyms breeding period and can attempt second broods.
Sandpipers; Waders Shorebirds exhibit a range of breeding patterns, from
monogamy to polygyny (one male with more than one
Definitions female mate) and polyandry (one female with more than
Shorebirds are small- to medium-sized birds that often one male mate; phalaropes). In some species, all three
feed along shorelines, and they breed in a wide range of patterns can be exhibited, depending upon food resources.
habitats near water. They include snipes, godwits, stints, Most shorebirds are territorial when breeding and gregar-
sandpipers, phalaropes, jacanas, thick-knees, oyster- ious when not breeding, often forming large flocks to for-
catchers, avocets, plovers, and lapwings. age, migrate, and overwinter. Shorebirds have some of the
Sandpipers are a group of small familiar shorebirds that longest migration routes of all birds, with some species
often move along the shoreline, foraging by running in breeding in the Arctic and wintering at the southern tip
and out of the tide. of South America.
Waders, another name for shorebirds that is used in
Europe, are also called short-legged waders (herons and Behavior and habitat selection
egrets are long-legged waders). Breeding
Most shorebirds breed solitarily, although a very few nest
Introduction
in small colonies (e.g., stilts). While nesting, shorebirds
Shorebirds are small- to medium-sized birds that frequently rely on their cryptic coloration and hide their nests to
feed along shorelines, often following the waves in and out, avoid mammalian predators. They usually nest on the
picking up prey from the sand. They include snipes, god- ground, in the open sand, under vegetation, on rocky
wits, stints, sandpipers, phalaropes, jacanas, thick-knees, beaches, or in wet swales from the Arctic to Antarctica,
oystercatchers, avocets, plovers, and lapwings. Shorebirds although some species nest in wet marshes (e.g., snipes),
are in 13 families of birds, usually considered in the order build floating nests in marshes (e.g., jacanas), or nest in
Charadriiformes, which also contains gulls, terns, skim- sand or salt flats. A very few species, such as the Solitary
mers, and auks (Warnock et al., 2001). Although “shore” Sandpiper, Tringa solitaria, nest in trees.
is part of their name, they are not generally considered Many species are monogamous, maintaining the same
marine birds or seabirds because they do not spend mate from year to year, and both members of the pair
a significant part of their life cycle at sea (Burger, 1984). incubate and care for young. At least 25 species are
Phalaropes are the exception; they breed in the prairie pot- polygynous, including some sandpipers, snipes, and
hole region of North American and in the Arctic, migrate woodcocks. In a few species, such as Ruff (Philomachus
to the open ocean, and generally overwinter there before pugnax; Van Rhjn, 1991), males gather at a lek (small dis-
they return to their breeding grounds. Other shorebirds do play and dancing ground) and display to females. The
not wander far offshore, except to pass over the ocean dur- females then go off and breed on their own, incubating
ing migration. However, 58 % of shorebirds use marine without the help of a male. Polyandry occurs in fewer spe-
habitats at some time in their life cycle, and 39 % some- cies, perhaps because it is more difficult for the female to
times or always nest along the coast (Burger, 1984). lay a complete clutch for different males than it is for
Because shorebirds travel over large regions of the world a male to fertilize the clutches of several females.
during their annual cycle, use a full range of habitats in Typically, shorebirds make a nest scrap in the ground and
many biomes and climate zones, and nest in many different line it with pebbles, shells, grass, or leaves. They lay 1–4
habitats, they are ideal sentinels or bioindicators of global eggs that are pyriform (one end large and rounded, the other
environmental change (Piersma and Lindstrom, 2004). small and V shaped) which allows the four eggs to fit nicely
together with the four small ends toward the center. Clutch
Life history size is a result of their ability to produce the clutch, incuba-
Shorebirds have intermediate life spans, breed when they tion limitations, and limitations on caring for the young,
are 2–4 years old, usually lay four eggs, have intermediate such as time constraints on high Arctic species because of
incubation periods (3–4 weeks), raise one or more chicks the short Arctic summer (Colwell, 2010). Both parents
a year, and have short parental care periods of less than incubate, and when the chicks hatch, they leave the nest
586 SHOREBIRDS

with the parents within a very few days. The parents lead half of the shorebird species migrate. For these species,
the chicks to good foraging areas, often along the shore, their strategy is to nest in regions with abundant food sup-
in muddy swales, or in pools and ponds. Both incubation plies during the breeding season, to leave when food is no
periods and fledging periods (the time parents continue to longer plentiful, and to migrate to regions with abundant
guard the chicks, prior to their being able to fly away) relate food for the winter. They usually follow coastlines or go
to body size. In larger shorebirds, the incubation period and over the oceans (Morrison, 1984; Warnock et al., 2001).
fledging period are longer. Except for phalaropes, shorebirds rarely touch the ocean
Shorebirds are notorious for drawing predators from surface during migration, and many species spend signifi-
their nests with a series of distraction displays that involve cant time flying over the oceans. Migration is generally
feigning injury, a broken wing, or an inability to fly, only associated with wind patterns, and overwater routes provide
to fly away when the predator is drawn sufficiently far energetic savings compared to following the coast. In the
from the nest. Even some shorebirds that nest in loose col- advent of light-sensitive global location sensors, it has been
onies (e.g., stilts) will use distraction displays to entice possible to document long transoceanic migrations of sev-
predators to leave the area (Gochfeld, 1984). eral species, including nonstop flights of 7,600 km in
Ruddy Turnstone (Arenaria interpres; Minton et al.,
Foraging 2011). A round-trip migration flight of 26,700 km was
reported in Red Knots (Calidris canutus rufa), with contin-
Shorebirds breed on land and usually winter along the
uous 6-day flights of 8,000 km (Niles et al., 2010, Figure 1).
coasts, foraging in estuaries, bays, and along the tide line,
but they also make use of saltmarshes, agricultural land,
and other upland habitats for roosting or foraging at high Shorebird populations
tide (Evans-Ogden et al., 2007). They usually locate their Estimates of shorebird populations are difficult to deter-
prey visually by plucking it from the water, ground, or mine because most nest solitarily or in small breeding col-
other surface or by probing in the mud. Morphology onies and are usually scattered throughout the available
affects foraging methods. Shorebird species with larger habitat. Estimates can be determined by counting birds
bills can eat larger prey items, whereas those with longer along transects in suitable, known habitats, and by esti-
bills can probe deeper in the sand or mud. Those with mating populations on the wintering grounds using aerial
longer legs can forage in deeper water (Durell, 2000). counts (Morrison, 2006). Survival rates can be determined
On land and along the shore, shorebirds are generally with mark-recapture rates, either on the breeding grounds
omnivorous, eating a wide range of foods including or the wintering grounds (Sandercock, 2003). Determin-
insects, snails and clams, worms, and other invertebrates, ing survival, even for marked birds, is difficult because it
although some eat fish, fruit, seeds, and even carrion. is hard separate whether birds survived, simply returned
Seedsnipes eat only plant material, while at the other elsewhere, were undetected, or the marking method failed
extreme, sheathbills will eat carrion and penguin chicks (e.g., bands fell off, batteries died). Understanding popula-
(Warnock et al., 2001). tion trends of shorebirds, however, requires monitoring
Shorebirds are generally solitary during the breeding over long periods, which has been done for many species
season, when they forage solitarily as well. Some will even in North America (Bart and Johnston, 2012) and Australia
defend foraging territories (Burger and Olla, 1984). During (Clemens et al., 2012). An overview of North American
migration, and on the wintering grounds, shorebirds often shorebirds shows declines in 80 % of 35 species with data
form foraging and roosting flocks of hundreds to thousands. (Morrison et al., 2001).
Particularly dense foraging flocks often occur under cir-
cumstances where there is a superabundance of prey that Threats and conservation
is renewed regularly, as occurs with rising tides. Shorebirds
At least 21 % of the world’s shorebirds (32 of 155) are
forage and roost in dense flocks as an anti-predator strategy.
listed as species of conservation concerns by BirdLife
Foraging in flocks has the advantage of enough eyes to pro-
International (Piersma et al., 1997), and this number has
vide early warning of approaching predators, evasive
increased since then. The main threats to shorebird
actions of the flock, and predator swamping (whereby any
populations are habitat loss, human disturbance, commer-
one shorebird has a lower probability of being taken if it
cial harvesting of shorebird prey, hunting, pollution, and
is a flock member, rather than being solitary). Predators,
long-term effects from global warming and sea-level rise
such as hawks, however, can have a negative effect on for-
(Goss-Custard et al., 2000). While harvesting of the birds
aging shorebirds in terms of energy cost and lowered sur-
may not be a problem in much of the developed world,
vival (Goss-Custard et al., 2006), and they can affect both
shorebirds are still harvested in some places, such as South
the timing and the routes of migrating shorebirds
America.
(Lank et al., 2003).
Although Arctic and Antarctic breeding habitat is not
generally threatened (except by climate change, oil devel-
Migration and overwintering opment, sea-level rise), habitat for breeding, foraging, and
Shorebirds breeding in north temperate to Arctic habitats migrating shorebirds along temperate and tropical regions
migrate to warmer climates to overwinter, and well over is threatened by development, human disturbance, and
 SHOREBIRDS 587

Shorebirds, Figure 1 Geolocator output for Red Knot Y0Y: periods when the bird remained in the same location are shown in white;
the great circle distances of movements are shown in yellow. Flight path and stopover location of Red Knot Y0Y. Location key: 1,
Delaware Bay, United States; 2, James Bay, Canada; 3, Western Hudson Bay, Canada; 4, Baker Lake, Canada; 5, Churchill, Canada; 6,
Lesser Antilles; 7, Maranhão, Brazil; 8, Lagoa do Peixe, Brazil; 9, San Antonio Oeste, Argentina; 10, Uruguay-Brazil border; 11, Ocracoke,
North Carolina, United States (after Niles et al. 2010).

contaminants (Borgmann, 2011). Habitat for migrating (Niles et al., 2008). Without sufficient crab eggs, shore-
shorebirds is particularly threatened because of the move- birds do not gain enough weight to survive or breed once
ment of people to coastal regions of the world, and manag- they reach their Arctic breeding grounds (Morrison
ing habitat for both humans and shorebirds will take et al., 2007). Predators can also affect survival and weight
collaboration with a range of community stakeholders gain during migration (Goss-Custard et al., 2006)
(Burger and Niles, 2013). Development in Arctic regions, Global warming and sea-level rise provide a long-term
particularly for oil and gas, also provides a threat to threat to shorebirds because of the potential to render
shorebird nesting habitat (Kendall et al., 2011). breeding, migration, and overwintering habitats
The vulnerability of migrant and overwintering shore- unsuitable. Foraging habitat for shorebirds that feed along
birds is partly threatened by their migratory patterns and coasts is estimated to decrease dramatically, even over the
behavior. Many places serve as massive staging and stop- next few decades (Convertino et al., 2012). Assuming
over points during migration for thousands of shorebirds, a conservative global warming scenario of only 2  C over
including the Copper River Delta in Alaska, Monomoy the next century, Galbraith et al. (2005) predicted that
Refuge on Cape Cod in Massachusetts, and Delaware major intertidal habitat losses for shorebirds in bays in
Bay in New Jersey, as well as the Wadden (see WHSR, Washington, California, Texas, and New Jersey/Delaware
2013). At such sites, shorebirds are vulnerable not only would range from 20 % to 70 %. Sea-level rise, however,
to habitat loss, human disturbance, and predators but also may be even higher than the initially expected 1 m (Pfeffer
to food shortages. For example, the massive migration of et al., 2008). Such massive changes in intertidal areas will
shorebirds through Delaware Bay in the spring is threat- affect the amount of suitable foraging habitat for migrating
ened by depleted horseshoe crab eggs because of declines and wintering shorebirds, as well as those breeding along
in the crabs caused by overharvesting by bait fishermen coasts. Changes in the insect populations brought about by
588 SHOREBIRDS

global change will also decrease prey abundance and coastline fluctuations due to climate change. Ecological
availability (Lindstrom and Agrell, 2012). Processes, 1, 1–17.
Because most species of shorebirds are migratory and Donaldson, G., Hyslop, C., Morrison, R. I. G., Dickson, H. L., and
Davidson, I., 2000. Canadian Shorebird Conservation Plan.
often span large geographic regions of the world, their Ottawa: Canadian Wildlife Service, Environment, Canada.
conservation requires international efforts, such as the Durell, S. E. A. L. D., 2000. Individual feeding specialisation in
Western Hemisphere Shorebird Reserve Network shorebirds: population consequences and conservation implica-
(WHSR, 2013). There are two in-depth conservation plans tions. Biological Reviews, 75, 503–518.
for shorebirds of North America: a US plan (Brown et al., Evans-Ogden, L. J., Bittman, S., and Lank, D. B., 2007. A review of
2001) and a Canadian plan (Donaldson et al., 2000), and agricultural land Use by shorebirds with reference to habitat
conservation in the Fraser River Delta, British Columbia.
both contain a wealth of information on both threats Canadian Journal of Plant Science, 88, 71–83.
and solutions. These involve collaborations among states, Galbraith, H., Jones, R., Park, R., Clough, J., Herod-Julius, S.,
federal governments, international agreements, and Harrington, B., and Page, G., 2005. Global climate change and
treaties. sea level rise: potential losses of intertidal habitat for shorebirds.
Colonial Waterbirds, 25, 173–183.
Summary Gochfeld, M., 1984. Antipredator behavior: aggression and distrac-
tion displays of shorebirds. In Burger, J., and Olla, B. I. (eds.),
Shorebirds, small- to medium-sized birds that frequently Behavior of Marine Animals, Vol. 5: Shorebirds: Breeding
feed along shorelines, include snipes, godwits, stints, Behavior and Populations. New York: Plenum Press,
sandpipers, phalaropes, jacanas, thick-knees, oyster- pp. 243–288.
catchers, avocets, plovers, and lapwings. Most shorebirds Goss-Custard, J. D., Stillman, R. A., West, A. D., McGrorty, S.,
breed solitarily, although a few nest in small colonies. Durell, S. E. A. E. V. D., and Caldow, R. W. G., 2000. The role
of behavioural models in predicting the ecological impact of
They rely on their cryptic coloration and hiding their nests harvesting. In Goslng, L. M., and Sutherland, W. J. (eds.),
to avoid mammalian predation on their nests and eggs. Behaviour and Conservation. Cambridge: Cambridge Univer-
They nest on the ground, in the open sand, under vegeta- sity Press, pp. 65–82.
tion, on rocky beaches, or in wet swales from the Arctic Goss-Custard, J. D., Triplet, P., Sueur, F., and West, A. D., 2006.
to Antarctica. During migration and while overwintering, Critical thresholds of disturbance by people and raptors in forag-
shorebirds form flocks of hundreds to thousands and ing wading birds. Biological Conservation, 127, 88–97.
forage in large groups to exploit coastal prey. Some WHSR (Western Hemisphere Shorebird Reserve Network), 2013.
Western Hemisphere Shorebird Reserve Network List of Sites.
migrate long distances from Arctic breeding grounds to http://www.whsrn.org/site-profile.
the tip of South America. Threats to shorebirds include Kendall, S., Payer, D., Brown, S., and Churchwell, R., 2011.
human disturbance while nesting or foraging on coastal Impacts of climate change and development on shorebirds of
beaches and mudflats; predators, harvesting, habitat loss the Arctic National Wildlife Refuge. In Watson, P. T., Cade,
due to coastal and Arctic development, and sea-level rise. T. J., Fuller, M., Hunt, G. and Potapov, E., (eds.), Gyrfalcons
and Ptarmigan in a Changing World. Boise: The Peregrine
Fund, pp. 1–10. http://dx.doi.org/10.4080.gpcw.2011.0109.
Bibliography Lank, D. B., Butler, R. W., Ireland, J., and Ydenberg, R. C., 2003.
Bart, J., and Johnston, V. (eds.), 2012. Arctic Shorebirds in North Effects of predation danger on migration strategies of sand-
America: A Decade of Monitoring. Berkeley: University of pipers. Oikos, 103, 303–319.
California Press. Lindstrom, A., and Agrell, J., 2012. Global change and possible
Borgmann, K. L., 2011. A Review of Human Disturbance Impacts effects on the migration and reproduction of Arctic-breeding
on Waterbirds. California: Audubon. www.sfbay.org/news- waders. Ecological Bulletin, 47, 145–159.
general.php Minton, C., Gosbell, K., Johns, P., Christie, M., Klaassen, M.,
Brown, S., Hickey, C., Harrington, B., and Gill, R. (eds.), 2001. Hassell, C., Boyle, A., Jessop, R., and Fox, J., 2011. Geolocator
United States Shorebird Conservation Plan, 2nd edn. Manomet: studies on Ruddy Turnstones Arenaria interpres and Greater
Manomet Center for Conservation Science. Sandplover Charadrius leschenaultii in the East Asian-
Burger, J., 1984. Shorebirds as marine animals. In Burger, J., and Australasia flyway reveal widely different migration strategies.
Olla, B. I. (eds.), Behavior of Marine Animals, Vol. 5: Shore- Wader Study Group Bulletin, 118, 87–96.
birds: Breeding Behavior and Populations. New York: Plenum Morrison, R. I. G., 1984. Migration systems of some New World
Press, pp. 17–81. shorebirds. In Burger, J., and Olla, B. I. (eds.), Behavior of
Burger, J., and Niles, L., 2013. Shorebirds and stakeholders: effects Marine Animals, Vol. 5: Shorebirds: Breeding Behavior and
of beach closure and human activities on shorebirds at Populations. New York: Plenum Press, pp. 125–201.
a New Jersey coastal beach. Urban Ecosystems, 16, 657–673. Morrison, R. I. G., Aubry, Y., Butler, R. W., Beyersbergen, G. W.,
Burger, J., and Olla, B. (eds.), 1984. Behavior of Marine Animals, Donaldson, G. M., Gratto-Trevor, C. L., Hicklin, P. W., John-
Vol. 5: Shorebirds: Breeding Behavior and Populations. New ston, V. H., and Ross, R. K., 2001. Declines in North American
York: Plenum Press. shorebird populations. Wader Study Group Bulletin, 94, 34–38.
Clemens, R. S., Kendall, B. E., Guillet, J., and Fuller, R. A., 2012. Morrison, R.I.G, McCaffery, B.J., Gill, R.E., Skagen, S.K., Jones,
Review of Australian shorebird survey data, with notes on their S.L., Page, G.W., Gratto-Trevor, C.L., and Andres, B.A., 2006.
suitability for comprehensive population trend analysis. Stilt, Population estimates of North American shorebirds, 2006.
62, 3–17. Wader Study Group Bull, 111, 66–84.
Colwell, M. A., 2010. Shorebird Ecology, Conservation, and Morrison, R. I. G., Davidson, N. C., and Wilson, J. R., 2007.
Management. Berkeley: University of California Press. Survival of the fattest: body stores on migration and survival of
Convertino, M., Bockelie, A., Kiker, G. A., Munoz-Carpena, R., Red Knots, Calidris canutus islandica. Journal of Avian
and Linkov, I., 2012. Shorebird patches as fingerprints of fractal Biology, 38, 479–487.
 SHORELINE 589

Niles, L. J., Sitters, H. P., Dey, A. D., Atkinson, P. W., Baker, A. J., The shoreline has shifted in the past with changes in sea
Bennett, K. A., Carmona, R., Clark, K. E., Clark, N. A., Espoz, level or crustal movements (crustal uplift or sinking).
C., González, P. M., Harrington, B. A., Hernández, D. E., Kalasz,
K. S., Lathrop, R. G., Matus, R. N., Minton, C. D. T., Morrison,
R. I. G., Peck, M. K., Pitts, W., Robinson, R. A., and Serrano, Description
I. L., 2008. Status of the Red Knot, Calidris canutus rufa, in the Due to the glacial eustatic rise in sea level after the last ice
Western Hemisphere. Studies in Avian Biology, 36, 1–185.
Niles, L.J., Burger, J., Porter, R.R., Dey, A.D., Minton, C.D.T.,
age maximum some 20,000 years ago, the shoreline has
Gonzalez, P.M., Baker, A.J., Fox, J.W., and Gordon, C., 2010. been changing from about
120 m to the present position.
First results using light level geolocators to track Red Knots in Along many coasts, the past shorelines have shifted due to
the Western Hemisphere show rapid and long intercontinental crustal movements. In seismically active areas, crustal
flights and new details of migration pathways. Wader Study dynamics give rise to sequences of uplifted former shore-
Group Bull, 117(2), 123–130. lines, for example, in Japan (Ota, 1986) and New Zealand
Pfeffer, W. T., Harper, J. T., and O’Neel, S., 2008. Kinematic con- (Wellman, 1967). In former glaciated areas such as
straints on glacier contributions to 21st-Century sea-level rise.
Science, 5(321): 1340–1343. Fennoscandia, Scotland, and North America, the glacial
Piersma, T., and Lindstrom, A., 2004. Migrating shorebirds as isostatic process (Jemisson, 1882; De Geer, 1888–1890;
integrative sentinels of global environmental change. Ibis, 146, Hillaire-Marcel and Fairbridge, 1978; Mörner, 1979,
61–69. 1980) has tilted the shorelines from the center of glaciation
Piersma, T., Wiersma, P., and van Gills, J., 1997. The many to the periphery. Bravais (1840) was the first to record
Unknowns about plovers and sandpipers of the world: introduc- such tilted shorelines. Gilbert (1890) recorded isostati-
tion to a wealth of research opportunities highly relevant for
shorebird conservation. Wader Study Group Bulletin, 82, 23–33. cally deformed shorelines at the former Lake Bonneville
Sandercock, B. K., 2003. Estimation of survival rates of wader in America. There are also submarine shorelines, which
populations: a review of mark-recapture methods. Wader Study appear to represent minor stillstands in the postglacial rise
Group Bulletin, 100, 163–174. of sea level (see Carter et al., 1986). Sometimes it is possi-
Van Rhjn, J., 1991. The Ruff. London: T. & A.D. Poyser. ble to isolate the crustal and eustatic components in the
Warnock, N., Elphick, C., and Rubega, M. A., 2001. Shorebirds in the spectra of former shorelines such as for the last intergla-
marine environment. In Schreiber, E. A., and Burger, J. (eds.),
Biology of Marine Birds. New York: CRC Press, pp. 581–615.
cial–glacial cycle of coral reefs in New Guinea
(Chappell et al., 1996) and for the last deglacial phase in
Fennoscandia (Mörner, 1971).
Cross-references
Climate Change Bibliography
Coastal Bays
Coastal Wetlands Bravais, M., 1840. Sur les lignes d-ancien niveau de la mere dans le
Deltas Finmark. Compte Rendu Academie des Sciences de Paris, 10,
Estuarine Beaches 691–693.
Food Chain Carter, R. M., Carter, L., and Johnson, D. P., 1986. Submergent
Food Web/Trophic Dynamics shorelines in the SW Pacific: evidence for an episodic post-
Habitat Loss glacial transgression. Sedimentology, 33(5), 629–649.
Mean Sea Level Chappell, J., Omura, A., Esat, T., McCulloch, M., Pandolfi, J., Ota,
Nonpoint Source Pollution Y., and Pillans, B., 1996. Reconciliation of late Quaternary sea
Oil Pollution levels derived from coral terraces at Huon Peninsula with deep
Seabirds sea oxygen isotope records. Earth and Planetary Science Let-
Shoreline ters, 141, 227–236.
Shoreline Changes De Geer, G., 1888–1890. Om Skandinaviens nivåförändringar
under Qartärperioden. Geologiska Föreningens i Stockholm
Förhandlingar, 10, 366–379 (1888) & ibid, 12, 61–110 (1890).
Gilbert, K. G., 1890. Lake Bonneville. United States Geological
Survey Memoire, 1, 1–438.
SHORELINE Hillaire-Marcell, C., and Fairbridge, R. W., 1978. Isostasy and
eustasy in Hudson Bay. Geology, 6, 117–122.
Jamieson, T. F., 1882. On the cause of the depression and
Nils-Axel Mörner re-elevation of the land during the Glacial Period. Geological
Paleogeophysics and Geodynamics, Saltsjöbaden, Magazine, 9, 400–407.
Sweden Mörner, N. -A., 1971. Eustatic changes during the last 20,000 years
and a method of separating the isostatic and eustatic factors in an
uplifted area. Palaeogeography Palaeoclimatology,
Synonyms Palaeoecology, 9, 153–181.
Coastline Mörner, N. -A., 1979. The Fennoscandian uplift and Late Cenozoic
geodynamics: geological evidence. GeoJournal, 3(3), 287–318.
Definition Mörner, N. -A. (ed.), 1980. Earth Rheology, Isostasy and Eustasy.
A Collective Work of 47 Individual Papers from the Interdisci-
Shoreline is defined as the point or line where the sea plinary Symposium in Stockholm 1977 on “Earth Rheology
intersects the land. Considering tidal variations, it corre- and Late Cenozoic Isostatic Movements.” Geodynamics Project,
sponds to the mean sea-level position on the shore. Scientific Report No. 49. New York: Wiley.
590 SHORELINE CHANGES

Ota, Y., 1986. Marine terraces as reference surfaces in late Quater- Because of the dynamic nature of the shoreline boundary,
nary tectonics studies: examples from the Pacific Rim. Royal the definition must consider the shoreline in both
Society of New Zealand, 24, 357–375. a temporal and spatial sense (Boak and Turner, 2005).
Wellman, H. W., 1967. Tilted marine beach ridges at Cape
Turakirae, N. Z., In Ikebe, N. (ed.), Sea level changes and crustal An idealized definition of shoreline is that it coincides
movements of the Pacific during the Pliocene and post-Pliocene with the physical interface of land and water (Dolan
time. Journal of Geosciences, Vol. 10, pp. 123–129. et al., 1980). In reality, the shoreline position changes con-
tinually through time, because of cross-shore and along-
shore sediment movement in the littoral zone and
Cross-references especially because of the dynamic nature of water levels
Mean Sea Level at the coastal boundary (e.g., waves, tides, storm surge,
Uplifted Coasts setup, run-up, etc.).
The shoreline is the position of the land–water interface
at one instant in time. A shoreline may also be considered
over a slightly longer time scale, such as a tidal cycle,
SHORELINE CHANGES where the horizontal/vertical position of the shoreline
can vary between centimeters and tens of meters
A. C. Narayana (or more), depending on the beach slope, tidal range, and
Centre for Earth & Space Sciences, University of prevailing wave/weather conditions. Over a longer time
Hyderabad, Hyderabad, Andhra Pradesh, India scale, such as 100 years or more, the position of the shore-
line may vary by hundreds of meters or more (Komar,
Synonyms 1998). The shoreline is a time-dependent phenomenon
Coastline changes that may exhibit substantial short-term variability
(Morton, 1991).
The shoreline is a vital part of the coastal zone. The
Definition study of shorelines is very important to understand the
Shorelines change boundary conditions in an estuary or interactions between parts of the hydrosphere, atmo-
ocean due to sea level fluctuations, circulation patterns, sphere, and solid earth. The atmosphere is involved in
waves and tides, and the amount of sediment supply. The transferring the energy from wind to water, thereby caus-
shoreline is quite narrow, linear in extent, and always in ing waves, which in turn generate nearshore currents.
contact with the estuarine waterbody or sea. It is The gravitational attraction of the moon and sun on ocean
a physical interface of land and an estuary or sea and waters is responsible for the rhythmic rise and fall of tides.
defined in both temporal and spatial terms. As a dynamic system, shorelines continuously adjust to
any change that takes place, such as increased wave
Introduction energy or an increase or decrease in sediment supply.
Morphodynamics of a coast can be defined as the “mutual Due to the dynamic nature of the shoreline boundary, it
adjustment of topography and fluid dynamics involving can be used as a proxy to represent the “true” shoreline
sediment transport” (Wright and Thom, 1977) or, alterna- positions. The shoreline positions can be investigated
tively, the “dynamic behaviour of alluvial boundaries” of and classified based on (1) visual observation
fluid motions (De Vriend, 1991). The evolution of coastal (a previous high-tide line or the wet/dry boundary),
systems is the result of morphodynamics that develop in (2) tidal datum-based shoreline indicator (mean high water
response to change in external conditions (Wright and or mean sea level), and (3) application of image
Thom, 1977) and controlled by various factors such as processing techniques to extract proxy shoreline features
morphology, geology, and size of the catchment area; from digital coastal images (Boak and Turner, 2005).
nature of sediments; climate leading to rainfall and river Shoreline position measurements of different time periods
discharge at coastal zone; freshwater input; and coastal can be used to derive quantitative estimates of the rate of
hydrodynamics – waves, tides, and currents (Albert and progradation/retrogradation (Fenster et al., 1993). Human
Jorge, 1998). Many of the regional controls on sea level intervention in coastal regions has modified pristine coast-
involve long-term geological processes (subsidence, isos- lines around the globe by deforestation, cultivation,
tasy) and have a profound influence on controlling short- changes in habitat, urbanization, and upstream obstruc-
term dynamics. As sea levels fluctuate, the morphology tions to river flows. The rate of change in coastal land-
of a coastal zone will further evolve, changing the bound- forms and shoreline position is important in the
ary conditions of other coastal processes, viz., circulation, development of setback planning, hazard zoning, ero-
waves, tides, and deposition of sediments on shorelines. sion/accretion perspectives, sediment budgeting, and con-
The shoreline is a part of coastal land in contact with the ceptual/predictive modeling of coastal morphodynamics
estuary or sea and is continuous around ocean basins. It (Sherman and Bauer, 1993; Chandramohan et al., 1994;
has remarkable linear extent but is usually quite narrow. Al Bakri, 1996; Zuzek et al., 2003; Kumar and Jayappa,
The width of shorelines is defined by tidal flux – the zone 2009; Kumar et al., 2010). The discussion below focuses
between the lowest low tide and the highest high tide. on shoreline processes, erosion and deposition, impact of
 SHORELINE CHANGES 591

sea-level rise, and analysis and predictive methods of a topographical ridge along the landward side of a beach
shoreline changes. which may subsequently be partly submerged. The other
important characteristic feature of paleoshorelines is
Classification of shorelines cheniers, originally described by Howe et al. (1935) as
Depositional and erosional coastlines long, narrow, sandy ridges rising above the surrounding
A sedimentary environment and its sub-environments in marshes and forming the most conspicuous topographic
a coastal environment can be either erosional or deposi- features along the southwestern Louisiana coast. Cheniers
tional. Erosion, progradation, and reworking of sediments are characterized by gently dipping littoral, sublittoral and
are important shore and nearshore processes that modify washover deposits, and some overlying dunes. Cheniers
coastlines. The details are discussed below. and barrier shorelines can be identified by the shape and
extent of their respective deposits. A fundamental differ-
Submergent and emergent coasts ence in the depositional process is the proximity of
a sizeable sediment source which can overload the distrib-
In some coastal areas where uplift occurs, the sea level
utive fores, viz., waves and tidal currents, along the adja-
falls fast relative to the land. Sea level will rise when the
cent shoreline.
coastal region subsides, resulting in a net change in sea
level of as much as 30 cm per century.
Shoreline changes due to physical forcing
Wave-dominated coasts Shoreline processes operate on a narrow zone, and there-
Wave-dominated coasts, comprising accumulations of fore the shoreline migrates landward or seaward
detrital sands, undergo high levels of physical reworking depending on changing sea level or uplift or subsidence
interspersed with periods of burial before finally being of coastal regions. These cyclic and noncyclic processes
deposited as present-day coastal deposits. Waves and change the position of the shoreline over various time
wave-induced currents are the dominant mechanisms for scales, from the daily and seasonal interaction of winds
moving and depositing sands on shorefaces and beaches and waves to over thousands of years due to secular
of the open coast, although winds, river discharge, tidal sea-level changes. During sea-level transgression, for
currents, and Ekman flows act as transporting agents land- example, the shoreline migrates landward and vice versa
ward of the beach in estuaries and seaward of the during sea-level regression. Furthermore, shoreline
shoreface (Roy et al., 1994). In relation to the shoreface changes are not constant through time and frequently
and beach, open coastal types are determined by four reverse in sign, i.e., accretion to erosion, or vice versa.
factors: (1) substrate gradient, (2) wave versus tidal range, Most shorelines undergo patterns of erosion and accretion
(3) sediment supply versus accommodation (Swift and on a daily and seasonal basis and may be unidirectional or
Thorne 1991), and (4) rates of sea-level change. At one cyclic on a long-term basis. In the process, beach and near-
end are steep, high-energy, sediment-deficient coasts, shore sediments are deposited over vast regions. However,
and at the other end are low-gradient, low-energy coasts the physical processes such as tides, waves, and nearshore
(Roy et al., 1994). The behavior of wave-dominated coasts currents are most important for modifying shorelines.
can be explained under two concepts: (1) geological inher-
itance or imprint of various land-forming processes that Tides and tidal currents
have operated for a long period of geological time and In response to the gravitational attraction of the moon and
(2) wave-formed coastal deposits operating over shorter sun, some shorelines experience two almost equal high
periods of time (Roy et al., 1994). tides and two low tides each day, called a semidiurnal tide.
Some locations experience only one high and one low tide
Clastic and carbonate coasts each day (called a diurnal tide), whereas some other loca-
Clastic coasts are characterized by the relative abundance tions experience two uneven tides a day or sometimes
of river materials and reworked by waves and tides. Car- one high and one low each day (called a mixed tide).
bonate coasts are those where calcareous sediment is pro- These regular fluctuations in the estuarine or ocean surface
duced, transported, deposited, lithified, and eroded. They vary from a few centimeters to >15 m. A complete tidal
significantly differ from clastic coasts. cycle includes a flood tide that progressively covers
more and more of a nearshore area until high tide is
Paleoshorelines reached, followed by ebb tide, during which the nearshore
A general history of relative changes in sea level and area is once again exposed. These regular fluctuations in
shoreline migration has been documented on the continen- sea level constitute one largely untapped source of energy
tal shelves of the world oceans. Relic barrier shorelines of as do waves, currents, and temperature differences in
Pleistocene age have been widely reported landward of seawater.
their modern analogues, while submerged Holocene bar- Tidal ranges are also affected by shoreline configura-
rier remnants are common features on modern continental tion. In offshore areas, where the direction of flow is not
shelves. Barrier shorelines are characteristic features of restricted by any barriers, the tidal current is rotary; that
depositional environments. Barriers can originate from is, it flows continuously, with the direction changing
592 SHORELINE CHANGES

Shoreline Changes, Figure 1 Wave dynamics in the nearshore region. Well-developed surf zone and beach are also shown.

through all points of the compass during the tidal period. such it transfers energy in the direction of wave move-
This rotation, caused by the earth’s rotation and unless ment. The diameters of the orbits that water follows in
modified by local conditions, is clockwise in the Northern waves diminish rapidly with depth, and at a depth of about
Hemisphere and counterclockwise in the Southern Hemi- one-half wavelength (L/2), called the wave base, they are
sphere. In estuaries and/or straits, or where the direction of essentially zero. Thus, at a depth exceeding wave base, the
flow is more or less restricted to certain channels, the tidal water and seafloor are unaffected by surface waves
current is reversing; that is, it flows alternately in approx- (Figure 1).
imately opposite directions with an instant or short period When a deepwater wave approaches the shore and
of little or no current (called slack water) at each reversal moves into shallow water, the resulting friction and com-
of the current. During the flow in each direction, the speed pression reduce the forward speed of the wave. Therefore,
varies from zero at the time of slack water to a maximum, when the wave “feels the sea bottom,” it slows down, and
called strength of flood or ebb, about midway between the the accompanying reduction in the wavelength and speed
slacks. Tidal currents generally have little modifying results in increased height and steepness as the wave
effect on shorelines along straight coasts, except in narrow energy is condensed in a smaller water volume. The influ-
passages where tidal current velocity is strong enough to ence of depth on the propagation of waves increases with
erode and transport sediment. It works as one of the continued shoaling until it becomes the dominating factor.
sediment-transporting agents and to prevent the blockage Breakers form in the surf zone because the water
of passageways created by sediment deposition via the particle motion at depth is affected by the bottom. Orbital
action of nearshore currents. motion is slowed and compressed vertically, but the orbit
Several processes such as landslides, earthquakes, and speed of water particles near the crest of the wave will
volcanic eruptions in the oceans generate large waves not slow down appreciably. The particles at the wave crest
and tsunamis that can devastate coastal areas, but most move faster toward the shore than the rest of the wave
natural process activities on shorelines are accomplished form, resulting in the curling of the crest and the eventual
by wind-generated waves, especially storm waves. Waves breaking of the wave. The two most common types of
are directly or indirectly responsible for most erosion, sed- breakers are plungers and spillers. Spilling breakers move
iment transport, and deposition in coastal areas. forward with a foaming turbulent crest, while plunging
breakers form on narrow, steep beach slopes. The more
Shallow-water waves and breakers common spilling breakers are found over wider, flatter
Waves are disturbances that cause energy to be transported beaches, where the energy is extracted more gradually as
through a medium (e.g., air or water); they are defined the wave moves over the shallow bottom. The spilling
with respect to their height, length, frequency, and wave breakers last longer than the plungers, because they lose
period (Figure 1). When wind blows over water, the fric- energy more gradually.
tion generated between the two transfers energy from the
wind to the water causing the water surface to oscillate. Nearshore currents
When waves move across a water surface, the water Wave action in and near the breaker zone carries mass
moves in circular orbits but shows little or no net forward transport of water shoreward as longshore currents, rip
movement. Only the waveform moves forward, and as currents, and the longshore movement of the expanding
 SHORELINE CHANGES 593

heads of rip currents. In the nearshore zone, where the

breaker zone and surf zone are located, the water from
breaking waves rushes forward and then flows seaward
as backwash. Longshore currents are one important aspect
of breaking waves, and these waves have the ability to
generate currents that flow parallel to the shoreline. They
occur because each wave thrusts the water forward when
it breaks. Longshore currents are long and narrow, and
they flow in the same general direction as the approaching
waves. Longshore currents are particularly important in
transporting and depositing sediment in the nearshore
zone. Rivers are one of the major sources of littoral drift
and the annual discharge of sediments to the seas. Along
the Indian coasts alone, the annual sediment discharge to
the ocean is about 1.2 billion tonnes, which accounts
roughly 10 % of the global sediment flux to the world
oceans (Subramanian, 1993).
Monsoons play an important role in shoreline changes
and configurations, particularly along Asia and Southeast Shoreline Changes, Figure 2 Gentle to moderate beach slope
Asian seas. The Indian coast experiences two monsoons – with beach cusps in the background, near Kotepura, west coast
the southwest (summer) monsoon (June–September) and of India (Photo: K.S. Jayappa).
the weaker northeast (winter) monsoon (October–Decem-
ber). During the southwest monsoon, the coastal current is
in a clockwise direction, while during the northeast mon- sectors, and littoral sediment transport direction. Wave
soon, the current is in a counterclockwise direction. As refraction and the resulting longshore currents are the pri-
a result, the longshore current is stronger and toward the mary agents of sediment transport and deposition on
south during the southwest monsoon (Narayana et al., shorelines. Tides also play a role as they rise and fall,
2001). Natural processes such as waves, littoral currents, and the position of wave attack shifts onshore and off-
offshore relief, rainfall, and sea-level changes are respon- shore. Rip currents play no role in shoreline deposition,
sible for erosion/accretion of the coast and subsequent but they do transport fine-grained sediments (silt and clay)
shoreline changes. Surging, spilling, and plunging brea- offshore through the breaker zone. The rate of littoral sed-
kers with wave heights of 2–3 m occur along the iment transport depends on the angle of wave approach,
monsoon-dominated coasts. In the monsoon season, wind wave energy, intensity of longshore currents, and sedi-
energy is much greater, resulting in larger amplitude ment supply. There is a well-established view that the
waves and strong littoral currents. Infragravity and far net littoral transport rate along the beaches ranges from
infragravity edge waves, coupled with strong reflections near zero to 7.65  105 m3/year (Shore Protection Manual,
and undertow, play an important role in the hydrodynam- 1984). Littoral transport determines the areas of coastal
ics of the coastline (Tatavarti et al., 1996). Sediments lying erosion and deposition, and it influences the morphology,
on the nearshore bed are disturbed and eroded during the orientation of coastal landforms, and evolution of the
monsoon season because of the larger and strong waves coast.
and undertow. Therefore, sediments are mobilized into Mudbanks (fluid muds) generally occur on coastlines
suspension and transported. As the low-frequency supplied with large quantities of river-discharged muds
motions are three-dimensional, they carry suspensions lat- which advect and diffuse across and along the shelf and
erally (Tatavarti et al., 1999), thereby resulting in varia- nearshore environment. Coastlines such as Surinam and
tions in shoreline positions. Further, whenever the wind French Guiana are dominated by river-derived suspended
activity is strong, well-formed cusps with spacing of mud along foreshore, shoreface, and shelf. However, the
15–20 m alongshore with vertical amplitudes of about of mudbanks of the southwest coast of India are not associ-
0.5 m are formed (Figure 2). ated with river mouths but occur as discrete features on
otherwise sandy shorelines. These mudbanks are associ-
ated with attenuated nearshore wave energy and high
Deposition along shorelines suspended-sediment concentrations during and for some
Depositional coasts are steady or growing because of their time after the summer monsoon period (June–September).
sediment accumulation rate. Rivers are one of the major Their impact on wave energy minimizes coastal erosion
sources of sediment supply and littoral drift of sediments and provides extensive deposition along the shore in the
in the coastal seas. Net sediment transport direction can mudbank/fluid mud regimes.
be inferred on the basis of accumulation of sediment on Extreme events such as tsunamis also contribute to
the updrift and erosion on the downdrift directions of extensive deposition of sediments on shorelines
breakwaters, shifting of river mouths, length of drift (Narayana et al., 2007).
594 SHORELINE CHANGES

Shoreline Changes, Figure 3 Cliffed and irregular shoreline protected with seawall near Kannur, southwest coast of India (Photo:
Avinash Kumar).

Shoreline erosion units – “soft-rock cliffs” – are exposed to wave action, they
Coastal erosion is generally related to wave energy, shore- have a tendency to be unstable and to rapidly retreat.
line material, coastal topography, and the direction of the Coastal cliffs generally form by undercutting due to marine
approaching waves with respect to the shoreline direction. erosion followed by subsequent collapse of large rocky
The breaking waves and currents in the nearshore zone are boulders (Woodroffe, 2002). Predictions of coastal cliff
responsible for the transport of coastal sediments resulting recession are essential for an appraisal of cliff protection
in shoreline change. The first effect of erosion on a newly options and for coastal land-use planning (Hall et al., 2000).
exposed coast is intensification of the coastline’s irregu- Erosive forces can produce wave-cut shores (Figure 4).
larity (Figure 3). In the long run, shoreline processes tend Wave intensity and the resistance of shoreline materials to
to straighten an initially irregular shoreline. Wave refrac- erosion determine the rate at which a sea cliff or shoreline
tion causes more wave energy to be expended on head- retreats landward. Due to hydraulic action and abrasion at
lands and less on embayments. Thus, headlands erode, their bases, sea cliffs slope abruptly from land into the
and some of the sediment yielded by erosion is deposited ocean, their steepness usually resulting from the collapse
in the embayments. of undercut notches. Thus, sea cliffs retreat gradually
The shoreline changes in India suggest that erosion/ and leave a beveled surface called a wave-cut platform
accretion is cyclic. Beach width reduces between 15 and that slopes gently seaward. Wave-cut platforms above
50 m during June and August because of intense mon- sea level are known as marine terraces.
soonal erosion, whereas beaches attain maximum width Sea cliffs generally retreat irregularly because some of
during February–April because of accretion along the the constituent material is more resistant to erosion than
coasts like Western India. The intensive tropical mon- other material. Headlands are seaward-projecting areas
soons cause large-scale shoreline erosion. However, the of the shoreline, eroded on both sides by wave refraction
sand that was lost during the monsoon is regained and (Figure 5).
accreted during the post-monsoon periods.
Erosional coasts are those in which the dominant pro- Shoreline changes due to natural processes
cesses remove coastal material. The nature of beach mate- Sandy shorelines are generally dynamic and exhibit tem-
rial plays an important role in modifying the poral and spatial changes. Sediment supply, littoral drift,
characteristics of incoming waves. This is a matter of prime and secular sea-level changes are the main factors that
concern with regard to the character of the beach because influence shoreline changes and the formation of different
the resistance of beach sands to erosive forces depends on coastal landforms, while river flow and wave breakers
particle size. Erosion creates steep or vertical slopes known play a significant role in shaping and orientating them
as sea cliffs. Globally, about 80 % of the open coast (Kunte and Wagle, 1991; Narayana and Priju, 2006).
is backed by sea cliffs (Bird, 2000). Wherever the The shoreline configuration is influenced by an acceler-
rocky shores containing weakly resistant sedimentary ated or decelerated accretion of sediment. Accelerated
 SHORELINE CHANGES 595

Shoreline Changes, Figure 4 Cliffed shoreline cut by monsoonal waves.

Shoreline Changes, Figure 5 Irregularly retreating shoreline. Headland of the shoreline projecting seaward is eroded on both sides
by wave refraction.

accretion or decelerated erosion results from greater sedi- mechanisms determine the areas of coastal erosion and
ment deposition, whereas decelerated accretion or acceler- accumulation, factors such as intensity of monsoons influ-
ated erosion suggests greater sediment transport (Morton, ence the erosion and accretion patterns along the west
1979). Deposition and erosion of beach sediments depend coast of India (Narayana et al., 2001). Therefore, it is
on shoreline configuration, source and sink of sediment, important to understand how sediments from various
and the hydrodynamics of the nearshore region. Although sources on the beaches are reworked and redistributed by
the overall direction of sediment transport and its the nearshore hydrodynamic processes.
596 SHORELINE CHANGES

Shoreline Changes, Figure 6 Wave-cut notches and erosion on the downdrift of a river mouth in Karnataka, India (Photo:
K.S. Jayappa).

A strong relationship was reported between the vari- For a cliff of intermediate height, wave-cut notches neither
ability of rainfall and sediment transport, where high sed- reach the cliff edge nor induce collapse as rapidly as in
iment discharges are recorded with high rainfall (Syvitski a high cliff, and the metastable profile can be maintained
and Morehead, 1999). Further, intensive monsoons make longer. Various mechanisms leading to detachment of
the sea rough, with high wave activity, and erosion of materials from the parent rock include mass movement,
the sediment along the coast, resulting in change of the seepage erosion, surface erosion (rain flash and wind ero-
shoreline configuration. In summary, intensive monsoon sion), and wave attack (abrasion, hydraulic action, and
rainfall and sediment derived from inland areas influence fluid shear by uprushing waves during large storms)
the configuration and position of the shoreline. The ocean- (Sunamura, 1991). The presence of cap rocks facilitates
ographic regime is dominated by meteorological forcing multiple rotational slides (Bromhead, 1979) that charac-
rather than tidal forcing along the Asian coasts, where teristically produce high-magnitude but low-frequency
tidal range is 1 m. If the accumulation of sediment at recession events (Brunsden and Jones, 1980). Groundwa-
the updrift arm completely balances the erosion at the ter reservoirs confined to permeable strata that overlie or
downdrift end, a “straight” inlet develops. A curved spit, interbed with impermeable units produce seepage erosion
projecting toward the inlet, suggests that spits display at the cliff face (Hutchinson et al., 1981) and also facilitate
marked changes in form and alignment in response to rel- major mass movements (Denness, 1971).
ative sea-level variations, sediment supply, and wave cli-
mate (Firth et al., 1995).
Recession of cliffed coasts is the cumulative result of Impact of sea-level rise (SLR)
a number of interacting forces and activities. It can be Sea-level rise during the late Pleistocene and Holocene
measured or estimated from identified, common, or analo- dramatically altered the physiography of the coastlines
gous cliff features and sequentially plotted over the lon- around the world. With the onset of sea-level rise, around
gest possible time periods (Malcolm and Janet, 1997). 18 ka, the coastlines began to migrate landward
When waves attack a permeable cliff base, notches of var- (Vanderburgh et al., 2010). The high rate of sea-level rise
ious shapes develop depending upon the wave conditions during early to mid-Holocene time, high sediment dis-
(Figure 6). Formation of wave-cut notches on the lower charge, and wave energy regime favored the preservation
part of cliff faces leads to collapse of the upper part of of transgressive depositional sequences. A reduction in
the entire cliff and rapid retreat. In the case of a high cliff, the rate of sea-level rise 3 ka, with a subsequent stabili-
the rate of retreat is even greater because the overlying zation of sea level 2.4 ka, resulted in a change from
weight exerts pressure on the roof of the notches. transgression to regression in most coastal regions of the
 SHORELINE CHANGES 597

world, as reported by Vanderburgh et al. (2010) for Shoreline changes due to anthropogenic activities
Columbia River littoral zone. Anthropogenic activities such as construction of coastal
Sea-level rise (SLR) today has been largely attributed to structures (harbors, breakwaters, seawalls, and vented
global warming. Global warming has added water to the dams across rivers), mining of sand and shells, and urban-
oceans by melting ice in the polar regions, but the greater ization and industrialization mainly affect the shoreline
contributor is thought to be thermal expansion of the configuration and coastal morphology. Bulkhead- and
oceans, a rise in sea level due to increasing water temper- revetment-type seawalls have been built along the eroding
ature. It has vulnerable and direct impact on coastal com- shores. Sandbags and gabions have also been used in these
munities (10 % of population lives within an elevation erosion-prone areas (Figure 7). However, they can induce
of 10 m above mean sea level) (McGranahan et al., severe erosion in several locations along the coast and
2007). Although sea level has been rising since the end increase the beach slope in front of the seawall as well as
of the last glaciation (nearly 11,000 years), the rate of on adjacent beaches. Seawalls often produce rubble along
sea-level rise has increased over the past 200 years as aver- the coast and degrade recreational beaches with episodic
age temperatures have increased. Sea level has risen damages. Groin trap littoral drift resulting in the accumu-
10–25 cm in the past 100 years, and it is predicted to rise lation of sediment on the updrift side and erosion on the
another 50 cm over the next century. Tide gauge data indi- downdrift side. A large quantity of sediment is arrested
cate that the global sea level has risen, on average, by by these structures, causing a deficit in sediment supply
1.5–2.0 mm/year in the last century (Miller and Douglas, along the coast.
2004), and since 1993, the rate has increased to 3 mm/year Extensive mining of sand and lime shells in the estuar-
(Church and White, 2006). There are large regional varia- ies and river mouths also leads to accelerated erosion
tions of sea level (Cazenave and Llovel, 2010). Since along the coastline. Reduction or loss in supply of sedi-
2003, the mean rate of global sea-level rise has declined ment affects the dynamic equilibrium of beaches and has-
to 2.5 0.4 mm/year (Ablain et al., 2009). tens erosion. In the last few decades, rapid urbanization
The absolute rate of sea-level rise in a region is mainly and industrialization such as construction of houses,
due to two factors: (1) the increase in volume of water in fish-processing units, major oil refineries, etc., have accel-
the ocean basins as a result of increasing glacial ice melt- erated changes in the coastal region. Tourism and recrea-
ing and (2) the thermal expansion of near-surface seawater tion are other human activities that often damage natural
(Milne et al., 2009; Stammer et al., 2013). Several studies vegetation, which can enhance coastal erosion and change
indicate that eustatic sea level will continue to rise because shoreline configuration.
of global warming caused by increasing concentrations of
greenhouse gases in the atmosphere. More locally other
processes must be considered including vertical land Shoreline-change analysis and predictions
motions such as subsidence or uplift due to tectonic and methods
volcanic activity, subsidence due to sediment loading, Research on coastal changes provides important environ-
ground water pumping, and oil and gas extraction mental indicators for coastal management (Welch et al.,
(Woppelmann et al., 2007). 1992; Stokkom et al., 1993). Coastal mapping methods
The IPCC Report suggests that the global sea level will are valuable tools to understand shoreline changes. Shore-
be 600 mm by 2100 AD. This means the annual rate of line mapping provides critical shoreline data for models
increase would be 6.45 mm. Furthermore, if the ice caps used to represent shorelines in the geographic database.
continue to melt, there could be a 1 m rise of eustatic sea Shoreline-change analysis methods can be applied. By
level by the end of the twenty-first century (Pfeffer et al., knowing the data acquisition method, the inherent errors
2008). Sea-level rise has a direct impact on shoreline that normally exist in the underlying measurement pro-
changes due to a higher shift in the zone of wave action cesses can be identified and modeled. Also, by knowing
on the beach. This would lead to shoreline recession, the models used to represent shorelines in the geographic
which will be larger on gentler slopes. Bruun (1962) devel- database, the level of abstraction of the real world inherent
oped a model which estimates shoreline recession with in these models can be recognized. This directly influ-
respect to rise in sea level. The effect of sea-level rise will ences shoreline-change analysis results.
be manifested by greater erosion of beaches and bluffs, The most effective and economic instruments used in
increased flooding, inundation of low-lying areas, intru- shoreline mapping and shoreline-change monitoring are
sion of salt water into aquifers, and higher water tables satellite sensors, Global Positioning Systems (GPS), and
(Gornitz, 1991; Nicholls and Leatherman, 1995). all-weather sensors (Li et al., 2001). Shorelines can be
Sea-level rise will strongly impact most coastal landforms extracted from the stereo-matched and geo-referenced
(e.g., beaches, lagoons, estuaries, deltas, coral reefs, man- aerial photographs both manually and automatically. The
groves, etc.), but the impacts would be spatially variable manual extraction of shoreline features is a process that
depending on local factors. Low-lying areas of developing involves digitizing the water and land interface, which is
countries are likely to be the most greatly impacted known as the instantaneous shoreline at the time of aerial
(Nicholls et al., 2007). photography. The automatic shoreline extraction process
598 SHORELINE CHANGES

Shoreline Changes, Figure 7 Fury of the monsoon waves slashing the coastline with intensive erosion. Surf zone between the wave
break and shoreline is well developed. Seawall is built as a protective feature (Photo: K.S. Jayappa).

involves the classification of the gray values in the (Owens, 1985; Deepika et al., 2013). This method makes
processed aerial photographs to obtain the water and land use of successive shoreline data available over time, which
interface. Shoreline-change studies using remote sensing enables assessment of future shoreline changes by
techniques are highly accurate and cost-effective. reviewing the spatiotemporal changes of the shoreline.
Satellite-imaging systems have increasingly improved
image resolution; the new generation of high-resolution
satellite imagery, such as QuickBird and IKONOS which Shoreline-change rate calculation methods
has a resolution of 1 m with stereo imaging capability, pro- Due to the shifting of shoreline position and human influ-
vides an example (Fritz, 1996; Li, 1998). An investigation ences on coastal processes and sediment sources, it is crit-
of shoreline mapping using such high-resolution satellite ical to determine whether the long- or short-term rates of
images demonstrates a promising mapping accuracy of shoreline change reflect present-day shoreline dynamics.
2 m and a great reduction in the number of ground control This analysis may be complicated in areas that exhibit
points required (Li et al., 2001). trend reversals (erosion to accretion, and vice versa), or
where human activities, such as revetment construction,
have affected sediment sources and altered shoreline pro-
Shoreline-change analysis cesses. An understanding and proper application of
Shoreline change can be accurately evaluated by short-term shoreline changes and long-term data are criti-
subdividing the shoreline into smaller segments by creating cal components for effective shoreline management. Pro-
transects at right angles to a master shoreline. Shoreline fessional judgment and knowledge of natural and human
changes along the transects can be computed and further impacts are essential in determining whether the long- or
used to predict future shoreline changes (Carter, 1986; short-term data should be used for management purposes
Kumar et al., 2010). This method has been adopted over particularly in areas that exhibit significant or frequent
the years to establish the correspondence between shoreline shoreline trend reversals or areas that have been exten-
models acquired at different times to predict shoreline sively altered by human activities.
change (Fenster et al., 1993; Maiti and Bhattacharya, The shoreline-change calculation and prediction tech-
2009; Kumar et al., 2010). Rates of change are then niques allow the stability of a long-term trend relative to
employed to summarize historical shoreline movements intermediate (>50 years) and short-term (decennial) trend,
and to predict future positions based on the perceived his- thereby relating the past with the expected future shoreline
torical trends. The method commonly used especially by positions. This section describes the various statistical
coastal land planners and managers to predict future shore- methods used to calculate shoreline-change data, as well
line changes is an extrapolation of a constant rate of change as the methodology used to generate the baseline and
 SHORELINE CHANGES 599

transect locations. Various methods of determining shore- Linear regression (LR)

line rates of change have been described by Dolan A linear regression rate-of-change statistic can be deter-
et al. (1991) and Kumar et al. (2010), which are widely mined by fitting a least squares regression line to all shore-
considered the definitive works on the subject. All line points for a particular transect. LR is the most reliable
methods used for calculating shoreline rates of change method to predict future shoreline positions and their asso-
involve measuring the differences between shoreline posi- ciated confidence intervals, if measurement errors and
tions through time. Rates of shoreline change are a linear trend of erosion were the only determining factors
expressed in terms of distance of change per year. over the longest possible period of shoreline position
Methods, such as end-point rate, average of rates, linear (Crowell et al., 1997; Douglas and Crowell, 2000). Linear
regression, jackknife, and average of eras rates, are regression can reveal if an association exists, and in partic-
adapted in estimation of shoreline changes. ular (via the r value) what fraction of the variance of the
dependent variable (shoreline position) is attributable to
the independent variable (time). This method uses all the
End-point rate (EPR) available data from many data sets to find a line, which
End-point rate can be calculated by dividing from the two has the overall minimum of the squared distance to the
end points – the earliest and latest positions – the distance known shoreline.
of shoreline movement by the time elapsed between the To calculate the rate of change and to predict the future
earliest and latest measurements at each transect. The shoreline position using linear regression, the oldest
future shoreline position for a given date can be estimated shoreline position is chosen as a baseline or zero (0) posi-
using the rate and intercept (Fenster et al., 1993): tion to measure the amount of shoreline shift (Maiti and
Bhattacharya, 2009; Kumar et al., 2010). With reference
Y ¼ mX þ B ð1Þ
to this baseline, progradation of the shoreline is consid-
where Y denotes the shoreline position, X the date, B the ered as a positive value, and recession is considered as
intercept, and m the rate of shoreline movement. Given a negative value. The change in shoreline position rate is
shoreline data sets, numbered in ascending order by date, calculated by the linear regression equation y ¼ a + bt,
the EPR intercept is where y is the shoreline shift during the year t, with
y ¼ 0 for t ¼ X. The regression coefficient (b) represents
BEPR ¼ Yn
mEPR  Xn ð2Þ shoreline-change rate, and the correlation coefficient (r) is
a measure of goodness of fit of the equation to the present
data. Based on the number of samples, the statistical
significance is to be considered at the 80 % level of
Average of rates (AOR)
confidence (if number of samples is small) and 95 % con-
This method involves calculating separate end-point rates
fidence level in the case of large numbers of samples as
for all combinations of shorelines when more than two are
suggested by Allan et al., (2003).
available and can be extended to incorporate the accuracy
of the shoreline position data and the magnitude of the rate
of change. Foster and Savage (1989) developed an equa- Jackknife
tion to determine if any given EPR meets a minimum time The jackknife method is used as an iterative linear regres-
criterion (Tmin): sion that calculates a linear regression fit to shoreline data
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi points with all possible combinations of shoreline points,
ðE1 Þ2 þ ðE2 Þ2 leaving out one point in each iteration. The slopes of the
Tmin ¼ ð3Þ linear regression lines are averaged to yield the jackknife
R1 rate. The advantages of the jackknife are similar to linear
where E1 and E2 are the measurement errors in the first and regression; the jackknife is also less influenced by outliers
second point, respectively, and R1 is the EPR of the lon- of data clusters. The main disadvantage of the jackknife is
gest time span for a particular transect. a lack of increased statistical value given the typically
The advantages of using AOR are that all the EPRs that small numbers of shoreline data points used to derive
survive the minimum time span equation are used and it a shoreline rate of change. The statistical utility of the
allows calculation of the time-dependent variance from jackknife is best realized with an order of magnitude of
the average of rates. The two primary disadvantages of (more) data points.
using AOR to compute long-term trend are as follows:
(1) there is no computational norm for modeling the Summary
minimum time span equation, and (2) the results are Shoreline, a part of coastal land in contact with estuarine
sensitive to the choice of values selected to represent the or ocean waters, undergoes various morphodynamical
measurement errors (Dolan et al., 1991). Foster and changes. The shorelines migrate landward or seaward
Savage (1989) do not recommend AOR as a general depending on changing sea level or uplift or subsidence
computational method, but it can be used as a method of of coastal regions. The cyclic and noncyclic processes
verification in combination with EPR and LR. change the position of shorelines over time scales, from
600 SHORELINE CHANGES

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602 SHORELINE UNDULATIONS

Cross-references description of the nearshore hydrodynamics is required

Anthropogenic Impacts for modeling shoreline undulations.
Beach Management
Beach Processes Bibliography
Estuarine Beaches
Shoreline Kaergaard, K., and Fredsoe, J., 2013. Numerical modeling of shore-
line undulations part 2: varying wave climate and comparison
with observations. Coastal Engineering, 75, 77–90.
López-Ruiz, A., Ortega-Sánchez, M., Baquerizo, A., and Losada,
M. A., 2012. Short- and medium-term evolution of shoreline
SHORELINE UNDULATIONS undulations on curvilinear coasts. Geomorphology, 159–160,
189–200.
Miguel Ortega-Sánchez, Alejandro López-Ruiz,
Asunción Baquerizo Azofra and Cross-references
Miguel A. Losada Rodríguez Coastal Barriers
Andalusian Institute for Earth System Research, Coastal Landforms
University of Granada, Granada, Spain Sand Ridge
Sediment Transport
Shoreline
Synonyms Spit
Shoreline sand waves

Definition
Shoreline undulations are medium to large spatial scale SIDE-SCAN SONAR IMAGING OF SEDIMENT
shoreline geomorphic features with longshore dimensions BEDLOAD
ranging from hundreds to thousands of meters and ampli-
tudes from tens to hundreds of meters.
Juan A. Morales and Irene Delgado
Department of Geology, Excellence Campus of Marine
Description Sciences (CEIMAR), University of Huelva,
Shoreline undulations are generally classified under rhyth- Huelva, Spain
mic coastline features, although many examples are nei-
ther periodic nor regularly spaced (López-Ruiz et al., Synonyms
2012). They can be episodically or persistently found Side-imaging sonar of bottom sediments; Side-scan sonar
along many shorelines all over the world, including regu- of bed morphology; Side-scan sonar of sedimentary beds
lar rectilinear or slightly curved beaches and river mouths
and estuaries. They are also frequently found associated
with sudden changes in the orientation of the coast such Definition
as at spits (Kaergaard and Fredsoe, 2013) and near human Side-scan sonar is a type of acoustic sonar system used to
infrastructures. Many authors refer to these features as obtain clear images of the surface of underwater floors.
shoreline sand waves, but the latter are generally consid- The system works by means of a beam of acoustic pulses
ered rhythmically spaced and migrating alongshore. It is which open vertically as a fan shape to both sides of the
generally assumed that, when an undulation is present at ship. Each acoustic pulse reflects on the floor and returns
the shoreline, the bathymetry is parallel to this undulation to the sensor. The intensity of the received reflected pulse
down to a certain depth. depends on the nature and morphology of the bottom.
The mechanism(s) behind the formation of shoreline The conjunction of all the received pulses can build an
undulations is still not well understood. The main working accurate image of the bed.
hypothesis in recent years is that coastlines with a wave Sediment bedload refers to coarse sedimentary particles
climate dominated by very oblique incidence commonly like sand and gravel that are transported very close to the
feature large-scale undulations, suggesting that this insta- bed because the relationship between the grain size and
bility mechanism could be mainly responsible for the for- flow velocity is near the transport threshold.
mation of regularly spaced shoreline undulations. Recent
advances reveal that the variation of alongshore sediment Introduction
transport with the angle formed by the wave crests and the Side-scan sonar imaging is a geophysical method used in
coastline, as well as the surf zone width, plays a major role marine engineering, underwater archaeology, and military
in the development of shoreline undulations. This process applications (mining detection). It is also used for
is important at estuarine and river mouths, littoral spits, sedimentological interpretations of bed configurations
and curved coastlines and close to human interventions (Wright et al., 1987; Anthony and Leth, 2002; Kuijpers
(e.g., jetties) where the nearshore wave regimen is inten- et al., 2002; Gómez et al., 2010). In this case, the applica-
sively modified. Whatever the case may be, detailed tion of the acoustic geophysical techniques enables the
 SIDE-SCAN SONAR IMAGING OF SEDIMENT BEDLOAD 603

study of the sedimentary dynamics in estuaries and coastal image of the bed. The intensity of the acoustic response
marine areas. The use of side-scan sonar is vital in estua- of the floor depends on the nature (reflectivity and texture)
rine research because it allows the rapid scanning of large of the bed material and the orientation that presents the bed
areas of the estuarine floor to yield images or records for surface with respect to the acoustic pulse. The correct
interpretation of the beds. interpretation of the records requires an accurate position-
Side-scan sonar uses acoustic pulses of frequency that ing of the images. For that, the system is normally
ranges from 100 to 1,000 kHz (Blondel, 2009) emitted connected with a GPS, obtaining a geo-referenced
from transductors located in a submerged towfish that is position of each point of the recorded image.
connected by a cable to a surface vessel (Figure 1). The A systematic navigation allows the scanning of wide
acoustic pulses are reflected in the bed and return to underwater surfaces, and the accurate geo-position of
the receptors also located in the towfish (Figure 2). The successive records is the base to build geo-referenced
towfish sends the received information to the vessel unit, photomosaics of the estuarine bed. Images obtained by
which processes the information to be transformed in an side-scan sonar can be as precise as a photograph of the
bed and clearly reveal sedimentary features and bedforms.
Side-scan sonar imaging allows the study of the litho-
logical, bathymetric, and morphological characteristics
of the estuarine floor. It also allows the determination of
the geometry, distribution, dimensions, and orientation
of the fields of bedforms and facilitates the analysis and
characterization of the flow regime. Analysis of the
temporal variations of these characters can significantly
increase the knowledge base on sediment dynamics in
estuaries.

Estuarine bedforms and tidal flow regime

Bedforms are morphological features of sandy or gravelly
beds that are normally rhythmical and that can range in
size from a few centimeters to more than 15 m. Estuarine
beds exhibit abundant bedforms with different morphol-
ogies and sizes dependent on the type of estuary, tidal flow
regime, dimensions of the estuarine channels, and the
Side-Scan Sonar Imaging of Sediment Bedload, Figure 1 Side- location of the study area with respect to the longitudinal
scan sonar equipment. zones of the estuary. The smallest of these bedforms are

Side-Scan Sonar Imaging of Sediment Bedload, Figure 2 Principles of side-scan sonar functioning.
604 SIDE-SCAN SONAR IMAGING OF SEDIMENT BEDLOAD

Side-Scan Sonar Imaging of Sediment Bedload, Side-Scan Sonar Imaging of Sediment Bedload,
Figure 3 Transition from a plane bed located in the channel Figure 5 Large sandwaves with straight crests showing
margin (left) to sinuous-crested megaripples (right). This is an superimposed metric-scale sinuous megaripples. During spring
example of the transition from low flow regime in the channel tides, large sandwaves are developed in the deep estuarine
margins to medium flow regime in the deeper part of the channels. During neap tides, only small bedforms can migrate,
estuarine sand bars. covering the larger sandwaves.

time of action of ebb and flood. As a response to the

reversing tidal current, the sense of asymmetrical forms
can also reverse.
Subaqueous dunes have been described by Duck
et al. (2001), Van Lancker et al. (2004), and Morales
et al. (2006). Similar estuarine bedforms have been described
in North American estuaries by Fenster et al. (1990),
Sherwood and Creager (1990), and Woodruff et al. (2001).
Examples of tidal estuarine bedforms with interpreta-
tion of the flow regime and bedload transport are
presented in Figures 3, 4, and 5.

Summary
Side-scan sonar is an acoustic technique to obtain accurate
images of underwater beds. In estuarine channels these
images reveal a variety of bedforms that enable determina-
tion of the dominant flow regime and hydrodynamic
processes, especially related to bedload transport.
Side-Scan Sonar Imaging of Sediment Bedload,
Figure 4 Transition from sinuous megaripples (left) to medium
sandwaves (right). Dark patterns located between different Bibliography
crests correspond with mud accumulations developed in the Anthony, D., and Leth, J. O., 2002. Large-scale bedforms, sediment
bedform runnels. The flow regime increases from the shallow distribution and sand mobility in the eastern North Sea off the
bars to the deeper part of the estuarine channel. Mud is Danish west coast. Marine Geology, 182, 247–263.
deposited during slack tide. Ashley, G. M., 1990. Classification of large scale subaqueous
bedforms: a new look at an old problem. Journal of Sedimentary
Petrology, 60, 160–172.
ripples; the medium-scale bedforms are megaripples or Blondel, P., 2009. The Handbook of Sidescan Sonar. Dordrecht:
dunes; the largest metric-scale forms are sandwaves. Springer.
There is a demonstrated relationship between the river/ Carling, P. A., Golz, E., Orr, H. G., and Radecki-Pawlik, A., 2000.
The morphodynamics of fluvial sand dunes in the River Rhine,
stream regime and the type, dimensions, and orientation near Mainz, Germany. Part I. Sedimentology and morphology.
of the dominant bedforms (e.g., Ashley, 1990; Harbor, Sedimentology, 47, 227–252.
1998; Carling et al., 2000). In estuaries, the bedforms Duck, R. W., Rowan, J. S., Jenkins, P. A., and Youngs, I., 2001.
clearly correlate with the tidal current velocity and the A multi-method study of bedload provenance and transport
 SOFT COMPUTING 605

pathways in an estuarine channel. Physics and Chemistry of the as problem specification in order to arrive at meaningful
Earth, Part B: Hydrology, Oceans and Atmosphere, 26, solutions (Zadeh, 1994).
747–752.
Fenster, M. S., Fitzgerald, D. M., Bohlen, W. F., Lewis, R. S., and
Baldwin, C. T., 1990. Stability of giant sand waves in Eastern Soft Computing
Long Island Sound, U.S.A. Marine Geology, 91, 207–225.
Gómez, E. A., Cuadrado, D. G., and Pierini, J. O., 2010. Sand trans- Following a series of works by Professor Lotfi Zadeh since
port on an estuarine submarine dune field. Geomorphology, 121, the mid-1960s, usage of the term “soft computing” became
257–265. prominent in the early 1990s (Zadeh, 1994). The main tools
Harbor, D. J., 1998. Dynamics of bedforms in the lower Mississippi of soft computing are neural networks, fuzzy logic, evolu-
River. Journal of Sedimentary Research, 68, 750–762. tionary algorithms (genetic algorithms, differential evolu-
Kuijpers, A., Hansen, B., Hühnerbach, V., Larsen, B., Nielsen, T.,
and Werner, F., 2002. Norwegian Sea overflow through the
tion), and probability embedded fuzzy tools. Support
Faroe–Shetland gateway as documented by its bedforms. vector machines, meta-heuristic and swarm intelligence,
Marine Geology, 188, 147–164. and colony optimization, particle swarm optimizations,
Morales, J. A., Delgado, I., and Gutierrez-Mas, J. M., 2006. and chaos theory also fall under the purview of soft comput-
Sedimentary characterization of bed types along the Guadiana ing (Karray and de Silva, 2004). Apart from the use of
Estuary (SW Europe) before the construction of the Alqueva a single tool like neural network or fuzzy logic, their com-
dam. Estuarine, Coastal and Shelf Science, 70, 117–131. binations such as neuro-fuzzy inference systems have also
Sherwood, C. R., and Creager, J. S., 1990. Sedimentary geology of
the Columbia River Estuary. Progress in Oceanography, 25, been beneficial (Azmathullah et al., 2008). Traditional logic
15–79. recognizes only two crisp values (e.g., true or false, yes or
Van Lancker, V., Lanckneus, J., Moerkerke, G., Hearn, S., Hoekstra, no) and accordingly assigns the value of 0 or 1 to the vari-
P., and Levoy, F., 2004. Coastal and nearshore morphology, ables in binary descriptions. On the contrary, the
bedforms and sediment transport pathways at Teignmouth multivalued fuzzy logic assigns a range of values in
(UK). Continental Shelf Research, 24, 1171–1202. between (0, 1) to the variables depending on the uncertainty
Woodruff, J. D., Geyer, W. R., Sommerfield, C. K., and Driscoll,
N. W., 2001. Seasonal variation of sediment deposition in the and can thus account for imprecision and partial truths in
Hudson River estuary. Marine Geology, 179, 105–119. data and system specification. Subjectivity can thus be
Wright, L. D., Prior, D. B., Hobbs, C. H., Byrne, R. J., Boon, J. D., replaced by objectivity in computations (Ross, 2004).
Schaffner, L. C., and Green, M. O., 1987. Spatial variability of Applications of soft computing tools in estuarine and
bottom types in the lower Chesapeake Bay and adjoining estuar- coastal fields have targeted function approximation, opti-
ies and inner shelf. Estuarine, Coastal and Shelf Science, 24, mization, control, system modeling, and pattern recogni-
765–784.
tion. A large number of applications pertain to the use of
neural networks, fuzzy logic, and genetic algorithms to
Cross-references estimate and forecast waves, tides, currents, storm surge,
Estuarine Sediment Composition sediment transport, salinity intrusion, foundation scour,
Mass Physical Sediment Properties structural optimization, control, and effects (Jain and
Sediment Transport Deo, 2006; Deo, 2010).
Sedimentary Structures

Bibliography
Azmathullah, H. M., Deo, M. C., and Deolalikar, P. B., 2008. Alter-
SOFT COMPUTING native neural networks to estimate scour below spillways.
Advances in Engineering Software, 39(2008), 689–698.
Deo, M. C., 2010. Artificial neural networks in coastal and ocean engi-
M. C. Deo neering. Indian Journal of Geo-Marine Sciences, 39(4), 589–596.
Department of Civil Engineering, Indian Institute of Jain, P., and Deo, M. C., 2006. Neural networks in ocean engineer-
Technology Bombay, Mumbai, India ing. International Journal of Ships and Offshore Structures,
1(1), 25–35.
Karray, F., and de Silva, C., 2004. Soft Computing and Intelligent
Synonyms Systems Design – Theory, Tools and Applications. Boston: Addi-
Artificial intelligence; Computational intelligence; son Wesley.
Evolutionary algorithms; Fuzzy logic; Machine learning; Ross, T., 2004. Fuzzy Logic with Engineering Applications.
Hoboken: Wiley.
Neural networks Zadeh, L. A., 1994. Fuzzy logic, neural networks and soft comput-
ing. Communication of the ACM, 37(3), 77–84.
Definition
Soft computing refers to those computational methods that
are inherently soft toward inexactness of data as well as Cross-references
that of problem specification. It is defined as the technique Sediment Transport
that targets the tolerance of the real world to uncertainty, Storm Surges
imprecision, inaccuracy, and partial truths in data as well Tides
606 SOFT SEDIMENT COMMUNITIES

SOFT SEDIMENT COMMUNITIES Summary

In the estuarine ecological literature, the terms “commu-
Francisco Barros nity” and “assemblage” are often used synonymously,
Laboratório de Ecologia Bentônica, Instituto de Biologia, but community not only refers to groups of species that
Universidade Federal da Bahia, Salvador, Bahia, Brazil co-occur in a similar habitat but also to the
interdependence between those species. In the absence
of experimental evidence of interdependence
Synonyms (or interaction), these groups should be called assem-
blages. Soft sediment communities are affected by the
Soft bottom communities physical, chemical, and biological characteristics of the
benthic habitat, and they are frequently used to assess
Definition human impacts on estuaries. Many primary consumers in
estuaries are part of benthic communities that provide
Estuarine soft sediment communities consist of groups of essential ecological functions to the system.
benthic species that frequently co-occur and show
interdependence.
Bibliography
Gray, J. S., and Elliott, M., 2009. Ecology of Marine Sediments:
Description From Science to Management. Oxford: Oxford University Press.
Soft sediments – mud, silt, and sand – cover a large por- Levin, L. A., Boesch, D. F., Covich, A., Daham, C., Erséus, C.,
tion of the world’s estuarine benthic environments and Ewel, K. C., Kneib, R. T., Moldenke, A., Palmer, M. A.,
provide habitat for a multitude of benthic invertebrate spe- Snelgrove, P., Strayer, D., and Welawski, J. M., 2001. The func-
tion of marine critical transitional zones and the importance of
cies. Soft sediments are three-dimensional environments sediment biodiversity. Ecosystems, 4, 430–451.
inhabited by many benthic species. It is well documented McLusky, D. S., and Elliott, M., 2004. The Estuarine Ecosystem:
that soft sediment communities are not only influenced by Ecology, Threats and Management. Oxford: Oxford University
environmental factors (e.g., sediment size, pH, organic Press.
content) but also by biotic factors (e.g., bioturbation) Rhoads, D. C., and Young, D. K., 1970. The influence of deposit
(see the pioneer work of Rhoads and Young, 1970). Bio- feeding organisms on sediment stability and community trophic
structure. Journal of Marine Research, 28, 150–178.
turbation is linked to deposit feeding, sediment reworking, Rosenberg, G., 2001. Marine benthic faunal successional stages and
construction of burrows and tubes, and irrigation, with related sedimentary activity. Scientia Marina, 65, 107–119.
important effects on biogeochemical processes in surface Underwood, A. J., 1986. What is a community? In Raup, M. P., and
sediments and at the sediment-water interface and on Jablonsky, D. (eds.), Patterns and Processes in the History of
redox conditions in deeper sediments (Rosenberg, 2001). Life. Berlin: Springer, pp. 351–367.
Estuaries provide essential ecological functions (e.g.,
nutrient cycling), and soft sediment communities are inte-
gral to these functions (Levin et al., 2001). A large fraction Cross-references
of primary consumers in an estuary are found in the bot- Benthic Ecology
tom sediments (McLusky and Elliott, 2004). The Biogenic Sedimentary Structures
macrofauna of estuarine soft sediment communities are Infauna
dominated by crustaceans, mollusks, and annelids, and
there are many studies showing that the structure of these
communities is an essential tool for assessing impacts on
estuaries. SOLDIER CRABS (MICTYRIDAE)
While the meaning of the term “soft sediment” is
straightforward, the meaning of the term “community” Joy Unno and Vic Semeniuk
has been a matter of debate for decades. This debate V & C Semeniuk Research Group, Warwick,
involves disagreement about populations versus commu- WA, Australia
nities as the appropriate level of organization to study
(see Underwood, 1986 and references therein). In the estu-
arine ecological literature, the terms “community” and Definition
“assemblage” are often used synonymously. However, The mictyrid soldier crab is easily identified by a number
the term “community” not only refers to groups of species of distinctive characteristics including its relatively small
that co-occur in a similar habitat but also to the body size (marble to golf ball diameter); blue coloring;
interdependence between those species. In contrast, the prominent black eyes on short stalks; round, spider-like
term “assemblage” refers to a collection of co-occurring appearance; chelipeds (front claws) orientated in the verti-
species that does not require links to a specific cal plane; and forward-walking locomotion style. McNeill
habitat, and it does not imply interdependency (Gray and (1926) provides a more detailed taxonomic description of
Elliott, 2009). the genus.
 SOLDIER CRABS (MICTYRIDAE) 607

Introduction rise. However, the substrate must be moist enough for the
Soldier crabs are semiterrestrial, intertidal crustaceans crabs to be able to pelletize the sand. Similar to other inter-
well known for their tendency to form spectacular wander- tidal crabs, such as Dotilla or Scopimera, soldier crabs are
ing droves or swarms of hundreds to thousands of brightly filter feeders, scooping sand into their water-filled buccal
colored (mainly blue) individual crabs on the tidal flat at cavities where the organic material is filtered off into the
low tide. There are eight recorded species of mictyrid gut and the heavier sand particles separated and
soldier crabs (Table 1), and their biogeographic range compacted into a discard pellet (Quinn, 1986).
incorporates coastal areas in the Indo-west Pacific region Unlike many other intertidal crabs such as Uca and
from the southern islands of Japan, through the China Sesarma, soldier crabs do not have permanent burrows
Sea and the Southeast Asian region, to Australia but create a subsurface circular air cavity by burrowing
(Figure 1). Climate zones inhabited by the different into the substrate from the surface in a cork-screw motion,
species range from tropical humid, to tropical arid, to sub- sealing off the top of the hole with sand pellets in
tropical and temperate zones. Tidal ranges are also diverse a characteristic rosette pattern. The crabs move through
with soldier crabs occurring in extreme macrotidal the subsurface of the sediment, dragging the air bubble
regimes of up to 8 m in tropical areas in northern King with them by pushing sand from one side of the bubble
Sound, Western Australia, to microtidal 2 m tides in higher to the other. Crabs will reside below the surface and carry
latitudes such as Hobart, Tasmania. Most soldier crab on feeding in the air bubble during high tide when the sub-
habitats occur in areas with semidiurnal or mixed tidal strate is saturated (Unno and Semeniuk, 2008). After sur-
cycles and less commonly with diurnal tides (Unno and face exposure at low tide, if the crabs have exhausted or
Semeniuk, 2009). lost their air bubble, they will emerge briefly and then cre-
Soldier crabs are common inhabitants of estuaries, and ate a reentry rosette while renewing their air cavity. It is the
an understanding of the habitat requirements, behavior, presence of the mobile air cavity that constrains the soldier
and life cycle of the crab combined with the knowledge crab to remain relatively close to the substrate surface, and
of the types of habitats occurring in estuaries explains as a result, the most important habitat requirement for sol-
why this situation occurs. Soldier crabs are important in dier crabs, and the one that determines where many soldier
estuaries as diet items for shore birds and fish, and as crab populations reside within estuaries, is that of wave
burrowing organisms for their major bioturbation effect energy. Waves influence the stability of the substrate, with
on substrates in terms of sediment turnover, benthic high energy waves creating megaripples and massive
metabolism, and chemical properties (such as oxygenation shoal movements that would disrupt soldier crab air cavi-
and nitrogen fluxes), and sediment structuring. ties, eliminating them from the area. Lower wave energy
can result in an increase in the mud content of substrates
to the extent that soldier crabs are eliminated from the
Habitat requirements of the soldier crab environment as they can no longer feed or create air cavi-
The highly visible emergent phase of the soldier crab as ties. The balance for the soldier crab habitat is one
swarms or “armies” on the tidal flat surface is only a short- between the appropriate level of wave energy and mud
term expression of what is largely a cryptic, benthic exis- influx (Unno and Semeniuk, 2009).
tence for the crab, as infauna within the substrate during
part of the low tidal and all of the high tidal periods. Sol-
dier crabs have an intimate relationship with their habitat Soldier crab behavior and life cycle
as they reside beneath the surface, living, feeding, and Once a soldier crab population has colonized a suitable
moving around in their subsurface habitat. Generally, they habitat, the complexity of soldier crab behavior and the
do not occur deeper than 30 cm unless pursued by preda- concomitant organismal structures (ichnos) created by
tors. The sediment characteristics of their habitat are the crabs become evident. The complexity of behavior
mostly fine to medium sand (quartz or calcareous) with and the resultant ichnological structures increases with
a small percentage of interstitial mud and organic matter. the age of the crab. After a planktonic larval stage, soldier
Mictyris longicarpus, the largest of the soldier crab crabs settle as juvenile recruits into the substrate and their
species, has been found in a more diverse range of sedi- presence is evinced by small clots of sand on the surface at
ment types from coarse or very coarse sand to muddy low tide. As the crabs molt and grow, juvenile crabs can be
sand. Sediment pellicular water and groundwater salinity seen emerging after exposure at low tide and creating
are circa marine salinity, but soldier crabs are small crater-like structures. Adult crabs have the greatest
osmoregulators and can tolerate considerable variations diversity of ichnological products with their emergent
in salinity (Barnes, 1967). With capillary tube lungs as phase and feeding activities on the surface resulting in
well as gills (Maitland and Maitland, 1992), soldier crabs short vertical subsurface shafts, exit holes, scrape marks,
are obligate air-breathers, not subtidal crabs, and therefore discard feeding pellets (“pseudofecal” pellets), and reentry
will drown if the frequency of inundation is too high. rosettes (Figure 2). Discard pellets can densely cover the
However, they do require a moist environment, and tidal flat for tens to hundreds of square meters. Infaunal
a shallow water table is necessary at low tide to allow for crabs that remain in the substrate at low tide but are feeding
pellicular water to be present in the substrate via capillary close to the surface are indicated by individual circular
608 SOLDIER CRABS (MICTYRIDAE)

Soldier Crabs (Mictyridae), Table 1 Recorded species of Mictyris Latreille, 1806, and their biogeographic range and images

Species Biogeographic range General features Image

Mictyris Australia: Cape York, Queensland to Wilson’s Typical size: 30 mm

longicarpus Promontory, Victoria Eyes: large
Latreille, 1806 Color: blue with white sides (branchial
regions); cream legs with narrow red
bands at base and middle joints

Mictyris Taiwan, Kinmen, China, Hainan Island, Typical size: 20 mm

brevidactylus Vietnam, Hong Kong, Southeast Asia, Eyes: medium
Stimpson, 1858 Singapore, Indonesia, Karakelong Island, Color: blue, paler blue sides; cream legs
Ambon Island, Bawean Island with broad red bands at base of walking
legs (Photo: Hsi-Te Shih)

Mictyris platycheles Australia: Moreton Bay, Queensland, northern Typical size: 20 mm

H. Milne Edwards, New South Wales, to Tasmania Eyes: medium
1852 Color: dark blue with reddish-purple
sides; reddish-cream legs

Mictyris livingstonei Australia: Cookstown, Queensland to Trial Typical size: 15 mm

McNeill, 1926 Bay, New South Wales Eyes: small
Color: blue with lighter sides; cream legs

Mictyris Australia: northern King Sound to Shark Bay, Typical size: 15 mm

occidentalis Unno, Western Australia Eyes: medium
2008 Color: blue with paler blue or pinkish
sides; cream or orange legs

Mictyris guinotae Japan: Ryukyu Islands (Nansei Shoto) Typical size: 15 mm

Davie, Shih & Chan, Eyes: medium
2010 Color: blue, with paler blue or brownish
sides; cream legs (Photo: Shawn Miller)

Mictyris Australia: northern King Sound, Western Typical size: 15 mm

darwinensis Unno Australia to Cape York, Queensland Eyes: medium
and Semeniuk, 2011 Color: slaty blue, with pale reddish or
brown sides; cream legs
 SOLDIER CRABS (MICTYRIDAE) 609

Soldier Crabs (Mictyridae), Table 1 (Continued )

Species Biogeographic range General features Image

Mictyris West coast of Thailand: Andaman coast Typical size: 15 mm

thailandensis Davie, between Ranong Province and Pakbara Beach, Eyes: medium
Wisespongpand & Satun Province Color: light blue, with pale blue to cream
Shih, 2013 sides; white chelipeds and fawn walking
legs (Photo: Puntip Wisespongpand)

Soldier crabs function ecologically as major

bioturbators and aerators of sediment in the intertidal eco-
system and as food items both for avifauna at low tide and
for nekton on the high tide. Their daily ingestion of the
substrate and cycling of groundwater in conjunction with
a physiological tendency to accumulate heavy metals sug-
gests that they can act as bioindicators of pollution events.
They have an effect on meiofauna assemblages and nutri-
ent fluxes within the substrate over an area that may be
kilometers in extent, depending on the scale of the habitat.

Soldier crabs recorded in estuaries

Soldier crabs have been documented in estuaries through-
out their Indo-west Pacific biogeographic range and stud-
ied in regard to various aspects of their ecology including
activity patterns, physiology, burrowing behavior,
swarming behavior, feeding behavior, reproductive
behavior, population dynamics, benthic metabolism, and
the effect of soldier crab feeding on sediment and
meiofauna (Cameron, 1966; Kelemec 1979; Farrelly and
Greenaway, 1987; Maitland and Maitland, 1992; Dittman,
1993; Shih, 1995; Dittman, 1998; Rossi and Chapman,
2003; Sadao, 2003; Webb and Eyre, 2004; Takeda,
2005). The importance of soldier crabs in estuaries has
been recognized for their role as burrowing organisms in
rapidly reworking sandy substrates and effecting oxygen-
ation, nitrogen fluxes, and other chemical changes. They
effect sediment turnover and sediment structuring to
a depth generally of 10–15 cm (Unno and Semeniuk,
2008), though they can occur to a depth of 30 cm.
River mouths are often locations for major towns or cit-
ies, and consequently researchers are located close to their
study sites. In Japan and Southeast Asia, mountainous ter-
rain and monsoonal climate result in short rivers draining
Soldier Crabs (Mictyridae), Figure 1 Map of coastal central Asia to the coast and developing estuaries. Most estuaries are of
and Southeast Asia and Australia showing the distribution of the the ria, coastal plain (flooded valley), or deltaic estuarine
eight species of Mictyris. types and are bordered by dense stands of mangroves
(Fairbridge, 1980). For instance, M. guinotae has been
studied on shore-parallel platform tidal flats adjacent to
pustular structures and elongate pellet-roofed tunnel work- mangroves in the Nadasa River estuary on Iriomote-jima,
ings appearing on the tidal flat surface. Dense tunnel work- Japan, and, similarly, Shih (1995) observed
ings may coalesce to form extensive mats of pellet-roofed M. brevidactylus on sandy tidal flats bordering mangrove
tunnels over the tidal flat resembling “pustular mats.” woodlands in the Tanshui River estuary in Taiwan.
610 SOLDIER CRABS (MICTYRIDAE)

Soldier Crabs (Mictyridae), Figure 2 Ichnological products of Mictyris. (a) Meandering and linear pustule structures of Unno and
Semeniuk (2008), which are pellet-roofed, shallow, horizontal tunnels. (b) Pustules produced by crabs re-excavating their air bubble.
(c) Exit hole, feeding scrape marks, and discard feeding pellets (“pseudofecal” pellets). (d) Close-up of a reentry rosette, comprising
a central plug and an outer ring that has a vague curved, radial structure, formed by crab burrowing for reentry into the sediment.

For the Australian species, a multitude of rivers every 5–10 years during times of flood; an example is
draining eastwards from the Great Dividing Range and the Gascoyne River estuary where soldier crab
Blue Mountains provide many estuarine habitats for sol- populations reside on deltaic (strand plain) shoals and
dier crab species along the eastern coastline. upstream on sandy creek banks and point bar shoals.
M. longicarpus has been well studied in the Brisbane
River and Pine River estuaries draining into Moreton
Bay, Queensland (Cameron, 1966; Dittmann, 1998) and Range of habitats available in estuaries
inhabits many estuarine environments from northern Soldier crabs inhabit a wide variety of large-scale coastal
Queensland to southern Victoria (Figure 1). M. livingstonei settings, including estuaries, barred lagoons, tidal creeks,
occurs from northern Queensland to the Macleay River delta strand plains, tidal flats, beach/dune shores, and lime-
estuarine complex in Trial Bay, New South Wales. stone barrier coasts. An estuary by definition is the area
M. platycheles occurs along on the central and southern where freshwater from rivers intermix with marine water
mainland Australian coast, as well as in numerous estuar- on a seasonal or intermittent basis resulting in a variation of
ies along the northern, eastern, and southern coastlines of salinity ranging from marine to brackish to fresh depending
Tasmania (Figure 1). In the Northern Territory and north- on the fluvial input. Factors influencing the development of
ern Western Australia, drainage from the highlands and local habitats for soldier crabs within an estuary include the
tectonic blocks result in extensive river outflow to the large-scale geomorphology of the coast, coastal and marine
coast forming estuaries wherein M. darwinensis resides processes, and fluvial hydrology and sediment supply.
(e.g., Keep River estuary). M. occidentalis inhabiting the The presence of soldier crabs in an estuary depends on
northwest Western Australian coastline is an exception to climate setting and coastal setting of the estuary, the type
the soldier crab trend towards dominantly estuarine inhab- of estuary, as well the sedimentological and hydrological
itation. Within the crab’s biogeographic range, the climate regime prevalent in the estuary. An estuary may have
is arid to semiarid resulting in the development of few many local habitats suitable for soldier crabs but can be
large rivers and estuaries. Rivers are dominated by marine situated in a region that due to remoteness from existing
salinities and only function intermittently as estuaries soldier crab populations, or lack of regional currents to
 SOLDIER CRABS (MICTYRIDAE) 611

deliver larval recruits, is depauperate of mictyrid crabs.

The Murchison River estuary in Western Australia is such
an example – despite having suitable local sand shoals, it
is isolated by the Zuytdorp Cliffs and rocky coastline from
the nearest soldier crab population 200 km to the north at
Shark Bay (Unno and Semeniuk 2009). Of the various
types of estuaries, the ria or flooded river valley and the
bar-and-lagoon types (with an inlet opening to the ocean)
are most likely to host soldier crab populations as they
provide sheltered, low-wave energy environments.
There is a salinity gradient within an estuary from fresh-
water to brackish water near, at, or within the river mouth
to brackish water to marine water in the central estuarine
reaches, to marine (and seasonally brackish water) at or
near the estuary inlet. Soldier crabs inhabit estuaries from
the marine saline parts to the brackish water parts, occupy-
ing tidally exposed sandy substrates between the Mean
Low Water Neap (MLWN) and Mean High Water Spring
(MHWS) tidal levels.
With respect to sandy substrates, at the local scale, sol-
dier crabs inhabit a wide range of sheltered (low-wave
energy) stable sandy habitats (Figure 3), including sand
bars, recurved spits, shoals, and depressions leeward of
spits; low gradient sloping beaches; shore-parallel shoals;
sheltered areas of sand on rock pavements; sheltered areas
among rock on sandy beaches; low-mid-high tidal sand
flats; tidal flats on shore-parallel sand platforms and sandy
estuarine river channel banks; mid-channel sand shoals,
mid-bay or mid-estuarine sand shoals; banks, shoals, and
point bars of tidal creeks; shoals, sand flats, and point bars
of intra-estuarine deltas; localized sand flats shoreward or
seaward of mangroves; sandy to muddy sand intertidal
platforms adjacent to and within mangrove woodlands;
sand flats among mangroves; and flood-tidal delta shoals
(Unno and Semeniuk, 2009).

Summary
Soldier crabs are common intertidal crabs inhabiting suit-
able sandy habitats within estuaries in the Indo-west Pacific
region. The sheltered low-wave energy environments pre-
sent in estuaries include habitats such as sand bars, recurved
spits, aligned sandy beaches, tidal flats on shore-parallel
sand platforms, sandy estuarine river channel banks,
mid-channel sand shoals, mid-bay sand shoals, intra-
estuarine delta and tidal delta shoals, and sandy to muddy
sand intertidal platforms adjacent to mangrove woodlands.
The fluctuating water salinities within estuarine environ-
ments are tolerated by soldier crabs through osmoregulatory
mechanisms. While notable for their habit of swarming in
large “armies” of small blue crabs on the tidal flat at low
tide, they largely live infaunally in round air cavities within Soldier Crabs (Mictyridae), Figure 3 Idealized map of an
the substrate, relatively close to the surface. Development estuary showing a selected range of habitats that Mictyris
of the crab from juvenile to adult is accompanied by increas- inhabits. The salinity of their seaward habitats is marine (and
ingly complex behavior and corresponding complexity in may be seasonally brackish); the salinity of their near-riverine
ichnological products and emergent behavior. and deltaic habitats is brackish (and may be seasonally marine).
612 SPECIES RICHNESS

Bibliography Unno, J., and Semeniuk, V., 2009. The habitats of the Western
Australian soldier crab Mictyris occidentalis Unno 2008
Barnes, R. S. K., 1967. The osmotic behaviour of a number of
(Brachyura: Mictyridae) across its biogeographical range. Jour-
grapsoid crabs with respect to their differential penetration of
nal of the Royal Society of Western Australia, 92, 289–363.
an estuarine system. Journal of Experimental Biology, 47,
Webb, A. P., and Eyre, B. D., 2004. The effect of natural populations
535–551.
of the burrowing and grazing soldier crab (Mictyris longicarpus)
Cameron, A. M., 1966. Some aspects of the behavior of the soldier
on sediment irrigation, benthic metabolism and nitrogen fluxes.
crab, Mictyris longicarpus. Pacific Science, 20, 224–234.
Journal of Experimental Marine Biology and Ecology, 309, 1–19.
Davie, P. J. F., Shih, H.-T., and Chan, B. K. K., 2010. A new species
of Mictyris (Decapoda, Brachyura, Mictyridae) from the Ryukyu
Islands, Japan. In Castro, P., Davie, P. J. F., Ng, P. K. L., and de Cross-references
Forges, B. R. (eds.), Studies on Brachyura: A Homage to
Danièle Guinot. Leiden: Brill. Crustaceana Monographs, Vol. Blue Crabs
11, pp. 83–105. Fiddler Crabs
Davie, P. J. F., Wisespongpand, P., and Shih, H.-T., 2013. A new spe-
cies of Mictyris Latreille, 1806 (Crustacea: Decapoda: Brachyura:
Mictyridae) from the Andaman coast of Thailand, with notes on its
ecology and behaviour. Zootaxa, 3686(1), 065–076.
Dittmann, S., 1993. Impact of foraging soldier crabs (Decapoda: SPECIES RICHNESS
Mictyridae) on meiofauna in a tropical tidal flat. Revista de
Biologia Tropical, 41, 627–637. Rafael Riosmena-Rodriguez, Gabriela Andrade-Sorcia
Dittmann, S., 1998. Behaviour and population structure of soldier and Nestor M. Robinson
crabs Mictyris longicarpus (Latreille): observations from a tidal
flat in tropical north Queensland Australia. Senckenbergiana
Programa de Investigación en Botãnica Marina,
Maritima, 28(4–6), 177–184. Departamento Académico de Biología Marina,
Fairbridge, R. W., 1980. The estuary: its definition and geodynamic Universidad Autónoma de Baja California Sur, La Paz,
cycle. In Olausson, E., and Cato, I. (eds.), Chemistry and Bio- Baja California Sur, Mexico
geochemistry of Estuaries. Chichester: Wiley.
Farrelly, C., and Greenaway, P., 1987. The morphology and vascu- Synonyms
lature of the lungs and gills of the soldier crab Mictyris
longicarpus. Journal of Morphology, 193, 285–304. Number of species per area/region/ecosystem
Kelemec, J. A., 1979. Effect of temperature on the emergence from
burrows of the soldier crab, Mictyris longicarpus (Latreille). Definition
Australian Journal of Marine and Freshwater Research, 30, Species richness is the simplest way to describe biotic
463–468.
Maitland, D. P., and Maitland, A., 1992. Penetration of water into
community and regional diversity (Maguran, 1988). It
blind-ended capillary tubes and its bearing on the functional refers to the number of species in an area, biotic commu-
design of the lungs of soldier crabs Mictyris longicarpus. Jour- nity, or ecosystem. Species richness does not take into
nal of Experimental Biology, 163, 333–344. account the abundances of the species or their relative
McNeill, F. A., 1926. A revision of the family mictyridae. Studies in abundance distribution, but rather the number of species
Australian Carcinology No. 2. Records of the Australian in a particular area considering their phylogenetic differ-
Museum, 15, 100–128, pls. ix–x. ences as part of the diversity (Smith and Smith 2001). Dif-
Quinn, R. H., 1986. Experimental studies of food ingestion and
assimilation of the soldier crab, Mictyris longicarpus Latreille ferent species concepts have been used in species richness
(Decapoda, Mictyridae). Journal of Experimental Marine Biol- studies (i.e., biological, ecological, evolutionary, and phy-
ogy and Ecology, 102, 167–181. logenetic species concepts). An unified species concept
Rossi, F., and Chapman, M. G., 2003. Influence of sediment on has recently been proposed by de Queiroz (2007).
burrowing by the soldier crab Mictyris longicarpus Latreille. Jour-
nal of Experimental Marine Biology and Ecology, 289, 181–195. Description
Sadao, K., 2003. Effect of soldier crab Mictyris longicarpus on
chemical properties and microflora of mangrove forest. The observed species richness is affected by the general
Mangurobu ni kansuru Chosa Kenkyu Hokokusho Heisei, 14 area of sampling, heterogeneity of the habitat, trophic
Nendo, 281–291. structure of the area, geographic region, and season of
Shih, J. T., 1995. Population densities and annual activities of Mictyris sampling. The species richness can vary considerably in
brevidactylus (Stimpson, 1858) in the Tanshui Mangrove Swamp different habitats, seasons, and geographic regions. The
of Northern Taiwan. Zoological Studies, 34, 96–105.
Takeda, S., 2005. Sexual differences in behaviour during breeding addition of new species with increasing sampling effort
season in the soldier crab (Mictyris brevidactylus). Journal of can be shown by a species accumulation curve (Bower
the Zoological Society of London, 266, 197–204. and Zar, 1995). Increasing the area sampled can also
Unno, J., 2008. A new species of soldier crab, Mictyris occidentalis increase the observed species richness both because large
(Crustacea: Decapoda: Brachyura: Mictyridae) from Western areas are environmentally more heterogeneous than small
Australia, with congener comparisons. Journal of the Royal areas and because more individuals may inhabit these
Society of Western Australia, 91, 31–50.
Unno, J., and Semeniuk, V., 2008. Ichnological studies of the West- areas. Species richness is a fundamental measurement of
ern Australian soldier crab Mictyris occidentalis Unno 2008: community and regional diversity, and it underlies many
correlations of field and aquarium observations. Journal of the ecological models and conservation strategies (Gotelli
Royal Society of Western Australia, 91, 175–198. and Colwell, 2001).
 SPECIES ZONATION 613

With species richness studies, ecologists often employ Introduction

rank abundance curves, which are graphs ranking the most Species zonation is the occurrence of species or groups of
abundant species to the least abundant (Adams, 2009). species in distinct bands or zones coincident with or
They can be shown as a plot of number of species vs. the related to environmental gradients. It is a common feature
number of individuals on a logarithmic scale that usually observed within estuaries where the distribution and abun-
yields a normal distribution. This is because environments dance of its biota respond to the combined and interactive
are usually undersampled, especially in high diversity effects of steep environmental gradients and food avail-
systems or regions. Singlets make up the middle ability and to interspecies competition, herbivory, and pre-
peak of the distribution, and the more sampling that is dation (Paine, 1974; Pennings and Callaway, 1992;
conducted, the more the curve will shift to the right. The Levinton, 1995). The distribution and abundance of indi-
number of unsampled species in cases of undersampling vidual species (as a result of their tolerances to the com-
can be roughly estimated using the Chao estimator, in bined effects of the environmental gradients) results in
which Sestimate ¼ Sobserved + F12/2F2 where F1 is the compositional changes in biotic assemblages across these
number of singletons sampled and F2 is the number of gradients. For vegetation, species zonation is often mani-
doublets. There is also a method of estimating what per- fest as visually distinct vegetation communities; for
cent of the total species is represented in a sample, rock-inhabiting communities, it is commonly manifest as
called Good’s coverage estimator, in which Coverage visually distinct bands of different encrusting shelly
¼ 1
(the number of individuals in species/total number organisms, epibenthos, and algae. For infauna, it is mani-
of individuals). These estimator equations allow fest as compositional and abundance changes in the com-
researchers to determine how their limited sampling munities across the habitat as determined by sampling and
relates to the entire sampled population (Chao, 2005). mapping. In areas of less steep environmental gradients,
species often occur in less differentiated formations, such
Bibliography as mottled mosaics or with diffuse zonation.
Adams, J., 2009. Species Richness: Patterns in the Diversity of Life. The description and discussion of species zonation pro-
New York: Springer Praxis Books. vided here is for a positive estuary that grades from fresh-
Bower, J. E., and Zar, J. H., 1995. Field and Laboratory Methods water at the river entrance to marine at the estuary mouth,
for General Ecology. Dubuque, IA: William C. Brown but the principles of species response also apply to inverse
Publishers. estuaries where evaporation from the surface water
Chao, A., 2005. Species richness estimation. In Balakrishnan, N., exceeds the freshwater runoff entering the estuary.
Read, C. B., and Vidakovic, B. (eds.), Encyclopedia of Statistical
Sciences. New York: John Wiley and Sons, pp. 7909–7916.
de Queiroz, K., 2007. Species concept and species delimitation. Variation on zonation in different environmental
Systematic Biology, 56(6), 879–886. situations
Gotelli, N. J., and Colwell, R. K., 2001. Quantifying biodiversity:
procedures and pitfalls in the measurement and comparison of
In estuaries where there is a strong seasonal to perennial
species richness. Ecology Letters, 4, 379–391. delivery of freshwater via subterranean seepage from
Maguran, A. E., 1988. Ecological Diversity and Its Measurement. supratidal environments (Semeniuk, 1983; Cresswell,
Princeton: Princeton University Press. 2000), the gradient of increasing salinity and pore-water
Smith, R. L., Smith T. M., 2001. Elements of ecology pearson edu- content upslope can be locally reversed at the contact of
cation. In Addison Wesley Longman S. A. (eds.). Inc. the tidal flat with the supratidal zone. Where frequency
of inundation and concomitant change in wave energy
Cross-references and tidal current energy also result in a differentiation/
partitioning of sediment grain sizes across the tidal flat
Shannon-Weaver Diversity Index
(from sand in low tidal areas to mud in high tidal areas),
there may be three gradients across the tidal flat, that of
salinity, pore-water content, and sediment grain size. As
a consequence, biota responding to these gradients in con-
SPECIES ZONATION cert often forms distinct zonation across the flat.

Vic Semeniuk1 and Ian Cresswell2 Heterogeneous distribution of habitats and lack of
1
V & C Semeniuk Research Group, Warwick, WA, species zonation
Australia In many estuaries there are mosaics of habitats and
2
Land & Water National Research Flagship, CSIRO, mosaics of biotic assemblages and occurrences of species
Hobart, TAS, Australia relating to these habitats. Due to the particular and deter-
minative environmental conditions present in a habitat,
Definition the species composition of biotic assemblages can vary
Species zonation is the occurrence of species or groups of markedly between habitats. Estuaries with heterogenous
species in distinct bands or zones coincident with or distribution of habitats can result in a mosaic of biotic
related to environmental gradients. assemblages corresponding to these habitats within the
614 SPECIES ZONATION

Species Zonation, Figure 1 Zonation of benthic mollusks from river to sea across an estuary showing response to an environmental
gradient of open-water salinity (information from Semeniuk and Wurm, 2000). The change in mollusk composition is from species-
depauperate, fluvial-dominated assemblages to species-rich, marine-dominated assemblages.

estuary. The distribution of assemblages across and along Saline conditions can be lethal to biota adapted for
the estuary in these cases is not zoned; e.g., an estuary freshwater conditions, and freshwater can be lethal or
comprised of deepwater mud basins, shallow-water debilitating to biota adapted for saline conditions. So, the
subtidal shore-parallel sandy platforms, tidal flats, and salinity of open estuarine water or that of sediment pore
hummocky tidally exposed shoals may have distinctive water (influencing those biotas residing in the substrate
fauna and flora in each of these habitats, but a transect or having their roots in the sediment) controls the occur-
across the estuary will not illustrate species zonation nor rence and functioning of the various species in an estuary
assemblage zonation but rather habitat-specific occur- and results in their occurrence or absence and in their dif-
rences of assemblages. In contrast, species zonation, if ferential abundance. In the case of substrates, some biotas
present in such estuaries, may be evident within a given are adapted solely for inhabiting sandy substrates, and the
habitat that has environmental gradients within the habitat. occurrence of mud interferes with their feeding or respira-
tory processes while, conversely, organisms adapted to
quiescent muddy conditions cannot tolerate mobile sandy
Some environmental determinants underpinning substrates. In the case of inundation, how frequently an
zonation area in the tidal zone is inundated will influence the distri-
The main environmental determinants for forcing species bution of those biotas that require nearly continuous inun-
zonation that are examined in this work are pore-water dation for survival (Pennings et al., 2005). The gradient of
salinity, open-water salinity, substrate type, and inunda- inundation affects the extent that tidal zones are exposed
tion. Each of these factors is discussed below in relation to solar radiation, winds, evaporation, and groundwater
to effects on species zonation. In the real world, it is the draining (which depletes sediment moisture content), all
combination of each of these physical environmental gra- of which influence the survivorship and population
dients, plus the interaction between species, that deter- dynamics of a given species.
mines the final zonation of species. Other environmental
factors and gradients also influence zonation; these Open-water salinity gradient
include wave energy, tidal currents, extent of water turbid- The salinity of open water in an estuary controls species
ity, water depth, temperature, degree of light penetration, occurrence, distribution, and zonation and is expressed
pH, and nutrients. They influence the survivorship of in a major gradient along an estuary (Figure 1). Salinity
a given species, determine the suitability of a habitat for can vary from marine near the estuary entrance to fluctuat-
an organism, and also affect the microbiota in the environ- ing marine and brackish water and freshwater in the cen-
ment that then influence the occurrence of macrofauna and tral estuary, to freshwater proximal to the river mouth or
macroflora. up-channel in a river. This salinity gradient results in
 SPECIES ZONATION 615

Species Zonation, Figure 2 Zonation of crustacean assemblages within habitats (a shore-parallel subtidal sand platform and
a subtidal basin) crossing an environmental gradient of open-water salinity from upper estuary to lower estuary and deltaic,
demonstrating the partitioning of the habitat into biotic zones (information from Semeniuk, 2000). In this case, the habitats are
shore-parallel subtidal sand platforms and a basin (with similar substrates along their length) that cross three salinity fields in an
elongated estuary.

major biotic compositional changes, expressed as zona- and biochemical signature than open estuarine water in
tion along the length of an estuary, from marine- terms of pH, Eh, cationic content, nutrients, and dissolved
dominated assemblages at one end to freshwater assem- gases. Focusing on pore-water salinity, while open-water
blages at the other, with the marine and brackish water/ salinity may influence the occurrence and distribution of
freshwater central environment with species that are epifauna and nekton, pore-water salinity has influence
adapted to fluctuating salinity conditions and those that on the survivorship and distribution of infaunal benthic
can tolerate a wide range in salinity (Semeniuk and Wurm, organisms and on plants, because the fauna is directly in
2000). contact with pore waters, and plants have their roots
immersed in such water and draw on pore waters for tran-
spiration and nutrient transfer (Pennings and Callaway,
Pore-water salinity gradient 1992; Silvestri et al., 2005; Unno and Semeniuk, 2009).
Pore-water can be different from open-water salinity in Pore water of higher salinity adversely affects fauna and
that it may remain stasohaline (relatively constant), while flora not adapted to those conditions and hence can elimi-
overlying open estuarine water fluctuates from marine to nate species. Conversely, pore water that is not saline
brackish water to freshwater. In addition, being in contact enough can also physiologically affect fauna and flora
with sediment, its geochemistry, and its organic matter, adapted to more saline conditions which are then elimi-
pore water has a more diagnostic and variable chemical nated from this “fresher” zone.
616 SPECIES ZONATION

Species Zonation, Figure 3 Zonation of polychaete assemblages within habitats (a shore-parallel subtidal sand platform and
a subtidal basin) crossing an environmental gradient of open-water salinity from upper estuary to lower estuary and deltaic,
demonstrating the partitioning of the habitat into biotic zones (information from Dürr and Semeniuk, 2000). In this case, the habitats
are shore-parallel subtidal sand platforms and a basin (with similar substrates along their length) that cross three salinity fields in an
elongated estuary.

Substrate environment for specialized fauna; such environments

Substrate grain sizes have a major effect on biota occur- are not well oxygenated and less transmissive to pore
rence, abundance, and functioning (McLachlan, 1996; waters and contain organic material interstitial to the sand
Semeniuk and Wurm, 2000; Unno and Semeniuk, 2009). particles. The Western Australian soldier crab, and its
Mobile sand, for instance, agitated by current and waves associated invertebrate community, is an example of fauna
and devoid of interstitial mud and organic matter, is adapted to inhabiting a stable-sand substrate specifically
a specific environment that only specialized fauna can with medium/fine sand and <10 % mud (Unno and
inhabit. Such environments are well oxygenated and Semeniuk, 2009). On the other hand, faunas that burrow
transmissive to pore waters and contain little or no organic and forage through organic-rich mud and build permanent
material interstitial to the sand particles. Faunas that burrows find mobile sand and even slightly muddy stable
require a proportion of mud to line their burrow walls, or sand unsuitable.
substrates that are stable (non-mobile) to build burrow
structures, or feed on microbiota interstitial to sand grains Inundation
find mobile substrates unsuitable. Stable sand not mobi- The frequency of inundation has a large influence on the
lized by currents and waves and containing some intersti- occurrence and abundance of species across tidal flats
tial mud and organic matter provides a specific (Pennings and Callaway, 1992; Mitsch and Gosselink,
 SPECIES ZONATION 617

Species Zonation, Figure 4 Well-zoned mangroves on a tidal flat (a shore-parallel macrotidal intertidal mud flat in Darwin, Australia).
The distinct color-evident mangrove bands from seaward with the main species identified being Sonneratia alba followed by
Avicennia marina, a dark zone of Rhizophora stylosa followed by Bruguiera exaristata, then Avicennia marina and subordinate Ceriops
tagal, and (most landward) Avicennia marina heath with samphires. The width of the mangrove band from seaward to the mangrove-
free salt flat is 340 m.

Species Zonation, Figure 5 Profile showing well-zoned mangroves on a tidal flat in the Lawley River Estuary, Western Australia, in
relation to tidal levels and to groundwater salinity (cf., Semeniuk, 1983).

1993). The higher parts of a tidal flat are exposed for lon- tidal flat is also subject to increasing internal drainage
ger periods than low tidal parts. The gradient from low such that higher flats at the time of low tide have drier sub-
tidal flat to high tidal flat is thus subject to increasing strates than lower flats. This gradient of increased salinity
effects of solar radiation and wind and hence evaporation and decreased pore-water content from low tidal flats to
and desiccation. The gradient from low tidal flat to high higher tidal flats results in absence and presence of species
618 SPECIES ZONATION

Species Zonation, Figure 6 Well-zoned saltmarsh on a tidal flat (a shore-parallel microtidal intertidal mud flat in the Western Port,
Victoria, Australia). The distinct color-evident vegetation bands with the main species identified from seaward are a seaward fringe of
Avicennia marina with a shore-parallel band of mangrove-free patches within the mangrove zone, followed by a lighter-toned zone
of fine parallel bands composed of mixed assemblages of Sarcocornia quinqueflora and Samolus repens and then a wide relatively
dark “samphire” zone of Tecticornia arbuscula assemblage; the most landward zone is shore-parallel copses of Melaleuca ericifolia
(cf. Bridgewater, 1975). The maximum width of the saltmarsh band in this image is 250 m.

Species Zonation, Figure 7 Profile showing well-zoned saltmarsh on the shores of the Leschenault Inlet Estuary, Western Australia,
with the main species identified in relation to tidal levels and to groundwater salinity. The landward edge of the high tidal saltmarsh
receives freshwater seepage that dilutes the groundwater hypersalinity (cf. Cresswell, 2000; Pen et al., 2000).
 SPECIES ZONATION 619

Species Zonation, Figure 8 Simplified diagram modified from Semeniuk and McNamara (2009) to specifically illustrate the
environmental tolerances of selected species in subtidal and tidal estuarine and marine environments in terms of sand, muddy sand,
and mud substrates. Some species are habitat restricted, because of grain size influences, food sources, and salinity (e.g., the mollusks
Bulla quoyi and Sanguinolaria biradiatai, the crabs Mictyris occidentalis, Scopimera inflata, and Uca flammula), while others inhabit
a range of substrate types (e.g., the mollusks Tellina deltoidalis and Nassarius burchardi). Some species cross environmental
boundaries, occurring in estuarine and marine settings. This diagram does not address depth occurrence or wave and tidal effects on
species occurrence.

and a zonation of species across the tidal flat. For example, Examples of species zonation
experimental results have shown that for salt-marsh vege- One of the best examples of species zonation in response
tation, periodicity of inundation can be a major determi- to tidal flat environmental gradients is afforded by man-
nant of survival, as it determines oxygen availability, and groves and saltmarshes (Figures 4, 5, 6, and 7; and
only plants adapted to these conditions are able to tolerate Chapman, 1938; Bridgewater, 1975; Tomlinson, 1986;
such environments (Pennings and Callaway, 1992; Mitsch Bridgewater and Cresswell, 1993; Cresswell and Bridge-
and Gosselink, 1993). water, 1998; Emery et al., 2001; Pennings et al., 2005).
From mean sea level (MSL) to the high tidal mark, man-
Broad-scale zonation groves form zones in response to frequency of inundation,
An estuary may be comprised of a sequence of habitats salinity of pore water, pore-water content, and sediment
that intergrade from river to sea, forming a gradient of grain size. For instance, in a region comprised of six man-
environments. In this context, biota responding to the gra- grove species (Avicennia marina, Aegialitis annulata,
dient of changing environments will form gradational Aegiceras corniculatum, Bruguiera exaristata, Ceriops
zones. For example, the gradient from river to estuarine tagal, and Rhizophora stylosa), with strong environmental
basin involves tidal sand, subtidal muddy sand, and deeper gradients of salinity and inundation, there is zonation of
water mud, with a gradient of open-water salinity of fresh- the species in terms of composition, vegetation structure,
water to brackish water. Here, species zonation reflects the and plant physiognomy. The species are zoned from
gradient or sequence of habitat types, as well as the along- MSL to the high tidal mark with Avicennia marina found
estuarine environmental gradient of salinity and substrate where the pore-water salinity is 40 ppt, followed by a zone
grain size. This is species zonation across multiple habitats of Rhizophora stylosa where the pore-water salinity is
(Figures 2 and 3). 45 ppt, then by a zone of Rhizophora stylosa and
Bruguiera exaristata where the pore-water salinity is
55 ppt, a zone of Ceriops tagal where the pore-water salin-
Finer-scale zonation ity is up to 85 ppt, and finally a landward zone of
Zonation within a habitat occurs when there are strong and Avicennia marina where the pore-water salinity is up to
steep environmental gradients. For instance, on tidal flats 90 ppt (Semeniuk, 1983). Likewise, various species of
where daily tidal exposure introduces inundation, evapo- saltmarsh form ecological zones in response to inundation
ration, salinity, and moisture content, the local species frequency and pore-water salinity on the tidal flats of
respond to these environmental conditions and occur in estuaries.
distinct and discrete bands. Where there is an aggregate There are similar patterns of zonation on tidal flats and
of species in the habitat, these will form a distinct zone, in shallow-water habitats in estuaries for invertebrates
i.e., species zonation across a habitat. Species zonation such as fiddler crabs, other Brachyura, and mollusks, with
also can occur in subtidal habitats in response to environ- the biota exhibiting zonation in response to environmental
mental gradients of water depth, light availability, and sed- gradients (Crane, 1975; Chakraborty and Choudhury,
iment types. 1985; Dittmann, 2000; Dürr and Semeniuk, 2000;
620 SPECIES ZONATION

Species Zonation, Figure 9 Zonation of mollusks in an estuary across the various habitats in upper estuarine, middle estuarine, and
deltaic fields (information from Semeniuk and Wurm, 2000). Species illustrating zonation are: Hydrococcus brazieri, Arthritica semen,
Acteocina sp., Bedeva paivae, Nassarius burchardi, Sanguinolaria biradiata, Spisula trigonella, Tellina deltoidalis, and Xenostrobus
securis.

Semeniuk, 2000; Bezerra et al., 2006). Species with broad For instance, within a suite of adjoining habitats varying
environmental and habitat tolerances are able to exist from sand to muddy sand to mud, and varying from saline
within a wider range of habitats or tolerate greater to hypersaline, or varying from subtidal to tidal flat, some
fluctuation in environmental changes and thus cross over species occur across habitat boundaries, while others
zones, while environmentally restricted species do not. are environmentally restricted (Figure 8). Species zonation
 SPECIES ZONATION 621

within habitats in estuaries determined by environmental Cresswell, I. D., 2000. Ecological significance of freshwater seeps
gradients is shown in Figure 9 for mollusks. along the western shore of the Leschenault Inlet estuary. Journal
of the Royal Society of Western Australia, 83, 285–292.
Cresswell, I. D., and Bridgewater, P. B., 1998. Major plant commu-
nities of coastal saltmarsh vegetation in Western Australia. In
Summary McComb, A. J., and Davis, J. A. (eds.), Wetlands for the Future.
Species zonation is a common feature within estuaries South Australia: Gleneagles Publishing, pp. 297–326.
where the combined and interactive effects of steep envi- Dittmann, S., 2000. Zonation of benthic communities in a tropical
ronmental gradients and food availability and to interspe- tidal flat of north-east Australia. Journal of Sea Research, 43,
cies competition, herbivory, and predation result in 33–51.
Dürr, V., and Semeniuk, T. A., 2000. Long-term spatial dynamics of
compositional changes in biotic assemblages across these polychaetes in Leschenault Inlet estuary. Journal of the Royal
gradients. For vegetation and for rock-inhabiting biota, it Society of Western Australia, 83, 463–474.
is often manifest as visually distinct communities occur- Emery, N. C., Ewanchuk, P. J., and Bertness, M. D., 2001. Compe-
ring in bands. For infauna, it is expressed as compositional tition and salt-marsh plant zonation: stress tolerators may be
and abundance changes in the communities across the dominant competitors. Ecology, 82, 2471–2485.
habitat. In areas of less steep environmental gradients, Levinton, J. S., 1995. Marine Biology: Function, Biodiversity, Ecol-
ogy. Oxford: Oxford University Press.
species often occur in less differentiated formations, such McLachlan, A., 1996. Physical factors in benthic ecology: effects of
as mottled mosaics or with diffuse zonation. changing sand particle size on beach fauna. Marine Ecology
Species zonation and changes in biotic assemblages can Progress Series, 131, 205–211.
be expressed across the whole of the estuary responding to Mitsch, W. J., and Gosselink, J. G., 1993. Wetlands, 2nd edn.
an along-estuarine environmental gradient of salinity, New York: Van Nostrand Reinhold, pp. 189–265.
from freshwater to marine, and a gradient in substrate Paine, R. T., 1974. Intertidal community structure: experimental
studies on the relationship between a dominant competitor and
grain size. For zonation within a given habitat, while there its principal predator. Oecologia, 15, 93–120.
may be a range of factors that influence species zonation Pen, L., Semeniuk, V., and Semeniuk, C. A., 2000. Peripheral wet-
that include wave energy, tidal currents, extent of water land habitats and vegetation of Leschenault Inlet estuary. Jour-
turbidity, water depth, temperature, degree of light pene- nal of the Royal Society of Western Australia Special Issue on
tration, pH, and nutrients, some of the main environmental the Leschenault Inlet Estuary, 83, 293–316.
determinants for forcing species zonation are pore-water Pennings, S. C., and Callaway, R. M., 1992. Salt marsh plant zona-
salinity, open-water salinity, substrate type, and inunda- tion: the relative importance of competition and physical factors.
Ecology, 73, 681–690.
tion. These affect the survivorship of a given species, Pennings, S. C., Grant, M.-B., and Bertness, M. D., 2005. Plant
determine the suitability of a habitat for an organism, zonation in low-latitude salt marshes: disentangling the roles of
and also affect the microbiota in the environment that flooding, salinity and competition. Journal of Ecology, 93,
influence the occurrence of macrofauna and macroflora. 159–167.
One of the best examples of species zonation in estuaries Semeniuk, V., 1983. Mangrove distribution in Northwestern Aus-
is afforded by mangroves in tropical regions and saltmarsh tralia in relationship to freshwater seepage. Vegetatio, 53, 11–31.
Semeniuk, T. A., 2000. Small benthic Crustacea of the Leschenault
on tidal flats in temperate regions in response to tidal flat Inlet estuary. Journal of the Royal Society of Western Australia,
environmental gradients. For example, in mangroves with 83, 429–441.
strong environmental gradients of salinity and inundation, Semeniuk, V., and McNamara, K. J., 2009. The power of stratigra-
there is zonation of the species in terms of composition, phy in determining biological evolutionary patterns. Journal of
vegetation structure, and plant physiognomy. the Royal Society of Western Australia, 92: 407–430.
Semeniuk, V., and Wurm, P. A. S., 2000. Molluscs of the Leschenault
Inlet estuary – their diversity, distribution, and population dynam-
ics. Journal of the Royal Society of Western Australia Special Issue
Bibliography on the Leschenault Inlet Estuary, 83, 377–418.
Bezerra, L. E. A., Dias, C. B., Santana, G. X., and Matthews- Silvestri, S., Defina, A., and Marani, M., 2005. Tidal regime, salin-
Cascon, H., 2006. Spatial distribution of fiddler crabs (genus ity and salt marsh plant zonation. Estuarine Coastal Shelf Sci-
Uca) in a tropical mangrove of northeast Brazil. Scientia ence, 62, 119–130, doi:10.1016/j.ecss.2004.08.010.
Marina, 70, 759–766. Tomlinson, P. B., 1986. The Botany of Mangroves. Cambridge:
Bridgewater, P. B., 1975. Peripheral vegetation of Westernport Bay. Cambridge University Press. Cambridge Tropical Biology
Proceedings of the Royal Society of Victoria, 87, 69–78. Series.
Bridgewater, P. B., and Cresswell, I. D., 1993. Phytosociology, and Unno, J., and Semeniuk, V., 2009. The habitats of the Western Aus-
phytogeography of Western Australian salt marshes. Fragmenta tralian soldier crab Mictyris occidentalis Unno 2008 (Brachyura:
Flora Geobotanica Supplementum, 2, 609–629. Mictyridae) across its biogeographical range. Journal of the
Chakraborty, S. K., and Choudhury, A., 1985. Distribution of fid- Royal Society of Western Australia, 92, 289–363.
dler crabs in Sundarbans mangrove estuarine complex, India.
In Bhosale, L. J. (ed.), Proceedings of National Symposium on
Biology, Utilization and Conservation of Mangroves. Kolhapur:
Shivaji University Press, pp. 467–472. Cross-references
Chapman, V. J., 1938. Studies in salt-marsh ecology: sections I to
III. Journal of Ecology, 26, 144–179. Mangroves
Crane, J., 1975. Fiddler Crabs of the World (Ocypodidae: Genus Saltmarshes
Uca). Princeton: Princeton University Press. Tidal Flat Salinity Gradient
622 SPIT

Wave refraction in multiple directions often induces the

SPIT formation of a complex spit system.
Wenyan Zhang
MARUM - Center for Marine Environmental Sciences, Bibliography
University of Bremen, Bremen, Germany Evans, O. F., 1942. The origin of spits, bars and related structures.
Journal of Geology, 50, 846–863.
Synonyms U.S. Army Corps of Engineers, 1984. Shore Protection Manual,
4th edn. Washington, DC: Department of the Army, U.S. Corps
Barrier spits; Headland spits; Sandspit of Engineers.
Definition
A spit is a coastal landform, a depositional ridge, or an Cross-references
embankment of sediment (Evans, 1942) with one end Bar
attached to a headland of the coast that serves as the source Barrier Spits
of sediment (proximal end) and the other end extending Beach Processes
into open water (distal end). It is younger than the head- Coastal Barriers
land to which it is attached. Coastal Cliffs
Coastal Landforms
Headland Breakwaters
Description Sediment Resuspension
Although spits can form and are maintained in a variety of Wave-Driven Sediment Resuspension
environmental settings, they develop most readily in large
lakes and wave-dominated coasts with a small tidal range,
which provides optimum conditions for undisturbed spit
development. Offshore waves normally approach the surf STORM SURGES
zone of a coast at an oblique angle. A combination of
shore-oblique swash caused by the incoming waves and
Harry C. Friebel
shore-normal backwash caused by gravity creates
U.S. Army Corps of Engineers, Philadelphia District,
a longshore drift of sediment which is further strengthened
CENAP-EC-EH, Philadelphia, PA, USA
by longshore currents generated by wave breaking.
Sediment is entrained by strong turbulence induced by
wave breaking, and it is transported down-drift along the Synonyms
coastline by longshore currents. Longshore sediment trans- Inundation; Surge
port rate remains constant if a uniformity of waves and near-
shore isobaths exist along the coastline (USACE, 1984). Definition
Net deposition of sediment occurs where the longshore
uniformity is broken by a decrease of wave energy. This is Storm surge refers to the increased water level above the
normally caused by a deepening of the bathymetry or predicted astronomical tide due to storm winds and atmo-
a change of the coastline orientation. In the latter case, spheric pressure changes.
the boundary constraint of the longshore currents by the Storm surge is principally produced by a storm’s wind
coastline no longer exists, and the currents are veered by stress that pushes water in the same direction as the wind
a barotropic pressure induced by wave radiation stress. (CHL, 2013). Low atmospheric pressure associated with
On the side to which the currents are veered, turbulence the storm makes a minor contribution to the overall storm
is dissipated by calm waters which cannot entrain the full surge. Storm surge is calculated by subtracting the astronom-
load. Much of the sediment is deposited as a result, ical tide from the observed water level during the storm. It is
forming a submerged bar. This submerged bar subse- considered the most significant hazard to life and property, as
quently acts to maintain the original direction of the the increased water level permits large waves to break inland
longshore currents and also serves as a reservoir for sedi- of the “normal” surf zone. Storm surge is complex and depen-
ment accumulation. Deposition of sediment on the sub- dent upon many contributing factors (NHC, 2013) including:
merged bar will not cease until a uniformity of waves Intensity: The greater the wind speed, the greater the wind
and nearshore isobaths is again achieved. Eventually stress and surge. Strengthening or weakening of the
a spit develops above water by this process. storm before landfall will play a significant role in the
When the submerged bar in front of a spit expands, it overall surge.
may cause significant wave refraction which deflects the Forward Speed: In a cyclonic storm, the relative wind
longshore currents around the distal end of the spit to form speed at any point is the vector sum of the local wind
a hook or a secondary recurved spit. A change in speed and the forward speed of the storm. For example,
prevailing wind direction may also cause similar effects. a cyclonic storm with a maximum wind speed of
 STRATIGRAPHY OF ESTUARIES 623

80 mph and forward speed of 50 mph will have relative whole estuary that represents the amalgamation of small-
winds of 130 and 30 mph on opposite sides of the storm. scale stratigraphic suites into estuarine-longitudinal and
Timing: The time of the storm with relation to the tide estuarine-transverse mosaics. At both scales, there are
makes a significant impact on the magnitude of storm a range of environmental factors and processes which con-
surge. For example, consider an area with an 8 ft normal tribute to a striking spatial variability of stratigraphy. Prior
tide range. If storm surge is 10 ft, at low tide the addi- to describing the small-scale stratigraphy and the litho-
tional water level would only be 2 ft above the normal logic detail of the large-scale stratigraphy, the setting of
high tide elevation; however, if the surge occurs at high estuaries and the processes and determinative factors lead-
tide, the water level would be 10 ft above the normal ing to sediment generation and accumulation are
high tide elevation at that time. described to provide an understanding of the development
Storm Size: The greater the storm size (radius of maximum of sedimentary and stratigraphic suites.
winds speed), the larger the surge and area the surge The landform settings and origin of estuaries are vari-
will impact. able, ranging from incised valleys, such as rias and fjords,
Angle of Approach: The angle of the storm path relative to to flooded valleys on coastal plains, to barred rias, and to
the coast will play a significant role in determining the barred coastal plain lagoons that have a riverine input
overall surge. Comparing two similar storms, the surge and an ocean outlet. Depending on their original geomor-
from a storm moving along the coast will be signifi- phology, the extent of inundation by the Holocene
cantly less than one making direct landfall. postglacial transgression, and local wave and wind energy,
Central Pressure: For significant storms (i.e., hurricanes), the forms of estuaries range from open estuarine bays, to
the central pressure on the water surface makes a small narrow estuarine bays or gulfs, to inlets that are partly
but finite contribution to the overall surge. barred or nearly fully barred basins. Estuaries also occur
in a wide spectrum of climates and reside in different
Bibliography coastal energy settings dominated either by waves, tides,
Coastal and Hydraulics Laboratory (CHL), 2013. U.S. Army Corps or wind or combinations of the three. Sediments derive
of Engineers. U.S. Department of Defense (available at http:// from a range of drainage basin types and sizes, and from
chl.erdc.usace.army.mil/glossary). Accessed 22 April 2013. different source rock provenances in different climatic set-
National Hurricane Center (NHC), 2013. National Oceanic & tings, resulting in variable volumes of sediment influx,
Atmospheric Administration (NOAA), U.S. Department of and different types and sizes of sediment particles. As
Commerce (available at http://www.nhc.noaa.gov). Accessed such, estuaries exhibit diverse attributes such as size and
1 May 2013.
shape, sediment sources, sediment influx rates, hydrody-
namic/sedimentary dynamics, and, of course, stratigraphic
Cross-references packages. Reflecting this variability in setting, coastal and
Extratropical Storms estuarine processes, and estuarine form (Figure 1), there
Extreme Events (Hurricanes) are a wide range of stratigraphic packages that can occur
in estuaries.
One of the settings in which estuaries also occur is in
the outlet mouths of distributary channels of large marine
STRATIGRAPHY OF ESTUARIES deltas, but in this context, the stratigraphy therein is that of
the enclosing delta (see Gould, 1970; Coleman et al.,
1970; Allen, 1970; Reineck and Singh, 1980) and will
Vic Semeniuk not be described here.
V & C Semeniuk Research Group, Warwick, The essence of an estuary is that it is a river-to-marine
WA, Australia transitional environment where marine salinity is measur-
ably diluted by (riverine) freshwater in a valley tract, an
Definition inlet, a coastal lagoon, or an embayment, producing
The stratigraphy of estuaries is the vertical and lateral a salinity gradient from the river to the sea. In terms of
array of sediments that occur in smaller-scale environ- hydrochemistry, biota, and processes, there are a riverine
ments within an estuary, as well as longitudinally and component toward the landward part of an estuary and
transversely across the whole of an estuary. a marine component toward the seaward part (Figure 2).
This salinity gradient from river to sea is mirrored in the
Stratigraphy of estuaries: overview and settings gradients in sedimentary processes, sediment types,
Estuarine stratigraphy can be viewed at two scales: firstly, facies, and stratigraphy. While this is a hydrochemical per-
the environmentally distinct small-scale stratigraphy, ception, geologists have emphasized the sedimentologic/
reflecting processes and products of internal estuarine stratigraphic aspects of an estuary (Dalrymple
geomorphic units and their sedimentary environments, et al., 1992), viewing the basin of an estuary as
and, secondly, at the larger scale, the stratigraphy for the a sedimentary sink for sediments deriving from both
624 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 1 Diagram showing a range of estuaries in terms of geometry and scale. These estuaries also occur
in a range of different climates (from tropical to temperate) and oceanographic settings (tide-dominated vs. wave-dominated). Some
of the estuaries shown here are used later to illustrate the variety and styles of stratigraphic fills, viz., the Thames, the Gironde, the
Peel-Harvey Estuary, the Lawley River Estuary, the Swan-Canning Estuary, the Walpole-Nornalup Estuary, the Leschenault Inlet
Estuary, and the Fitzroy River Estuary.
 STRATIGRAPHY OF ESTUARIES 625

Stratigraphy of Estuaries, Figure 2 A typical funnel-shaped estuary with its hydrochemical fields and marine, estuarine, and fluvial
processes that occur longitudinally along its length that determine the sedimentologic processes and responses (modified after
Dalrymple et al., 1992).

fluvial and marine sources and containing facies The riverine system is usually a shallow water system.
influenced by tide, wave, and fluvial processes. In this By channel flow and floods, it delivers sand (usually
context, the essence of an estuary is that it is a river-to- quartz sand), terrigenous mud (usually clay minerals and
marine transitional environment, which is reflected in the quartz silt), and gravel to the estuary. While these sedi-
change from riverine conditions (with its assemblage of ments are mainly located in the estuary headwaters, they
lithofacies) to marine conditions (with its assemblage of can be dispersed into the central parts of the estuarine
marine lithofacies), with the estuarine basin being basin, graded in grain size from coarsest sand at the deltas
a unique assemblage of facies (Figure 2). and river mouths to fine and very fine sand away from the
Sedimentologically, an estuary acts as a basin, deltas. Riverine mud is the sediment type that is most
semi-protected or nearly fully protected from the sea. widely dispersed, and, since it is carried in suspension, it
Therein, fluvial sediment can be delivered and largely can be deposited some distance from the river mouth.
trapped. Marine processes deliver sediment at its seaward The central estuary is a shallow water to moderately
portions in an open bay setting, or by marine coastal trans- deepwater system. The central estuary generates sand
port processes though narrow inlets, or by washover (as foraminifera, algal fragments, and fragmented inverte-
across a low barrier. Estuarine processes within the estua- brate skeletons), gravel (as invertebrate skeletons), mud
rine basin itself operate to develop intra-basinal sediment, (as comminuted thin-shell fragments, disintegrated algal
transport and disperse sediment, and develop sedimentary skeletons, diatoms, and sponge fragments), and
suites from the materials delivered from fluvial, marine, biogenically built structures such as biostromes (e.g., mus-
and intra-basinal estuarine sources. The magnitude of the sel beds), bioherms (e.g., oyster reefs and worm-tube
tidal range for the region where the estuary resides, and/or reefs), and weed-built, weed-constructed, or weed-trapped
the extent that the shape of the estuary magnifies the tidal sediment sheets. These particle types and biogenic sedi-
range, will determine how far upstream tidal effects mentary products are mainly located in the central part
are experienced and to what degree tidal patterns will of the estuary, while plant products and biostromes/
influence sedimentation patterns (Figure 2). bioherms specifically develop in the shallow water mar-
The sources and types of sediment that build ginal parts of the estuary. However, through intra-
stratigraphic sequences, their delivery system to the estuarine transport processes, the mud-sized and sand-
estuary, and where the sediment finally is emplaced in sized particles listed above can be dispersed into deeper
the estuary are described below in four systems, viz., the water parts of the central part of the estuary. As with flu-
riverine system, the central estuary, the marginal estuarine vial mud, mud-sized particles generated within the estuary
system, and the marine system. are the most widely dispersed.
626 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 3 Main sedimentary sources and pathways of sediment movement within the Walpole-Nornalup
Inlet Estuary, a twin ria system with three river inputs, illustrating the principles of sediment particle sources and transport.

The central estuary is also the location where mud column and transported into this low-energy area. As such,
delivered by rivers accumulates, because it is generally the sediments accumulating in the central estuary range
the deepest part of the estuary and a low-energy sink. In from muds to organic-matter-enriched muds. Without bio-
addition, the central basin is the site for accumulation of turbation, the muds in the central estuary are laminated.
organic matter generated by shallow water biotic assem- However, more generally, the muds in the central estuary
blages that might have been suspended in the water are thoroughly bioturbated. The central estuary is also
 STRATIGRAPHY OF ESTUARIES 627

Stratigraphy of Estuaries, Figure 4 Main sedimentary sources and pathways of sediment movement within the Leschenault Inlet
Estuary, an elongate shore-parallel estuarine lagoon barred by a dune barrier, and with river inputs restricted to its southern end, to
illustrate the principles of sediment particle sources and transport.

the site for accumulation of aeolian very fine sand and silt, The marginal estuarine system is a shallow water to
usually deriving from dune barriers seaward of the estuary. geomorphically emerged system. It receives exogenic sed-
In these circumstances, if the muds of the central estuary iment from a number of sources. Sediment may be
are laminated, particle-width laminae of aeolian sand and reworded and delivered from adjoining uplands, other
silt define some of the lamination. supratidal locations, or alongshore from elsewhere in the
628 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 5 The progressive obliteration of primary sedimentary structures in sediments reflecting the
relative balance between biota abundance and the rapidity of sedimentation. The primary sedimentary structures, once diagnostic of
an environment, can be reduced firstly to root-structured or burrow-structured sediments (within which vestiges of the former
structures are discernable) and then finally to a thoroughly bioturbated sediment (in which only grain sizes, grain types, and the
presence of biota can be determined for use as environmental indicators).

estuary (as sand, mud, or gravel), or it may consist of peat form shoreline spits, ribbons, or platforms on the leeward
from plants inhabiting shorelines and marginal shallow margin of the barrier. The tidal-delta plumes and washover
water environments, or carbonate mud (in marginal lobes encroach into the estuary, often migrating into and
lagoons) generated from algal meadows, or mud brought overlying the sediments of the deeper water basin.
in by suspension on the high tide or by storms. Depending The variety of sediment sources and processes that
on whether the surrounding uplands are rocky, preexisting emplace the particle types and sediment types in an estu-
older sedimentary deposits, or stranded estuarine deposits, ary is shown in a case study of the Walpole-Nornalup
the material reworked by sheetwash, shoreline erosion, or Inlet Estuary and the Leschenault Inlet Estuary both in
fluvial action may be lithoclast (rock) gravel, sand Western Australia (Figures 3 and 4). In the former, the
(usually quartz), and mud. These sediments are shed onto surrounding uplands are weathered Precambrian rock,
the supratidal to high-tidal parts of the estuary. The tidal to Cainozoic quartz sand, and a Quaternary dune barrier,
shallow subtidal marginal estuarine environments, and the rivers deliver sand and mud, and there is an
through prevailing wave action, storm, and tide activity, intra-basinal production of biogenic particles. In the lat-
generally comprise a sediment platform underlain by sand ter, the uplands are Pleistocene dune sands and
and (in low-energy settings) mud. The shallow water biota a Holocene dune barrier, and the rivers deliver sand and
thereon contributes shell and fragments, algal fragments, mud, and there is an intra-basinal production of biogenic
diatoms, and plant organic matter. particles. These examples provide case studies of sedi-
The marine system is usually a shallow water environ- ment sources and pathways, firstly, in a barred ria system
ment. The marine system delivers sand (as quartz and where there are three sites of river input, though the river-
marine skeletal material, viz., foraminifera, algal frag- ine input is axial, and, secondly, in a barrier-and-lagoon
ments, and fragmented invertebrate skeletons) and shell estuary system where river input is at one end of the estu-
gravel to the estuary. These particles are transported as arine lagoon.
sand sheets by waves, tides, and storms, across and into
the entrance of the estuary, or are funneled by waves and
tides through the narrow tidal inlet, or multiple inlets Small-scale local environment stratigraphy
(that breach the barrier), to form radiating to palmate In the estuarine environment where the sediment either is
plume(s) of sediment (the flood-tidal delta). Marine delivered exogenically or is generated intra-basinally, the
sediment is also transported during storms into the estuary local processes of wave action, tidal currents, fluvial cur-
across low barriers to form washover lobes or is reworked rents, wind, hydrochemical processes, shoreline freshwa-
from the leeward barrier by estuarine waves and tides to ter seepage, and biogenic activity (exoskeleton and
 STRATIGRAPHY OF ESTUARIES 629

Stratigraphy of Estuaries, Figure 6 The lithologies and sedimentary structures in the various sub-environments of a tide-dominated
estuary exemplified by an estuary such as the Gironde (Adapted from Allen and Posamentier, 1993). This diagram extends the results
of Allen and Posamentier (1993) for a Gironde-type estuary in that, where there is benthos abundant in the aquatic and tidal
sub-environments/facies and vegetation inhabits tidal and riverine sub-environments/facies, the effects of bioturbation have been
added.

endoskeleton production generating shell gravel, skeletal A significant factor in the development of facies-
sand, and biostromes and reefs, plant production, biotur- specific lithologies and their potential as diagnostic fea-
bation, and biomediated mineralization) result in tures in identifying environments is the extent to which
a variety of sedimentary processes and products. These bioturbation may overprint primary sedimentary struc-
include biogenic particles such as mud, sand, and gravel; tures. Many primary sedimentary structures such as lami-
partitioning of grain sizes of particles of both exogenic nation, wavy lamination, flaser bedding, and climbing
and intra-basinal origins; dispersal/transport of sediment; ripple structures, among others, are diagnostic indicators
shaping of bedforms and generation of sedimentary struc- of hydrodynamic conditions and formative environments
tures by wave action and tidal current; bioturbation and (Reineck and Singh, 1980) and can be used to identify spe-
root structuring; accumulation of sediment with its signa- cific estuarine environments. However, bioturbation by
ture lithology for a given facies; and (with lateral animals and/or plants can destroy the sedimentary struc-
progradation and vertical accretion) development of ture evidence, reducing various individual diagnostic
facies-specific stratigraphic packages. lithologies to a similar appearance. Figure 5 shows the
630 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 7 The range of sub-environments/facies that may occur in an estuary. The diagram is not specific
to a fluvial-, wave-, or tide-dominated estuary, but is generalized. Each of these sub-environments/facies may/will have diagnostic
sedimentary features and biota.
 STRATIGRAPHY OF ESTUARIES 631

Stratigraphy of Estuaries, Figure 8 The idealized diagnostic small-scale sedimentary features and their bioturbated equivalents of
the different sub-environments/facies of a wave-dominated estuary (Diagram based on estuarine model of Dalrymple et al. (1992),
with approach of Allen and Posamentier (1993), for illustrating the character of the small-scale facies).

process whereby distinct sedimentary structures that may the overprint of bioturbation because, depending on the
be or are diagnostic of specific environment are progres- occurrence and abundance of biota, either end product
sively obliterated by animal bioturbation or by root struc- can eventuate.
turing. Bioturbation overprinting and obliterating primary As noted earlier, viewed in an overall context, an estu-
sedimentary structures is typical in shoreline estuarine ary can be wholly river-dominated, wave-dominated, or
environments in humid climates where there is much veg- tide-dominated or can be river-dominated at its upstream
etation or in tropical climates where, in addition to shore- part and wave-dominated or tide-dominated at its seaward
line vegetation, there is a diverse and abundant benthos. In part. The extent that an estuary is dominated by river,
this context, the illustrations that follow are shown as wave, or tide processes and where these environments
small-scale primary sedimentary structures and also with occur will determine the nature of small-scale stratigraphy
632 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 9 The idealized diagnostic small-scale sedimentary features and their bioturbated equivalents of
the different sub-environments/facies of a tide-dominated estuary (Diagram based on estuarine model of Dalrymple et al. (1992),
with approach of Allen and Posamentier (1993), for illustrating the character of the small-scale facies).

at a given site. In this context, for instance, using the 1. The major estuarine-head delta(s)
Gironde Estuary from France as a model, Allen and 2. Subsidiary deltas
Posamentier (1993) highlighted the facies types in 3. Marginal estuary alluvial fans
terms of lithology and sedimentary structures along 4. Spits and their lagoons
tide-dominated estuaries and the small-scale stratigraphy 5. Marginal estuary environments, comprising plat-
diagnostic of each sub-environment within such estuaries forms, tidal flats, mangrove- and/or salt marsh-
(Figure 6). vegetated tidal flats, and biostromes
The main sedimentary environments, or facies, that 6. Central estuary basin, comprising deepwater
occur within an estuary from landward to seaward are as environments
follows (Figure 7): 7. Central estuary channels
 STRATIGRAPHY OF ESTUARIES 633

Stratigraphy of Estuaries, Figure 10 The idealized whole-of-estuary down-valley-tract longitudinal stratigraphy in a wave-
dominated estuary showing transgressive and progradational relationships (Diagram simplified and modified from Dalrymple et al.,
1992). While the lithologic suites related to setting in the estuary are generalized and not lithology specific, the lithologic details of
the various large-scale stratigraphic units will be determined by climate setting, wave energy, sediment types supplied, and biota.

Stratigraphy of Estuaries, Figure 11 The idealized whole-of-estuary down-valley-tract longitudinal stratigraphy in a tide-dominated
estuary showing transgressive and progradational relationships (Diagram simplified and modified from Dalrymple et al., 1992). While
the lithologic suites related to setting in the estuary are generalized and not lithology specific, the lithologic details of the various
large-scale stratigraphic units will be determined by climate setting, tidal range, sediment types supplied, and biota.

8. Tidal channels to a setting of a wave-dominated or a tide-dominated estuary

9. Tidal channels fringed by biostromes but rather to a “general estuary.” However, these types of
10. Central estuary shoals, some vegetated by mangrove environments (or estuarine sub-environments) or facies gen-
and/or salt marsh erate specific suites of sediments and, with accretion,
11. Margin of leeward side of the barrier a specific stratigraphy. This expression of stratigraphy at
12. Flood-tidal delta the specific environmental level or the level of geomorphic
13. Washover fans unit is small-scale stratigraphy, which focuses on the details
of sedimentary structures, lithologies, lithologic
Each of these environments (or estuarine interlayering, and biota imprints preserved/evident within a
sub-environments or facies) listed above has not relegated given facies, estuarine sub-environment, or geomorphic unit.
634 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 12 The idealized whole-of-estuary stratigraphy in a wave-dominated estuary showing lithofacies
types according to location within the estuary, but not as a progradational sequence (Diagram modified from Dalrymple et al., 1992).
Again, while the lithologic suites related to setting in the estuary are generalized and not lithology specific, the lithologic details of
the various large-scale stratigraphic units will be determined by climate setting, tidal range, sediment types supplied, and biota.

The overall estuarine geometry, as well as more specif- small-scale sedimentary features that are diagnostic of
ically the mid-estuary geomorphology and processes, the different sub-environments of a wave-dominated and
determines how facies within an estuary are formed and a tide-dominated estuary, respectively, and the small-scale
how they are interrelated, are juxtaposed, or overlap at sedimentary features of each where bioturbation has
the large scale. For instance, an estuarine basin that is become dominant.
semi-closed or meandering, both with limited fetch, will Small-scale stratigraphy is also affected by the climate
not generate intra-estuarine wind waves nor be subject to setting of the estuary itself in that climate influences the
ocean swell and waves that can penetrate into the estuary. development of lithology, particularly in the tidal and
Such estuaries will be dominated by fluvial processes and supratidal environments, where facies have become emer-
tidal processes, depending on whether tides are macrotidal gent (shoaled) to levels of the highest tide or to levels
or microtidal and how far the tides penetrate into the river- above high tide (supratidal) by storm sedimentation and
ine sector. However, estuaries with a large surface water inhabited by vegetation. The climate setting also can
area, even though barred or semi-enclosed, if located in determine, for instance, the biodiversity and abundance
a region of strong winds, can be wave-dominated inter- of shell material contributed to the estuarine facies, and
nally because of the large intra-estuarine fetch. Estuaries the extent and type of marginal facies that will be devel-
that are funnel-shaped and open to the sea in the appropri- oped and, thirdly, for stratigraphic sequences that have
ate orientation can be wave-dominated because they shoaled to tidal and supratidal levels, whether the facies
receive swell and wind waves from the open marine envi- that cap the sequences are peat (in humid climates), root-
ronment that penetrate far into the estuary. structured lithologies formed in salt marsh environments,
Small-scale stratigraphy at any given site is influenced such as root-structured and bioturbated sand, muddy sand,
by the extent to which an estuary is river-dominated, or mud (in humid to subhumid climates), or root-
wave-dominated, or tide-dominated and where these envi- structured, bioturbated, desiccated sand, muddy sand, or
ronments occur. Figures 8 and 9 illustrate the idealized clay-mineral mud (in semiarid to arid climates). Also, if
 STRATIGRAPHY OF ESTUARIES 635

Stratigraphy of Estuaries, Figure 13 The idealized whole-of-estuary stratigraphy in a tide-dominated estuary showing lithofacies
types according to location within the estuary, but not as a progradational sequence (Diagram modified from Dalrymple et al., 1992).
Again, while the lithologic suites related to setting in the estuary are generalized and not lithology specific, the lithologic details of
the various large-scale stratigraphic units will be determined by climate setting, tidal range, sediment types supplied, and biota.

calcareous algae are present in estuaries of drier climates, Climate setting of the river drainage basin determines to
carbonate mud may become a dominant or contributing a large extent the amount of weathering and erosion of
component of the lithologic suite. source materials for the rivers and, hence, the volume
and types of sediment and particle sizes that may be deliv-
ered to the estuary. Estuaries that have low influx of river-
Large-scale whole-of-estuary stratigraphy ine sediment will be sedimentologically depauperate, and
Large-scale stratigraphic accretion within an estuary is here the estuaries may consist of a rock-floored and rock-
determined by the volume of sediment input. Using valley walled basins or may have only a veneer of modern sedi-
tracts (as the receiving basin reservoir) as examples for ment on the postglacial unconformity. Where there is little
estuarine sedimentary fill, three types of sedimentary/ sediment delivery from marine environments, then,
stratigraphic sequences can be identified: (1) largely equally, the marine part of the estuary will also be sedi-
unfilled valley tracts and basins (wherein sedimentary ment depauperate. In the sediment veneers, facies changes
sheets and veneers occur), (2) partly filled valley tracts will occur reflecting the environments from river to sea,
and basins, and (3) fully filled valley tracts and basins. and the sediments of such estuaries will consist of a thin
The most significantly developed stratigraphic sequences sheet of riverine sediment, grading to sediment with an
are developed in those estuaries that have large volumes estuarine signature, to that of marine sediment in a river-
of sediment delivered by fluvial and marine sources and to-sea transect.
a significant intra-estuarine sediment contribution. These The stratigraphy of fully shoaled or nearly filled estua-
comprise the partly filled and wholly filled valley tracts rine basin sequences has a common pattern: a pre-estuarine
and basins. stratigraphy representing Pleistocene deposits, palaeosols,
636 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 14 A series of transverse complex stratigraphic transects from south to north along the Fitzroy
River Estuary. The transects show fluvial-dominated stratigraphy to the south (inset D), followed by complex relationship further
north between early Holocene mangrove-developed mud deposits with large-scale cut and fill of later Holocene shoaling sand-to-
mud tidal deposits (inset C), a shoaling Holocene sand-to-mud sequence in central parts (inset B), and veneers on pre-Holocene
sediments and rock to the (distal) north (inset A) (Information from Semeniuk, 1980, Semeniuk, 1981 and Semeniuk and Brocx, 2011).

or bedrock, overlain by fluvial deposits, and then estuarine shoals with some vegetated by mangrove and/or salt
deposits, followed by either a capping of deltaic deposits, marsh; margin of leeward side of the barrier; flood-tidal
post-estuarine alluvial plain deposits, or transgressive bar- delta; and washover fans) generates a specific and distinc-
rier dune deposits. tive suite of sediments and, with accretion, will develop
At the scale of the estuarine basin, each of the estuarine either a longitudinal sequence or a mosaic of stratigraphic
sub-environments listed above (viz., the major estuarine- units along and across the estuary basin, respectively, as
head delta(s); subsidiary deltas; marginal estuary alluvial will be shown later in the case studies. The extent that indi-
fans; spits and their lagoons; marginal estuary environ- vidual facies are dispersed, or migrate laterally or down-
ments, comprising platforms, tidal flats with some vege- slope, will determine the extent that they are recorded in
tated by mangrove and/or salt marsh, tidal flats with adjoining facies and the extent that interfingering,
biostromes; central estuary basin, comprising deepwater interlayering, or encroachment takes place and hence the
environments; central estuary channels; tidal channels; extent of the complexity of the whole-of-estuary
tidal channels fringed by biostromes; central estuary stratigraphy.
 STRATIGRAPHY OF ESTUARIES 637

Stratigraphy of Estuaries, Figure 15 Longitudinal stratigraphic transect in the Lawley River Estuary showing a mud-dominated
sequence. Fluvial deposits to the south are minimal on bedrock as compared to the thickness of the tidal and subtidal mud deposits
(Information from Semeniuk, 1983, Semeniuk, 1985a).

Stratigraphy of Estuaries, Figure 16 Longitudinal stratigraphic transect in the Klang-Langat Estuary showing a sand-to-mud
shoaling sequence, capped by freshwater peat (Modified from Coleman et al., 1970).

The expression of stratigraphy at the estuarine basin at the small scale. Most studies of estuarine stratigraphy
scale is a large-scale stratigraphy, which focuses on the have been at the basin scale of a large-scale stratigraphy
geometry and interrelationships of the various gross facies and, in particular, longitudinal stratigraphy (Dalrymple
within the estuary. For complex estuaries, the stratigraphy et al., 1992).
at the large scale will be an aggregate or amalgamation of To date, there has been an emphasis on valley-fill estu-
juxtaposed and/or onlapping facies that have been formed arine stratigraphy (Dalrymple et al., 1992) usually within
638 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 17 Longitudinal stratigraphic transect in the estuary of the Gironde showing a sand-and-mud
shoaling sequence and the relationship between the estuarine fill and the fluvial sediments (Modified from Allen and Posamentier,
1993).

a linear single valley tract, and this has provided valuable River Estuary of northern Australia, the Lawley River
models of the evolution and architecture of whole-of- Estuary of northwestern Australia, and the Leschenault
estuary stratigraphy as summarized in Figures 10, 11, 12, Inlet Estuary of southwestern Australia, among others,
and 13. Figures 10 and 11 show generalized stratigraphy stand as examples of contrasting stratigraphic fills in
in a wave-dominated and a tide-dominated estuary, in estuaries. While the longitudinal facies variation in an
a context of transgression, followed by sedimentary incised valley estuary provides some degree of heteroge-
progradation, with simplified lithologic suites. Figures 12 neity in such estuaries, the full range of heterogeneity, or
and 13 show generalized stratigraphy in a wave- even homogeneity, has not been addressed in these
dominated and a tide-dominated estuary, in a context models. For instance, heterogeneity can be determined
of the simplified lithofacies located according to the by deltas, i.e., on the number of deltas, their positions
sub-environments within the estuary, but not as a in the estuary and hence their stratigraphic contribution
progradational sequence. to the head and to the central parts of the estuarine
The stratigraphic models for estuaries, focused on embayment, their position relative to basin fetch, and
wave-dominated types and tide-dominated types within hence whether they are wave-, tide-, or fluvially domi-
linear, single valley tracts (Dalrymple et al., 1992; nated, and the volume and type of sediment delivered
Figures 10, 11, 12, and 13), have provided an overarch- by contributing river(s), and thus whether the deltas
ing view of their facies and stratigraphic architecture are mud-dominated, sand-and-mud-dominated, or sand-
and have presented the stratigraphic evolution of an estu- dominated. In the complex Peel-Harvey Estuary
ary in a conceptualized and uniform manner. However, of southwestern Australia, for example, the deltas facing
while this overarching approach for incised valley sys- the prevailing summer southwesterly wind waves
tems provides a useful model to characterize many estu- across a large estuary fetch are wave-dominated, while
arine sedimentary fills and their longitudinal one in the south of the estuary is fluvially dominated
stratigraphy, it does not provide a framework for, or (Semeniuk and Semeniuk, 1990a; Semeniuk and
explanation of, the full range of relatively homogeneous Semeniuk, 1990b). Similarly for the complex Walpole-
stratigraphy or for the longitudinally and transversely Nornalup Inlet Estuary of southwestern Australia, each
heterogeneous stratigraphy found in many other types of the deltas faces estuarine waters of differing fetch
of estuaries in the variety of climatic settings and other and wind-wave trains such that they develop different
coastal settings in which they occur. The South Alligator delta types and delta stratigraphy.
 STRATIGRAPHY OF ESTUARIES 639

Stratigraphy of Estuaries, Figure 18 Longitudinal stratigraphic transect in the estuary of the Thames showing a mud-dominated
sequence, with local lenses of peat and sand (the stratigraphic section has been interpreted and constructed from information in
Morgan, 2006, and Khan et al., 2011)

Estuarine stratigraphic patterns will also depend on prograded sandy beach ridges, while those sheltered from
hydrodynamic patterns, dispersal of sediments, estuarine the prevailing wave directions are mud-dominated. Also,
geometry, and factors which can result in cross-estuarine estuaries developed by marine flooding of a complex
heterogeneity or in facies asymmetry. For example, the meandering river on a coastal plain will result in
Leschenault Inlet Estuary of southwestern Australia, a geomorphically and sedimentologically complex estu-
a north–south-oriented shore-parallel estuarine lagoon, is ary, with a resultant complex whole-of-estuary stratigra-
subject to southwesterly wind waves that rework its east- phy. In summary, the heterogeneous geometry, setting,
ern shores such that it is sand-dominated, whereas the and internal features of estuaries may directly contribute
western shore is spit, chenier, mud, and muddy sand- to a heterogeneous stratigraphy.
dominated (Semeniuk, 2000). The Peel-Harvey Estuary Thus, heterogeneity, complexity, and variability in an
of southwestern Australia, mentioned above, provides estuary, in terms of shape, hydrodynamics (viz., river
another example. Although functioning ecologically and vs. internal estuarine processes vs. magnitude of tides,
hydrochemically as a single estuary, it is geomorphically ocean waves, and wind), and sediment sources and vol-
a compound estuary. It has developed heterogeneous stra- umes, will result in complex estuarine topography, inter-
tigraphy as a result of residing in a complex geological nal geomorphology, and facies. For instance, in shore-
framework, with local source materials, river dynamics, parallel elongate estuary, with a sand supply from eroding
fetch, and estuarine hydrodynamic processes (Semeniuk estuarine shores, and local sand (and not riverine sand),
and Semeniuk, 1990a). For instance, its shores facing under appropriate wind directions and wind waves, there
a long fetch are wave-dominated and composed of can be a major source of sediment transported alongshore
640 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 19 Generalized longitudinal stratigraphic transects in two estuary types from southeastern
Australia showing the style of stratigraphic filling in these sand-and-mud-filled estuaries (Modified from Roy et al., 1980 and Roy,
1984).

to form beach ridges and cuspate forelands, that can pro- sea-level highstands. Allen and Posamentier (1993), for
grade into the estuarine basin. Similarly, a shore-parallel instance, document the stratigraphic sequence of an
elongate estuary with a barrier dune migrating inland incised valley fill in the Gironde Estuary, France, record-
may transgress into the estuary or partition the estuary or ing a diverse assemblage of lithofacies (that are grouped
at the least contribute to estuarine shore facies. Also, elon- into lowstand, transgressive, and highstand system tracts)
gate estuaries with mobile sand along their shore (derived and sedimentation within the current estuary (in terms of
from an eroding shore mentioned earlier) can develop estuary mouth, estuary funnel, a zone of varying types of
accretionary spits and cuspate forelands that segment the estuarine channels, and the alluvial plain). The valley-fill
estuary (Zenkovitch, 1959; Bird, 1969), with the accre- stratigraphic sequence begins during the Würm global
tionary sedimentary bodies, manifesting as sand bodies sea-level lowstand, with accumulation of a continuous
“invading” from the estuary margins into the interior of unit of relatively thin fluvial gravel and coarse sand in
the estuary, locally encroaching over basinal muddy the thalweg of the incised valley. The transgressive system
deposits. In contrast, an estuary founded on a bifurcating tract, accumulated during the Holocene sea-level rise, com-
tributary system, but now largely barred by a barrier, prises the bulk of the incised valley fill and forms a
may be digitate/palmate in geometry, with complicated landward-thinning wedge of tidal-estuarine sands and
hydrodynamics resulting in complicated sedimentary muds. In the estuary mouth, these are overlain by a thick
dynamics and facies (of sandy spits, bars, cuspate fore- unit of coarse-grained tidal-inlet and tidal-delta sands. The
lands, and mud-filled lagoons leeward of these spits, bars, highstand system tract forms a seaward-prograding, tide-
and cuspate forelands), as the wind-wave fields interact dominated estuarine bayhead delta that has been gradually
with complex estuarine form and shore orientations. filling the estuary since the post-Holocene stillstand.
Complexity in stratigraphy of estuarine valley fills can In addition, complexity and variability can result from
also reflect the various depositional regimes in relation climate effects and climate setting, which can determine
to sea-level lowstands, transgressive phases, and factors such as the formation of peat, the extent of
 STRATIGRAPHY OF ESTUARIES 641

Stratigraphy of Estuaries, Figure 20 Transverse stratigraphic transects located along the length of the South Alligator River valley
tract in northern Australia showing a sand-and-mud-filled system and the stratigraphic architecture along different segments of the
estuary (Modified from Woodroffe et al., 1985, Woodroffe et al., 1986).
642 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 21 Location of stratigraphic transects in the twin ria Walpole-Nornalup Inlet Estuary. The map of
the estuary also shows the location of the three deltas that enter the estuary, the shallow water sand platforms, the flood-tidal sandy
delta, and the deepwater mud basin (Information from Semeniuk et al., 2011).

freshwater seepage from uplands bordering the estuarine geomorphically simple tide-dominated sand-and-
shore, the biodiversity and productivity of plant and ani- mud-filled Klang-Langat Estuary in Malaysia
mal biota, and the processes and products of evaporation. (Coleman et al., 1970; Figure 16)
Biogeography and animal productivity can determine the 3. Valley-fill stratigraphy in a macrotidal setting in a tem-
extent that biostromes and reefs are developed as strati- perate climate using the relatively geomorphically sim-
graphic units. Plant productivity can determine the extent ple tide-dominated estuary of sand-and-mud-filled
that peat in high-tidal and supratidal areas and seagrass- Gironde and the tide-dominated sand-and-mud-filled
trapped sand sheets in tidal and shallow subtidal areas Thames estuary (Allen and Posamentier, 1993; Mor-
are developed as stratigraphic units. A selection of exam- gan, 2006; Khan et al., 2011)
ples of the variability of large-scale whole-of-estuary stra- 4. Valley-fill stratigraphy in a microtidal to mesotidal set-
tigraphy from a range of case studies is provided below. ting in a temperate climate using the moderately
The examples for the large-scale whole-of-estuary stratig- geomorphically complex mixed wave and tide-
raphy are as follows (the shape and size of most of these dominated sand-and-mud-filled estuaries of southeast-
estuaries are shown in Figure 1): ern Australia (Roy et al., 1980; Roy, 1984)
5. Stratigraphy in a meandering valley, on an alluvial
1. Valley-fill stratigraphy in macrotidal settings in plain in a macrotidal setting in a tropical climate using
a tropical climate using the relatively geomorphically the relatively geomorphically complex tide-dominated
simple but sedimentologically complex tide- sand-and-mud-filled South Alligator River Estuary of
dominated sand-and-mud-filled Fitzroy River Estuary northern Australia (Woodroffe et al., 1985, 1986)
(King Sound) and the relatively geomorphically and 6. Valley-fill stratigraphy in a microtidal setting in a
sedimentologically simple tide-dominated mud-filled temperate climate along a wave-dominated coast using
Lawley River Estuary of northwestern Australia the relatively geomorphically complex intra-estuarine
(Semeniuk, 1980, 1981, 1983, 1985a; Semeniuk and wave-dominated sand-and-mud-filled Walpole-
Brocx, 2011; Figures 14 and 15) Nornalup Inlet Estuary (Semeniuk et al., 2011)
2. Valley-fill and gulf-fill stratigraphy in a macrotidal 7. Stratigraphy in a meandering valley on a coastal plain
setting in a tropical climate using the relatively in a microtidal setting in a temperate climate using
 STRATIGRAPHY OF ESTUARIES 643

Stratigraphy of Estuaries, Figure 22 Longitudinal stratigraphic profile along the southern basin of the Walpole-Nornalup Inlet
Estuary from a large delta to across the mud basin to the flood-tidal delta and two local transects across the other two deltas with
their leeward mud-and-peat-filled lagoons. Insets show more stratigraphic detail of the sandy deltas and flood-tidal delta of the main
transect. Information from Semeniuk et al. (2011). Note that two of the deltas are capped by peat or have peat in their stratigraphic
development.
644 STRATIGRAPHY OF ESTUARIES

Stratigraphy of Estuaries, Figure 23 Longitudinal stratigraphic profile along the Canning River delta in the Swan-Canning Estuary of
southwestern Australia showing a sand-and-mud fluvial system, a sand-dominated delta, and a mud-dominated estuarine basin.

the relatively geomorphically simple river-dominated Inlet Estuary and Klang-Langat Estuary); (4) they are
sand-and-mud-filled Swan-Canning Estuary of south- dominantly a mud-filled estuarine lagoon with sand mar-
western Australia ginal platforms and with a small component of deltaic
8. Stratigraphy in a coastal estuarine lagoon in deposits, and the estuarine fill will be a river-derived
a microtidal setting in a subtropical climate using the mud ribbon capped by estuarine sand sheets reworked
moderately geomorphically complex river-dominated from barrier dunes and Pleistocene dunes and
to tide-dominated sand-and-mud-filled Leschenault finally covered by sands of the retreating coastal barrier
Inlet Estuary of southwestern Australia (Semeniuk, dunes (the Leschenault Inlet Estuary); (5) they
2000) comprise small component of deltaic deposits in essen-
9. Stratigraphy in a compound estuary on a coastal plain tially a sediment-depauperate estuarine system (the Swan-
in a microtidal setting in a subtropical climate using Canning Estuary); and (6) they comprise complex
the relatively geomorphically complex river- compound systems with a variety of shoals and shoreline
dominated to wave-dominated sand-and-mud-filled protuberances that complicate the idealized transition
Peel-Harvey Estuary of southwestern Australia from delta to estuarine mouth shoals (the Peel-Harvey
(Brown et al., 1980; Semeniuk and Semeniuk, 1990a; Estuary) (Figures 17, 18, 19, 20, 21, 22, 23, 24, 25,
Semeniuk and Semeniuk, 1990b) and 26).
The examples span climates from tropical to temperate,
tidal regimes from macrotidal to microtidal, and sedimen- Summary and discussion
tologically from sand-and-mud accumulations to Stratigraphy of an estuary can be complex and dependent
mud-dominated systems. on a wide range of variables, from the large scale involv-
Of the range of examples provided above, the Gironde ing regional setting, estuary origin and shape, and ocean-
Estuary, those of southeastern Australia, and perhaps the ography to the small scale involving lithogenesis and
South Alligator River conform with the idealized model biota. While there is complexity in estuaries and much
of incised valley estuarine stratigraphy. The other estuar- variability between them, for each of the estuarine types,
ies provide variations and complications as follows: with their own hydrodynamic settings and sediment
(1) they are mud-dominated or mud-dominated with some sources, there appears to be a recurring pattern of
peat (Lawley River Estuary and the Thames, respec- (a limited range of) stratigraphic fills both at the small
tively); (2) they contain earlier Holocene deposits accu- scale of the environment-specific level and at the large
mulated in a different climate and different height of scale of the whole-of-estuary level.
MSL (Fitzroy River Estuary) and, as such, the strati- This contribution provided information on stratigraphy
graphic fill is not simple; (3) they are capped by peat of estuaries at two scales – that of the facies-specific scale
because of their humid climate setting (Walpole-Nornalup and that of the whole-of-estuary scale in longitudinal and
 STRATIGRAPHY OF ESTUARIES 645

Stratigraphy of Estuaries, Figure 24 Transverse stratigraphic transects located along the length of the Leschenault Inlet Estuary
showing a sand-and-mud-filled estuary and sand-and-mud deltas. Information from Semeniuk (2000). The mud fills the central
elongate estuarine basin, and sand underlies the shallow water platforms.

transverse profiles, in relatively stratigraphically homoge- large-scale whole-of-estuary stratigraphy, is the climate
neous estuaries, and in heterogeneous estuaries. setting of the estuary. Climate can result in a range of
A major factor in determining the lithology, and lithologic types and stratigraphic types, particularly in
both the small-scale facies-specific stratigraphy and the the tidal and supratidal environments, and it will
646 STRATIGRAPHY OF ESTUARIES

of vegetation bioturbation in tidal/supratidal environ-

ments, and contain a relatively depauperate diversity of
shell benthos in lower tidal and shallow subtidal environ-
ments. The stratigraphy of estuaries in drier climates may
have less peripheral vegetation and tend to preserve sedi-
mentary lamination. Also, if calcareous algae are present
in estuaries of drier climates, carbonate mud may become
a component of the lithologic suite.
Another major factor in determining the lithology, the
small-scale stratigraphy, and the large-scale stratigraphy
of estuaries is the coastal hydrodynamic setting of an estu-
ary and the intra-estuarine hydrodynamics, i.e., whether
the estuary is river-dominated, tide-dominated, or
set along a wave-dominated coast. Internal to an estuary,
the hydrodynamics determined by how far tides penetrate
into the estuary, and the wind patterns and fetch particu-
larly influence the array of estuarine facies that are
developed.
For incised valley systems, with an up-valley river
source, the stratigraphy is longitudinally graded because
of down-valley facies of estuaries, and this will be
a major factor in the development of the stratigraphic
architecture of an estuary. The nuances of ocean-side
facies development in the down-valley facies transition
will be determined by whether the oceanographic setting
is wave-dominated, tide-dominated, or mixed wave- and
tide-influenced. In wave-dominated environments, the
ocean-side environments will often develop a barrier,
which under strong wind conditions or coastal retreat con-
ditions may transgress over the estuarine deposits
(Semeniuk 1985b). The geomorphic perforations in the
barrier are sites of flood-tidal deltas, with their specific
stratigraphic geometry and lithologic signature. During
storms, the barrier may be breached or overtopped to form
washover sediment fans or lobes that encroach into the
estuary prograding over deeper water lithofacies. In tide-
dominated environments, the ocean-side facies will often
Stratigraphy of Estuaries, Figure 25 Location of stratigraphic develop tidally oriented shoaling sand bars. But while
transects in the complex compound Peel-Harvey Estuary. The
map of the estuary also shows location of the three deltas that there may be a recurring overall pattern to facies/strati-
enter the estuary, the shallow water partitioning shoals, the graphic architecture longitudinally along the whole of
flood-tidal sandy delta, the deepwater mud basin, and the estuary in both situations, the details of lithology in site-
complexity of shore types (Modified from Semeniuk and specific environments are determined or influenced by
Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b). the local effects of climate and by the biodiversity and
abundance of the biota.
Outside of incised valley systems, the stratigraphy of
determine the extent that fluvial sediment will contribute estuaries will be determined by the complexity of
to the filling of an estuarine basin. It will also have the array and mosaics of environments within the estuary
a particular effect on the sediments and stratigraphy of as a consequence, in part, of the type of estuary,
facies that have become emergent (shoaled) and inhabited e.g., shore-parallel estuarine lagoon, digitate/palmate
by tidal and supratidal vegetation. The shoaled stratigra- estuarine lagoon, meandering estuary on a coastal plain,
phy of estuaries in tropical humid climates, for instance, or compound estuary. The sedimentary facies in each of
may be capped by mangrove-vegetated mud, or at least these estuarine types will be determined by different
have a heavy imprint of vegetation bioturbation in tidal/ types of intra-estuarine processes such as wind-wave
supratidal environments, and also contain a rich diversity development in relation to fetch, the depth of water, the
of shell benthos in lower tidal and shallow subtidal envi- extent of shoaling, climate influences, and biota. These
ronments. The shoaled stratigraphy of estuaries in temper- types of estuaries provide the largest range of variability
ate humid climates or tropical humid climates may be of stratigraphic fills and architectural style of strati-
capped by peat, or by sand or mud, with a heavy imprint graphic fills.
 STRATIGRAPHY OF ESTUARIES 647

Stratigraphy of Estuaries, Figure 26 Longitudinal stratigraphic profile from the north to the south of the Peel-Harvey Estuary: from
the flood-tidal delta to across the mud basin, across the partitioning shoals, and to southern elongate digitate delta. The insets show
details of the two northern lobate deltas, the elongate digitate delta, the stratigraphically complex flood-tidal delta, and the
partitioning sandy sill (Information from Brown et al., 1980 and Semeniuk and Semeniuk, 1990a; Semeniuk and Semeniuk, 1990b).
648 STRUCTURALLY DOMINATED ESTUARY

Bibliography Semeniuk, V., and Brocx, M., 2011. King Sound and the tide-
dominated delta of the Fitzroy river: their geoheritage values.
Allen, J. R. L., 1970. Sediments of the modern Niger delta:
Journal of the Royal Society of Western Australia, 94, 151–160.
a summary and review. In Morgan, J. P. (ed.), Deltaic Sedimen-
Semeniuk, C. A., and Semeniuk, V., 1990a. The coastal landforms
tation – Modern and Ancient. Tulsa, Oklahoma: Society of Eco-
and peripheral wetlands of the Peel-Harvey estuarine system.
nomic Paleontologists and Mineralogists, pp. 138–151. Special
Journal of the Royal Society of Western Australia, 73, 9–21.
publication number 15.
Semeniuk, V., and Semeniuk, C. A., 1990b. Radiocarbon ages of
Allen, G. P., and Posamentier, H. W., 1993. Sequence stratigraphy
some coastal landforms in the Peel-Harvey estuary. Journal of
and facies model of an incised valley fill; the Gironde estuary,
the Royal Society of Western Australia, 73, 61–71.
France. Journal of Sedimentary Petrology, 63(3), 378–391.
Semeniuk, V., Semeniuk, C. A., Tauss, C., Unno, J., and Brocx, M.,
Bird, E. C. F., 1969. Coasts. Cambridge: M.I.T. Press.
2011. Walpole and Nornalup Inlets: Landforms, Stratigraphy,
Brown, R. G., Treloar, J. M., and Clifton, P. M., 1980. Sediments
Evolution, Hydrology, Water Quality, Biota, and Geoheritage.
and organic detritus in the Peel-Harvey estuarine system. Report
Perth: Western Australian Museum. http://museum.wa.gov.au/
to The Peel-Harvey Estuarine System Study Group. Perth, West-
store/museum-books/fauna/walpole-and-nornalup-inlets.
ern Australia: Environmental Protection Authority.
Woodroffe, C. D., Chappell, J. M. A., Thom, B. G., and Wallensky,
Coleman, J. M., Gagliano, S. M., and Smith, W. G., 1970. Sedimen-
E., 1985. Stratigraphy of the South Alligator tidal river and plains,
tation in a Malaysian high tide tropical delta. In Morgan, J. P.
Northern Territory. In Bardsley, K. N., Davie, J. D. S., and
(ed.), Deltaic Sedimentation – Modern and Ancient. Tulsa, Okla-
Woodroffe, C. D. (eds.), Coasts and Tidal Wetlands of the Austra-
homa: Society of Economic Paleontologists and Mineralogists,
lian Monsoon Region. Australian National University Northern
pp. 185–197. Special publication number 15.
Australia Research Unit Mangrove Monograph No. 1. pp. 17–30.
Dalrymple, R. W., Zaitlin, B. A., and Boyd, R., 1992. Estuarine
Woodroffe, C. D., Chappell, J. M. A., Thom, B. G., and Wallensky,
facies models; conceptual basis and stratigraphic implications.
E., 1986. Geomorphological Dynamics and Evolution of the
Journal of Sedimentary Research, 62, 1130–1146.
South Alligator Tidal River and Plains, Northern Territory.
Fairbridge, R. W., 1980. The estuary: its definition and geodynamic
Australian National University Northern Australia Research
cycle. In Olausson, E., and Cato, I. (eds.), Chemistry and
Unit Mangrove Monograph No. 3.
Biogeochemistry of Estuaries. Chichester: Wiley.
Zenkovitch, V. P., 1959. On the genesis of cuspate spits along
Gould, H. R., 1970. The Mississippi delta complex. In Morgan, J. P.
lagoon shores. Journal of Geology, 67, 269–277.
(ed.), Deltaic Sedimentation – Modern and Ancient. Tulsa, Okla-
homa: Society of Economic Paleontologists and Mineralogists,
pp. 3–30. Special publication number 15. Cross-references
Khan, N. S., Vane, C. H., Horton, B. P., and Fackler, S., 2011. A new
record of Holocene sea-level change in the Thames Estuary and Biogenous Sediment
its implications for geophysical modeling. British Geological Estuarine Sediment Composition
Survey. Publication – Conference Item (Poster). http://nora. Sediment Sorting
nerc.ac.uk/id/eprint/14256. Sediment Transport
Morgan, D., 2006. Modelling the Thames estuary. British Geologi- Washover Fans
cal Survey, Earthwise, 23, 20–21. Washovers
Pizzuto, J. E., and Rogers, E. W., 1992. The holocene history and
stratigraphy of palustrine and estuarine wetland deposits of
central delaware. Journal of Coastal Research, 8, 854–867.
Reineck, H. E., and Singh, I. B., 1980. Depositional Sedimentary STRUCTURALLY DOMINATED ESTUARY
Environments, 2nd edn. Berlin: Springer.
Roy, P. S., 1984. New South wales estuaries: their origin and evolu- David M. Kennedy
tion. In Thom, B. G. (ed.), Coastal Geomorphology in Australia. Department of Resource Management & Geography,
Sydney: Academic Press.
Roy, P. S., Thom, B. G., and Wright, L. D., 1980. Holocene The University of Melbourne, Parkville, VIC, Australia
sequences on an embayed high-energy coast: an evolutionary
model. Sedimentary Geology, 26, 1–19. Synonyms
Semeniuk, V., 1980. Quaternary stratigraphy of the tidal flats King Structurally built estuary; Tectonically produced estuary
Sound, WA. Journal of the Royal Society of Western Australia,
63, 65–78.
Semeniuk, V., 1981. Sedimentology and the stratigraphic sequence Definition
of a tropical tidal flat, North-Western Australia. Sedimentary A structurally dominated estuary is one where the shape
Geology, 29, 195–221. of the estuarine basin is primarily determined by the
Semeniuk, V., 1983. Mangrove distribution in Northwestern long-term geological history of the coast.
Australia in relationship to freshwater seepage. Vegetatio, 53,
11–31.
Semeniuk, V., 1985a. Development of mangrove habitats along ria Description
shorelines in north and northwestern Australia. Vegetatio, Two types of structural estuaries have been identified:
60, 3–23. (1) where the tectonic setting has determined the nature
Semeniuk, V., 1985b. The age structure of a Holocene barrier dune of the estuarine basin and (2) where the sediments within
system and its implication for sealevel history reconstructions in the estuary have been impacted by neotectonics after their
southwestern Australia. Marine Geology, 67, 197–212.
Semeniuk, V., 2000. Sedimentology and Holocene stratigraphy of deposition (Quivira, 1995). Structural estuaries most com-
Leschenault Inlet. Journal of the Royal Society of Western monly occur on tectonically active coasts where faulted
Australia Special Issue on the Leschenault Inlet Estuary, landscapes produce basins which can be flooded by
83, 255–274. the sea. Structural grabens, eroded volcanic calderas, and
 SUBLITTORAL ZONE 649

uplifted river valleys are all examples of systems that can processes operate in these systems resulting in the forma-
be considered to be structurally influenced (Hume and tion of subaqueous soils. Important processes in subaque-
Herdendorf, 1988; Hume, 2003). In highly jointed bed- ous soils include accumulation of organic matter,
rock, ria-type estuaries form, and these can also accumulation of sulfides (sulfidization), sedimentation,
be classified as structural estuaries (Kennedy, 2011). and bioturbation (soil mixing). Early studies of subaqueous
The infill of structurally dominated estuaries is complex, soils focused on methodologies to sample and characterize
being driven by sediment supply, accommodation space, these soil materials. Models were developed to link shallow
and process dominance (Kennedy, 2011), with those subtidal landscapes (i.e., flood-tidal deltas, washover fans,
sequences on tectonically active coasts preserving multi- submerged beaches) to specific soil types. Understanding
ple transgressive and regressive sequences in response to the processes, characteristics, and subaqueous soil-
vertical land movement (Wilson et al., 2007). landscape relationships aided in developing soil taxa to
classify the soils which could be used to map the subtidal
Bibliography components of estuaries. These taxa are incorporated
Hume, T., 2003. Estuaries and tidal inlets. In Goff, J. R., Nichol,
into the Entisol and Histosol orders of the US national
S. L., and Rouse, H. L. (eds.), The New Zealand coast. Palmer- soil classification system known as Soil Taxonomy
ston North: Dunmore Press. (ftp://ftp-fc.sc.egov.usda.gov/NSSC/Soil_Taxonomy/keys/
Hume, T. M., and Herdendorf, C. E., 1988. A geomorphic classifi- ebook/Keys_to_Soil_Taxonomy_11th_Edition.pdf).
cation of estuaries and its application to coastal resource More recent studies have focused on the application of
management – a New Zealand example. Journal of Ocean subaqueous soils information to estuarine management
Shoreline Management, 11, 249–274. issues such as shellfish and SAV restoration, identifying
Kennedy, D. M., 2011. Tectonic and geomorphic evolution of
estuaries and coasts. In Wolanski, E., and McLusky, D. (eds.), productive areas for shellfish aquaculture, carbon account-
Treatise on Estuarine and Coastal Science. Waltham: Academic, ing, and determining which areas can be dredged and the
Vol. 3, pp. 37–59. materials placed on the upland (Rabenhorst and Stolt,
Quivira, M. P., 1995. Structural estuaries. In Perillo, G. M. E. (ed.), 2012). The authors of the US National Coastal
Developments in Sedimentology. Amsterdam, Chap. 8: Elsevier, and Marine Ecological Classification Standard (CMECS;
pp. 227–239. http://www.csc.noaa.gov/digitalcoast/publications/cmecs)
Wilson, K., Berryman, K., Cochran, U., and Little, T., 2007.
A Holocene incised valley infill sequence developed on a recognized the value of its use and management interpreta-
tectonically active coast: Pakarae River, New Zealand. tions and recommended that the subaqueous soils approach
Sedimentary Geology, 197, 333–354. be employed to classify shallow subtidal substrates for use
and management purposes. Mapping subaqueous soils is
usually done in waters less than 5 m deep. Although the
Cross-references methods to create a subaqueous soils map are well devel-
Emergent Shoreline oped, mapping subaqueous soils is still in its infancy. Maps
Submerged Coasts of selected estuaries are available on the Atlantic coast of
Submergent Shoreline
Tectonic Eustasy
the USA (in Maine, Rhode Island, Connecticut, New York,
Uplifted Coasts New Jersey, Delaware, Maryland, and Florida).

Bibliography
Rabenhorst, M. C., and Stolt, M. H., 2012. Subaqueous soils:
SUBAQUEOUS SOILS pedogenesis, mapping, and applications. In Lin, H. (ed.),
Hydropedology: Synergistic Integration of Soil Science and
Hydrology. Waltham: Academic, pp. 173–204.
Mark H. Stolt
Department of Natural Resources, University of Rhode
Island, Kingston, RI, USA
Cross-references
Sediment Grain Size
Definition
The foundation of every estuarine ecosystem is the water
column and underlying substrate that support the plants
and animals living in these unique habitats. In most cases, SUBLITTORAL ZONE
the substrate is considered sediment, and the materials are
classified based simply on their grain size (i.e., mud, sand). Michael J. Kennish
In the late 1990s, pedologists began developing a soil sci- Department of Marine and Coastal Sciences,
ence approach to study, map, and classify shallow subtidal School of Environmental and Biological Sciences,
substrates as soils (http://nesoil.com/sas/index.htm). These Rutgers University, New Brunswick, NJ, USA
studies recognized that shallow subtidal substrate often
supports rooted plants (submerged aquatic vegetation; Synonyms
SAV) and that a range of physical, chemical, and biological Subtidal zone
650 SUBMERGED COASTS

Definition factors in the coastal configuration, one historical and

In an estuary, sublittoral refers to the zone immediately one contemporary. In the first, the coastal type is con-
below the eulittoral zone extending outward from the trolled by plate tectonics resulting in emergence-
neap low tide mark at the shoreline. In an ocean, sublitto- submergence categories. Historic factors must be assessed
ral refers to the zone extending from the neap low tide such as the nature of the sea-level change over the last
mark at the shoreline to the outer edge of the continental 10,000 years of the postglacial transgression. Additionally
shelf. variations in wave energy, tidal regime, and biogeographic
agents and processes must be considered. Valentin (1952)
Description incorporated these factors in an elegant diagram that com-
Sublittoral habitats in an estuary lie within the photic zone bined advancing and retreating coasts with emerging and
and thus are typically highly productive. Numerous flora submerging ones. Most of the submerging coasts correlate
and fauna inhabit the sublittoral zone forming rich biotic with coastal retreat or land erosion. The main shortcoming
communities (Day et al., 2012; Levinton, 2013). This is of these early classifications is the emphasis on geological
a subaqueous environment susceptible to the vagaries of control that places less significance on the role of hydro-
physical and chemical factors, resulting in variable spatial dynamic processes.
and temporal biotic responses (Allaby and Allaby, 1999; The coupling mechanisms in coastal submersion are
Kennish, 2001). sea-level rise and land subsidence. Between 20 and 6 kyr
ago, driven by the melting of Northern Hemisphere ice
caps, global sea level increased 120 m. This rise in sea
Bibliography level caused the rapid submergence of vast areas of the
Allaby, A., and Allaby, A. (eds.), 1999. A Dictionary of Earth continental shelf. In the last 6 kyr, global sea level has
Sciences, 2nd edn. Oxford: Oxford University Press. remained more or less stable during an interglacial period
Day, J. W., Crump, B. C., Kemp, W. M., and Yanez-Arancibia,
A. (eds.), 2012. Estuarine Ecology, 2nd edn. New York: (Pirazzoli, 1996). Despite being more variable in time and
Wiley-Blackwell. space, land subsidence may result from different tectonic
Kennish, M. J. (ed.), 2001. Practical Handbook of Marine Science, and neotectonic processes or by the accumulation and
3rd edn. Boca Raton: CRC Press. compaction of sediments. During the twentieth century,
Levinton, J. S., 2013. Marine Biology: Function, Biodiversity, many coastal areas experienced human-induced land sub-
Ecology, 4th edn. Oxford: Oxford University Press. sidence caused by groundwater withdrawal or oil and gas
extraction, among others. Regulation of river flow and the
Cross-references construction of breakwaters that cut longshore sediment
Benthic Ecology supply have also contributed to destruction and sinking
Littoral Zone of deltas (Bird, 1993). Climate models, which take into
Shoreline account increasing greenhouse effects, estimate that the
global temperature may increase 2  C and sea level may
increase 0.11–0.77 m over the next century, which will
cause significant coastal flooding and submergence
SUBMERGED COASTS (Church et al., 2001).
Typical submerged coasts are drowned river and glacial
valleys, often referred as rias, calas, fiords, fiards, sharms,
Lluís Gómez-Pujol
and sebkhas. Although many of them can fit within
Universitat de les Illes Baleares, Palma (Balearic Islands),
a broad morphological classification of estuaries, from
Spain
a hydrodynamic point of view, drowned valleys are not
Synonyms necessarily estuaries (Bird, 2008). Inlets formed by partial
submergence of river valleys are known as rias, whereas
Submergent coasts fiords are steep-sided inlets at the mouth of valleys that
were formerly glaciated. Inlets generated by Holocene
Definition marine submergence of formerly glaciated valleys and
Submerged coasts are defined as coasts formed by the rel- depressions in low-lying rock landforms are known as
ative submergence of a landmass via sea-level rise and/or fiards. These steep-sided valleys, incised in limestone
by land subsidence or by both factors. plateaus during low sea-level stages that have been
submerged during the Holocene, are known as calas
Description or calanques (Gómez-Pujol et al., 2013). Finally, in arid
Most early coastal classification schemes are based on the environments, such as those of the Red Sea and the
observation that coastal landforms are largely the product Arabian Gulf, wadis developed during the Holocene
of sea-level variations. Such classifications distinguish marine transgression, leading to the formation of long,
between emerged and submerged coasts (Johnson, narrow marine inlets termed sharms. When these valleys
1919). More sophisticated analyses have attempted to exhibit a branched embayment, they are known as sebkhas
establish coastal classifications by balancing two sets of (Castaing and Guilcher, 1995).
 SUSTAINABLE USE 651

Bibliography relative sea level. In combination with subsidence of

Bird, E. C. F., 1993. Submerging Coasts: The Effects of Rising Sea a delta environment, this may cause severe coastal prob-
Level on Coastal Environment. Chichester: Wiley and Sons. lems, as in Bangkok.
Bird, E. C. F., 2008. Coastal Geomorphology: An Introduction. Many old-tide gauges were installed in areas of heavy
Chichester: Wiley and Sons. harbor construction, and the added weight of the construc-
Castaing, P., and Guilcher, A., 1995. Geomorphology and sedimen- tion can cause local compaction of the sediments and asso-
tology of rias. In Perillo, G. M. E. (ed.), Geomorphology and
Sedimentology of Estuaries. Amsterdam: Elsevier, pp. 69–111. ciated subsidence. NOAA (2013) has 204 tide gauge
Church, J. A., Gregory, J. M., Huybrechts, P., Khun, M., Lambeck, stations scattered all over the globe where the mean rate
K., Nhuan, M. T., Quin, D., and Woodworth, P. L., 2001. of sea-level rise is +0.75 mm/year.
Chapter 11: Changes in sea level. In Intergovernmental Panel
on Climate Change, Climate Change 2001: The Scientific Basis, Bibliography
Contribution of Working Group I to the Third Assessment Report
of the Intergovernmental Panel on Climate Change. Cambridge: Daly, R. A., 1934. The Changing World of the Ice Age. New Haven:
Cambridge University Press, pp. 639–693. Yale University Press.
Gómez-Pujol, L., Gelabert, B., Fornós, J. J., Pardo-Pascual, J. E., Jelgersma, S., 1961. Holocene sea-level changes in the Netherlands.
Rosselló, V. M., Segura, F. S., and Onac, B. P., 2013. Structural Medded. Netherl. Geol. Stichting., Ser. C, VI: 7, 1–101.
control on the presence and character of calas: observations from NOAA, 2013. http://tidesandcurrents.noaa.gov/sltrends/sltrends.
Balearic Islands Rock Coast Macroforms. Geomorphology, 194, shtml
1–15.
Johnson, J. W., 1919. Shore Processes and Shoreline Development.
New York: Wiley. Cross-references
Piarazzoli, P. A., 1996. Sea-level Changes: The Last 20,000 Years. Climate Change
Chichester: Wiley and Sons. Mean Sea Level
Valentin, H. J., 1952. Die Kusten der Erde. Gotha: Justus Perthes. Shoreline Changes
Petermanns Geographische Mitteilungen Ergänzüngsheft, Vol. Submerged Coasts
246.

Cross-references
Deltas SUSTAINABLE USE
Mean Sea Level
Shoreline Changes Mafalda Marques Carapuço
Institute Dom Luiz, University of Lisbon, Lisbon,
Portugal

SUBMERGENT SHORELINE Synonyms

Sustainable Resource Use
Nils-Axel Mörner
Paleogeophysics and Geodynamics, Saltsjöbaden, Sweden Definition
Sustainable use is the use of resources in a way and at
Synonyms a rate that does not lead to long-term decline, thereby
Submarine shoreline; Submerged coasts; Submergence; maintaining its potential to meet the needs and aspirations
Subsided shoreline of present and future generations (adapted from UN,
1992).
Definition
A submergent shoreline is one that becomes flooded due Introduction
to coastal subsidence or sea-level rise. Natural resources are vital to the survival and develop-
The Dutch coast is well known for its long-term crustal ment of the human population and to the world economy.
subsidence. This has given rise to a sequence of submarine However, the way in which natural resources, both renew-
shorelines and a transgressing sea (Jelgersma, 1961). able and nonrenewable, are used and the speed at which
Sediment loading in delta areas can induce subsidence renewable resources are being exploited are rapidly erod-
(sedimento-isostasy) and the development of a submerging ing the planet’s capacity to regenerate the resources and
coast with a succession of submarine shorelines, drowned environmental services on which our prosperity and
forests, and flooded coastal zones. Eustatic sea-level rise growth is based (CEC, 2005). According to the Millen-
after the last Ice Age has been 120 m, which has caused nium Ecosystem Assessment report, over the past 50 years,
most coasts of the world to experience substantial inunda- humans have changed ecosystems more rapidly and
tion, leaving former shorelines and land surfaces in subma- extensively than in any comparable period of time in
rine positions (e.g., Daly, 1934). human history, largely to meet rapidly growing demands
Water extraction along coasts may also lead to human- for, namely, food and freshwater (MEA, 2005). If current
induced subsidence, often resulting in local increases of patterns of resource use are maintained, environmental
652 SUSTAINABLE USE

degradation and depletion of natural resources will con- (SERI et al., 2009). However, it is not just the natural
tinue. The issue has a global dimension. If the world as resources that must be sustainably managed. All compo-
a whole followed traditional patterns of consumption, it nents of ecosystems should be considered. Estuaries pro-
is estimated that global resource use would quadruple vide natural resources (e.g., fish and shellfish) but also
within 20 years (CEC, 2005; EC, 2009). The negative have recreational and aesthetic value such as fishing,
impact on the environment would be substantial. The bird-watching, and boating. Additionally, estuaries are
alternative can be to adopt a coordinated approach, antic- often the cultural centers of coastal communities, serving
ipating the need to shift to more sustainable-use patterns, as focal points for local commerce, recreation, celebra-
which can result in environmental and economic benefits tions, and traditions (Figure 1).
at a global scale (CEC, 2005). In addition to the resources, sustainable use should also
consider the ecosystems services (e.g., detoxification and
Sustainable use: the concept decomposition of wastes, stabilization and moderation of
the climate) (SCBD, 2000), which represent the benefits
In 1992, the Earth Summit was held in Rio de Janeiro
human populations derive, directly or indirectly, from eco-
(Brazil), with objectives built upon the hopes and achieve-
system functions (Costanza et al., 1997).
ments of Our Common Future Report (also known as the
Brundtland Report), in order to respond to pressing global
environmental problems (UN, 1987). Among other agree- Conditions for sustainable use
ments, the United Nations Convention on Biological The challenge of sustainable use is the following: revers-
Diversity was adopted (UN, 1992). This convention, con- ing the degradation of ecosystems while meeting the
sidered a key global instrument on conservation (Höft, increasing demands for their resources and services. In
2008), establishes three main goals: (1) the conservation 2004, the Secretariat of the Convention on Biological
of biological diversity, (2) the sustainable use of its com- Diversity identified the fundamental conditions that
ponents, and (3) the fair and equitable sharing of the ben- should be taken into account in structuring a sustainable-
efits from the use of genetic resources. The United Nations use approach (SCBD 2004). These include the following:
Convention on Biological Diversity conveys to decision-
1. Resources should be used in a manner in which ecolog-
makers that natural resources are not infinite. Thus, it sets
ical processes, species, and genetic variability remain
out a new philosophy for the twenty-first century: sustain-
above thresholds needed for long-term viability, and
able use (SCBD, 2000). While past conservation efforts
thus all resource managers and users have the responsi-
aimed at protecting particular species and habitats, the
bility to ensure that use does not exceed these capaci-
convention recognizes that ecosystems must be used for
ties. It is crucial that the ecosystem is maintained or,
the benefit of humans. However, this should be done in
in some cases, recovered, to ensure that those ecosys-
a way and at a rate that does not lead to the long-term
tems are capable to sustain the ecological services on
decline of biological diversity, thus promoting the sustain-
which both biodiversity and people depend.
able use of biodiversity (SCBD, 2000).
2. Ecosystems, ecological processes within them, species
variability, and genetic variation change over time
Sustainability and sustainable use whether or not they are used; therefore, governments,
The most commonly used definition of the term “sustain- resource managers, and users should take into account
able development” is found in the 1987 report, Our Com- the need to accommodate change, including stochastic
mon Future, of the World Commission on Environment events that may adversely affect biodiversity and influ-
and Development (WCED, 1987). In this report sustain- ence the sustainability of a use.
able development is defined as “development that meets 3. Under circumstances where the risk of converting nat-
the needs of the present without compromising the ability ural landscapes to other purposes is high, encouraging
of future generations to meet their own needs.” Achieving sustainable use can provide incentives to maintain hab-
this in practice requires that economic growth, social itats and ecosystems and the species within them.
progress, and environmental quality improvement go 4. Biodiversity provides many direct benefits and ecosys-
together. These three pillars cannot be developed in isola- tem services necessary for life. Increasingly, many
tion since they are strongly interdependent. Economic marine species are of value to pharmaceuticals for dis-
growth can provide the additional financial resources for ease prevention and cure. Finally, local communities
improving the quality of the environment and reinforcing and their cultures often depend directly on the uses of
social cohesion. Social policy underpins economic perfor- natural resources for their livelihoods. In all of these
mance and helps citizens to be responsible. Environmental instances, governments should have adequate policies
policy contributes to preserving the natural resource base and capacities in place to ensure that such uses are
of the economy and to enhance the quality of life (CEC, sustainable.
2003). Thus, the sustainable use of natural resources con- 5. The supply of biological products and ecological ser-
stitutes an effective tool to achieve sustainable develop- vices available for use is limited by intrinsic biological
ment; achieving sustainable patterns of resource use characteristics of both species and ecosystems,
is a key part of achieving sustainable development including productivity, resilience, and stability.
 SUSTAINABLE USE 653

Sustainable Use, Figure 1 Religious celebration in honor of Nossa Senhora do Rosário de Tróia, the patroness saint of fishermen of
Setubal (Sado estuary, Portugal).

Biological systems, which are dependent on cycling of project cycle, enabling timely adjustments, and as
finite resources, have limitations with respect to the a guide to structuring future projects more effectively.
goods they can provide and services they can render. Sustainable-use monitoring must involve the consider-
Although certain limits can be extended to some degree ation of governance, ecological (including environmental)
through technological breakthroughs, there are still and socioeconomic dimensions, as well as the interaction
limits and constraints imposed by the availability and between them (UNESCO, 2006). The indicators oriented
accessibility of endogenous and exogenous resources. to measure sustainable development are designated as sus-
6. To ameliorate any potential negative long-term effects tainable development indicators. The recent report titled
of resource uses, it is incumbent on all resource users Framework and Suggested Indicators to Measure Sustain-
to apply the precautionary principle in their manage- able Development (UNECE et al., 2013) presents an
ment decisions and to opt for sustainable-use manage- approach which aims to facilitate users’ choices through
ment strategies and policies favoring uses that provide large numbers of sustainable development indicators
increased sustainable benefits. available in literature (e.g., EC, 2007; UN, 2007).
Although this publication is primarily aimed at statisti-
cians, it may also be relevant for policymakers, as policy
Monitoring sustainability targets for sustainable development are increasingly being
Monitoring is the continuous or periodic process of formulated at national and international levels.
collecting and analyzing data to measure the performance
of a program, project, or activity. As an integral and con-
tinuing part of project/program management, monitoring Estuaries
provides managers and stakeholders with regular feed- An ecosystem with unequaled value
back on implementation and progress toward the attain- Estuaries are highly productive ecosystems which provide
ment of environmental objectives (UNESCO, 2006). a suite of resources and services (e.g., Nixon et al., 1986;
Monitoring enables management to take appropriate cor- Wilson and Farber, 2009; Barbier et al., 2011). Thus, estu-
rective action to achieve desired results. Effective moni- aries are an irreplaceable natural ecosystem that must be
toring requires baseline data, as well as indicators of managed carefully for the mutual benefit of all who enjoy
performance and related measurements, regular reporting, and depend on them. Thousands of species of birds, mam-
and a feedback mechanism for management decision- mals, and other wildlife depend on estuarine habitats as
making (UNESCO, 2006). places to live, feed, and reproduce. And many marine
Effective monitoring and evaluation are widely recog- organisms, including most commercially important spe-
nized as an indispensable tool in assuring that the manage- cies of fish, depend on estuaries at some point during their
ment objectives established are being achieved. If done development. Estuaries are the year-round home for many
well, a monitoring and evaluation plan and associated species (e.g., oysters), while other species move in and out
indicators serve both as a corrective function during the of estuaries on a seasonal basis for reproduction and
654 SUSTAINABLE USE

growth (e.g., salmon and shrimp) (Wilson and Farber, approach 6 billion people. Habitat destruction has
2009). Because they are biologically productive, estuaries far-reaching ecological consequences, modifying the
provide ideal areas for migratory birds to rest and refuel structure, function, and controls of estuarine ecosystems
during their long journeys. Additionally, numerous fish and contributing to the decline of biodiversity. Other antic-
and invertebrate species rely on the sheltered waters of ipated high-priority problems are excessive nutrient and
estuaries as nursery habitats (Vasconcelos et al., 2011). sewage inputs to estuaries, principally from land-based
Estuaries have important commercial value and their sources. These inputs will lead to the greater incidence
resources provide economic benefits for fisheries, tour- of eutrophication as well as hypoxia and anoxia. During
ism, and cultural activities. the next 25 years, overfishing is expected to become
The protected coastal waters of estuaries also support a more pervasive and significant anthropogenic factor,
important public infrastructure, serving as harbors and also capable of mediating global-scale change to estuaries.
ports vital for shipping and transportation. Estuaries also Chemical contaminants, notably synthetic organic com-
perform other valuable services. They are inherently pounds, will remain a serious problem, especially in
important to environmental and human health. Estuarine heavily industrialized areas. Freshwater diversions appear
capacity to filter pollutants not only serves to provide to be an emerging global problem as the expanding coastal
a healthy environment for marine creatures to thrive, but population places greater demands on limited freshwater
it contributes to cleaner coastal waters for beach-going supplies for agricultural, domestic, and industrial needs.
populations (Kildow, 2009). Wetland plants and soils also Altered freshwater flows could significantly affect nutri-
act as natural buffers reducing impacts by moderating the ent loads, biotic community structure, and the
effects of stormwater runoff including stabilizing soil to trophodynamics of estuarine systems. Ecological impacts
prevent erosion; filtering suspended solids, nutrients, and that will be less threatening, but still damaging, are those
harmful or toxic substances; and moderating water-level caused by introduced species, sea level rise, coastal subsi-
fluctuations (Castelle et al., 1992). dence, and debris/litter. Although all of these disturbances
can alter habitats and contribute to shifts in the composi-
Use of estuaries: opportunities and threats tion of estuarine biotic communities, the overall effect will
be partial changes to these ecosystem components.
Estuaries are areas with major economic potential because
Stevens (2010) also describes trends for estuaries but
of their strategic location close to seas and inland water-
focuses on climate change effects (Table 1). With rising
ways. Estuaries also provide some of the world’s most fer-
sea levels, estuaries will also be affected, causing changes
tile areas for food production. That is why navigation and
in these waterbodies as manifested by the loss of intertidal
port development, as well as agriculture and fisheries,
area, erosion of shorelines, and increased risk of flooding
have always been the engines of economic development
of low lying areas (Rossington, 2008 in Stevens, 2010).
of estuaries. Attracted by these resources, large numbers
Given the above, management strategies must be devel-
of people live in the vicinity of estuaries leading to the
oped to mitigate future impacts on estuaries.
growth of coastal cities and mega cities (Sekovski et al.,
2012). Of the 32 largest cities in the world, 22 are located
on estuaries (Ross, 1995). Five of the ten largest metropol- The future of estuaries: managing, restoring and
itan areas in the United States are centered along major monitoring
estuaries (NOAA, 1998 in Rice et al., 2005). For example,
It is a fact that increasing human activities in the coastal
New York is located at the mouth of the Hudson River
zone leave a significant human environmental footprint,
estuary; San Francisco is located on San Francisco Bay
leading to multiple stresses on estuaries and causing
which is an estuary for the Sacramento and San Joaquin
declines in water quality and overall ecosystem health.
rivers; and New Orleans is on the estuary of the Missis-
Included here are the effects of eutrophication, wastewater
sippi River. Unfortunately, this increasing concentration
inputs, chemical contaminants, freshwater diversions,
of people disturbs the natural balance of estuarine ecosys-
draining and ditching of wetlands, hardened shorelines,
tems due to environmental impacts caused by develop-
sediment/turbidity influx, inlet stabilization, introduced
ment (Sekovski et al., 2012), which threatens their
species, and fisheries overexploitation (Kennish, 2012).
integrity, and imposes increased pressures on vital natural
One course of remedial action is habitat restoration.
resources that endanger their susceptibility. For example,
Restoring habitats involves reestablishing natural ecosys-
along the Hudson River, New York, human presence and
tem processes by removing invasive species, reducing
activities have profoundly changed the estuary as a natural
pollution levels, and reintroducing indigenous flora and
ecosystem (Figure 2).
fauna. The goal is to rebuild the estuary as an ecosystem
that functions as it once did prior to impacts. Fortunately,
Trends estuarine ecosystems can often be restored, because of
Kennish (2002) describes the trends for estuaries until their adaptive and resilient capacity (Most et al., 2009).
2025. He suggests that estuaries will be most significantly Simenstad and Bottom (2002) recommend the following
impacted by habitat loss and alteration associated with principles that, although being developed in the context
a burgeoning coastal population, which is expected to of estuarine habitat restoration for salmon recovery for
 SUSTAINABLE USE 655

Sustainable Use, Figure 2 Hudson River, New York: Human presence and activities have profoundly changed the estuary as natural
ecosystem.

the Columbia River system, can be generally applied to structures at inappropriate or unsustainable locations. To
a wide range of restoration activities. the extent possible, engineered structures should be
avoided in restoration designs.
Protect first, restore second
Protection of existing habitat is critical to the success of Restore rather than enhance or create
estuarine restoration. To restore habitat in the absence of Past experience demonstrates that, compared to restora-
any overlying conservation program is counterproductive. tion, enhancement (where designed to increase one or
All restoration sites should be explicitly incorporated into more specific functions of a degraded habitat) or creation
a broad conservation framework that will ensure their of estuarine habitat is problematic and rarely leads to
long-term protection. Estuaries and estuarine habitats can self-sustaining ecosystems.
be restored only through a long-term stewardship
approach with the necessary constituencies, policies, and Develop a comprehensive restoration plan using
funding to support it. landscape ecology concepts to reestablish ecosystem
connectivity and complexity
Do no harm Unplanned, opportunistic approaches to restoration will
To ensure no net loss of habitat functions and to protect not suffice. Strategic planning is necessary across hierar-
unimpeded natural processes, restoration actions should chical scales, from watershed to estuarine habitats, in
achieve proposed benefits without degrading other eco- order to set a broad vision, articulate clear goals, and place
logical functions of natural habitats or broader local restoration activities in an ecologically sound, eco-
ecosystems. system context. Both ecological and socioeconomic
aspects of the estuarine landscape must be considered
Use natural processes to restore and maintain structure when selecting, designing, and locating restoration sites;
Restorative measures should reestablish the dynamics of restoration and conservation can only be effective if
estuarine hydrology, sedimentology, geomorphology, implemented within the human context. Restoration plans
and other habitat-forming processes that naturally create should be designed to restore ecosystem complexity and
and maintain habitat, rather than simply implanting habitat diversity. Public access to restoration sites should be
656 SUSTAINABLE USE

Sustainable Use, Table 1 Expect changes in estuaries from sea level rise (Stevens, 2010)

Changes within the environment Changes within an estuary

Higher air temperatures Increased evaporation and lower soil moisture affecting
runoff to estuaries
Increased fire risk for surrounding vegetation
Increased stratification of coastal lakes
Decreased rainfall or changes to Decreased runoff and its impact on environmental flows
rainfall patterns Average rainfall might stay the same but how and when it falls could change, i.e., rain falling in very
large storms less often
Increased fire risk for surrounding vegetation
Sea level rise Saline intrusion with dieback of freshwater wetlands
Larger or more frequent storm surges impact barrier ridges and/or salinity of estuaries (wave-
dominated estuaries)
Inundation and shoreline recession including:
 Erosion and landward recession of soft sandy shorelines, particularly where these are backed by
low-lying plains of soft unconsolidated sediments
 Modification of soft low-lying muddy estuarine and deltaic shores
 Acceleration of existing progressive erosion of soft clayey-gravelly shorelines
 Increased slumping of steep landslip-prone shorelines
Higher sea surface temperatures Changes to nutrient cycling
Changes to primary productivity
Changes to water temperature of coastal waters
Ocean acidification Changes to pH and pCO2
Ocean circulation wave patterns Changes to nutrient cycling
Changes to sediment dynamics and form of estuaries
Vector-borne diseases Change in the occurrence and distribution of vectors which utilize coastal waterways in their life
cycles

encouraged wherever appropriate and incorporated into the established performance criteria is essential to improve
restoration plans. However, they should be designed to restoration techniques and to achieve estuarine restoration
minimize impacts on the ecological functioning of the site. goals. All restoration designs should be monitored and,
based on the concept of adaptive management, altered if
Use history as a guide, but recognize irreversible change necessary to achieve desired end points and to insure that
Historic templates often provide the framework for resto- local projects are self-sustaining.
ration goals, as well as a perspective on how ecosystems
have been incrementally degraded. Tidal, fluvial, geomor- Use the best interdisciplinary science and technical
phic, and other naturally dynamic processes occur in knowledge and use science review processes
a landscape context. Understanding the historic landscape All available scientific and technical expertise should be
structure is essential to comprehending how restoration brought to bear on the complex problems of estuarine hab-
can be implemented strategically in the modern landscape itat restoration. Restoration should be planned, designed,
to promote the natural formation and maintenance of implemented, and monitored by an interdisciplinary, not
important habitats. Reconstructing the historical river, just multidisciplinary, group of experts. Physical (e.g.,
tidal floodplain, and estuarine structure does not necessar- hydrology, geomorphology, geophysics, sedimentology),
ily guarantee restoration success but will decrease chemical (e.g., sediment geochemistry), mathematical
uncertainty. (e.g., biostatistics), and engineering sciences should be
represented in addition to the essential biological disci-
Establish performance criteria based on specific plines (e.g., estuarine and fish ecology, landscape ecology,
objectives and monitor performance both individually and botany). An independent, peer-review panel should be
comprehensively established to evaluate scientific assumptions and perfor-
Monitoring and adaptive management are essential com- mance throughout the restoration process and to ensure
ponents of restoration and habitat management. Objec- restoration goals are achieved.
tives for restoration projects should be clearly stated, site These recommendations stress the importance of mon-
specific, measurable, and long-term, in many cases greater itoring. Monitoring is essential for assessing whether the
than 20 years. Performance criteria should derive directly action led to the hypothesized result and for providing
from these goals and should include both functional and managers and researchers with increasing knowledge
structural elements and be linked to suitable, local refer- about the feasibility and approaches to rehabilitation
ence (“target”) habitats. Scientific monitoring based on (Rice et al., 2005). According to Thom and Wellman
 SUSTAINABLE USE 657

Sustainable Use, Table 2 Examples of estuarine rehabilitation environmental concerns, and growing risk aversion). Such
activities by ecosystem type (Rice et al. 2005) a vision should be developed in close cooperation with all
parties that have a stake or a say in the development of the
Ecosystem Actions estuaries.
Tidal marsh Dike or levy breach or removal, excavation,
substrate addition, transplantation, fertilization, Estuarine technology: innovations in science and
hydrologic control (e.g., tide gates), grazer technology
control, competitor control, large woody debris Sustainable development of estuaries requires innovations
placement, wastewater and sediment discharge in the knowledge of natural systems’ behavior as well as in
control, chemical contaminant removal or the approach to planning and design. An important source
containment of innovation is the development in information and com-
Sea grass Transplantation, fertilization, excavation, substrate
addition, wastewater and sediment discharge munication technology. Advances in sensor and simula-
control, chemical contaminant removal or tion technologies may promote the development of more
containment accurate warning and forecasting systems. These technol-
Kelp Transplantation, substrate addition, grazer control, ogies also support the development of local- and global-
competitor control, wastewater and sediment scale monitoring and diagnostic systems. Integration of
discharge control knowledge from various disciplines may open new appli-
Mudflat Dike or levy breach or removal, excavation,
substrate addition, chemical contaminant cations as well.
removal or containment
Sand/gravel Substrate addition, excavation Estuarine governance: social and institutional
beach innovations
For development of estuaries to be more sustainable, it is
important to obtain societal acceptance and support for
this development. Good governance should foster shared
(1996), monitoring should be considered as a fundamental visions on sustainable development of estuaries. More-
part of a restoration project aiming to: (1) assess the per- over, conditions should be created for the actual imple-
formance of the restoration project relative to the project mentation of such visions through development projects.
goals, (2) provide information that can be used to improve Governance should also provide adequate arrangements
the performance of the project, and (3) provide informa- for maintenance of infrastructure to prevent early deterio-
tion to interested parties. ration of the infrastructure.
Rice et al. (2005) showed that the most common tech-
nique in estuarine rehabilitation is the return or the intro- Estuarine dialogue and forum: learning from others
duction of tidal inundation. Table 2 lists some examples Sustainable development of estuaries is an increasingly
of estuarine rehabilitation activities by ecosystem type. complex field which requires the contribution and cooper-
Rice et al. (2005) also present examples of physical, ation of many parties. Although there is no general strat-
biological, and chemical controlling factors and structural egy on how to best deal with many estuarine issues, it is
and functional attributes for use in estuarine rehabilitation important to learn from experiences elsewhere. To this
monitoring (Table 3). end, the exchange of knowledge and experiences should
be encouraged.
The way forward Best practices in the sustainable use of estuaries
Population growth, economic development, and climate Estuaries have characteristics in common, but there is also
change are factors that can significantly impact estuaries. much diversity in physical conditions, governance struc-
Efficient management of natural resources and services ture, and cultural background. Hence, there is no general
is a key to maintaining the integrity of estuaries. To approach on how to deal with estuarine issues. Neverthe-
achieve this, innovations are required – social, institu- less, some broad perspectives may be generated to deal
tional, and technological innovations (Most et al., 2009). with these issues. Most et al. (2009) identified the emerg-
Most et al. (2009) have examined approaches for achiev- ing “best practices” for deltas (that can be generalized for
ing sustainable development of estuaries, as recounted in estuaries) which should comprise a balanced mix of mea-
the four passages below. sures from the different response themes and reflect the
integrated nature and regional scale of estuarine
Estuarine vision: a shared view on sustainable development.
development
A shared vision on estuarine sustainable development Relieving the pressure on available space
should deal with all drivers of change in an estuary Spatial planning regulation may relieve some of the pres-
(population growth, economic development, and climate sure by redirecting urban development and economic
change) as well as with the relevant societal trends activities to less “crowded” and/or low-risk areas. In cases
(decentralization, privatization, participation, growing where spatial planning offers little solace, land
658 SUSTAINABLE USE

Sustainable Use, Table 3 Examples of physical, chemical, and biological variables for controlling factors and structural and
functional attributes that could be considered as potential metrics in estuarine rehabilitation monitoring (Rice et al. 2005)

Category Controlling factors Structural attributes Functional attributes

Physical
Hydrology Geomorphology, freshwater inflow, Tidal range, tidal prism, Fish presence/absence (access to habitat)
tidal regime hydroperiod, residence time
Geomorphology/ Geology, tidal regime, sedimentation Elevation, connectivity, channel Fish presence/absence (access to habitat)
topography complexity
Water Freshwater inflow, tidal regime, Temperature, salinity, dissolved Fish prey production (capacity of
characteristics nutrient concentrations, biochemical oxygen (DO), current habitat)
oxygen demand, residence time stratification
Soil/sediment Geology, tidal regime, sediment supply Grain size, organic carbon content, Sedimentation, organic carbon
nutrient concentrations, salinity, accumulation, nutrient accumulation
redox potential
Chemical
Nutrients and Freshwater runoff, point and sources, Nutrient concentrations, organic Primary production, invertebrate
organic matter marine upwelling, sedimentation carbon content community structure and production
Contaminants Point and nonpoint sources organic Chemical concentrations in Altered organism growth, reduced
carbon, hydrology sediment, water, and biota immune function
Biological
Emergent Elevation, tidal regime, salinity, soil Area, percent coverage, shoot Primary production, faunal utilization
vegetation composition, pore water salinity, density, biomass, height, species
competition, grazers richness, relative abundance
Submergent Elevation, substrate, light, temperature, Area, percent coverage, shoot Primary production, faunal utilization
vegetation salinity, nutrients, flow density, biomass
Benthic Substrate, elevation, temperature, Abundance, species richness, Biomass, presence in predator diet
invertebrates salinity, DO, chemical contaminants relative abundance, dominance
Fishes Temperature, salinity, DO, access, Abundance, species richness, Growth, fecundity, residence time,
flow, food availability, predation, relative abundance, dominance movement patterns, survival,
competition, harvest population structure, population
growth
Birds Access, food availability, nesting site Abundance, species richness, Growth, fecundity, residence time,
availability, predation, competition dominance survival, behavior, population
structure, population growth
Phytoplankton Light, temperature, salinity, nutrients, Abundance, species richness, Primary production
stratification dominance
Zooplankton Temperature, salinity, DO, flow, Abundance, species richness, Density, biomass, presence in predator
phytoplankton dominance diets

reclamation has proven to be an effective way to relieve Secure fresh water supplies
some of the pressure on space. Land reclamation also Many estuaries in the world currently face water shortages
offers good opportunities for implementation of the which may be worsened by climate change and pollution.
“building with nature” concept, meanwhile easily apply- Adaptation to land and water use will be an important way
ing new safety considerations. Multifunctional use of to respond to these shortages. This may include more effi-
areas, e.g., giving a water storage function to nature areas, cient water use and/or changes in cropping pattern and fer-
may further assist in relieving the pressure on space. tilization in agriculture. Pollution reduction programs and
establishment of flow requirements for estuaries are
needed. Their implementation may benefit by involve-
Improving resilience of estuaries ment of river basin agencies.
Vulnerability of societies to future climate change (such as
flood risks, droughts, and salinity intrusion) should be Upgrade aging infrastructure
reduced, preferably by making societies more resilient. Many estuaries have irrigation and drainage systems as
Resilience can be improved by preparedness, coping well as flood protection works, roads, water supply, and
strategies, and adaptation to changing conditions. This treatment facilities which require upgrading. Public and
requires a combination of willingness to change, appropri- private partnerships can provide solutions in those cases
ate technology, and community participation. Increasing where farmers, industries, and communities directly bene-
the robustness of infrastructure is another promising way fit from these infrastructure investments. However, for
to respond to the increase of vulnerability of estuarine protection schemes against floods and storm surges, other
areas as well as the growing aversion of risk. options may be more appropriate, such as introducing
 SUSTAINABLE USE 659

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Definition Cross-references
Symbiosis is defined as a lasting, intimate association Mutualism
between members of different species of organisms