C lim a te change and b iological ocean ograp h y o f

th e A rctic O cean

B y R. G radinger

Institu t fu r Polardkologie der ChristKiel,

Wischhofstr. 1-3, Geb. 12, D - 2 f l f 8 Kiel, Germany

Polar environments are characterized by unique physical and chemical conditions for
the development of life. Low tem peratures and the seasonality of light create one of
the most extrem e habitats on E arth. The Arctic sea ice cover not only acts as an
insulator for heat and energy exchange processes between ocean and atm osphere but
also serves as a unique hab itat for a specialized community of organisms, consisting
of bacteria, algae, protozoa and metazoa. The prim ary production of sea ice algae
may play a crucial role in the life cycle of planktonic and benthic organisms. Thus,
a reduction of the sea ice extent due to environmental changes will influence the
structure and processes of communities living inside the ice and pelagic realms.1

1. I n tr o d u c tio n
The Arctic Ocean is an enclosed sea area w ith two m ajor connections to the sur­
rounding seas, the shallow Bering Sea and the relatively deep Fram Strait. The Arctic
marine environment is in its present state one of the most extreme h ab itats on E arth.
Strong seasonal variations of some param eters, such as solar radiation, are in con­
tra st to the relative stability of others, such as w ater tem perature. Organisms living
in polar oceans are well adapted to these environmental conditions. Climatic changes
will therefore influence the structure of the Arctic marine communities, as already
indicated by the geological record: the Pleistocene warming a t approxim ately 1.5 Ma
coincides w ith a drastic increase in N orth Atlantic species and calcareous organisms
in the sediment record as a result of changes in the w ater mass exchange between
the Arctic Ocean and its surrounding seas (Clarck 1990).
The emission of trace gases to the atmosphere by anthropogenic activities may
lead to similar changes, but on much shorter time scales. Global models studying
the effect of CO 2 increase in the E a rth ’s atmosphere showed the largest tem perature
increase in the Arctic (Mitchell et al. 1990). Recent measurements from Alaska al­
ready dem onstrate a tem perature increase of approximately 1.5 °C during the past
decade (Oechel & Vourlitis 1994).
An increase in atmospheric, and consequently sea surface, tem perature will have a
high im pact on the Arctic marine epipelagic system. Based on present knowledge of
the Arctic marine pelagic system, possible changes will be discussed in this contri­
bution, but the developed scenario will be in no sense predictive. It is obvious th a t
the response of the Arctic marine ecosystem will largely depend on the fate of its
most characteristic realm, the perm anent sea ice cover.
Phil. Trans. R. Soc. Loud. A (1995) 352, 277-286 © 1995 The Royal Society
Printed in Great Britain 277 Paper
278 R. Gradinger
120

Chukchi Frobisher Barrow Transpolar

Sea Bay Strait Drift
fast ice fast ice fast ice pack ice
Figure 1. Algal biomass in Arctic sea ice. Data for Chukchi Sea from Clasby al. (1973);
Frobisher Bay from Grainger (1979); Barrow Strait from Smith al. (1989) and Transpolar
Drift from Gradinger (unpublished data).

2. T h e se a ice rea lm
Sea ice covers between 7 and 14 x 106 km2 of the Arctic Ocean (M aykut 1985).
Its existence largely influences the m aterial and energy exchange between ocean and
atmosphere and is therefore a crucial param eter in the modelling of environmental
changes in polar areas. In contrast to A ntarctica, about 50% of the Arctic sea ice
floes survive summer melting and thus reach thicknesses of more th an 2 m (for a
detailed comparison between Arctic and A ntarctic sea ice properties, see Spindler
1990).
Sea ice consists of a mixture of ice crystals and brine channels, which form a
three-dimensional network of tubes and channels with typical diameters of 200 |Lim
(Weissenberger et al. 1992) within the ice matrix. The brine salinity and the total
volume of the brine channels as percentage of the ice volume are dependent on ice
tem perature and total salt content. For example, a decrease in the ice tem perature
from —4 °C to —10 °C leads to growth of ice crystals and thus an increase in brine
salinity from 70 to 144 psu (Assur 1958), as well as a decrease in the brine volume.
Despite these harsh environmental conditions, a specialized community has devel­
oped and adapted to live within the brine channel system. Diatoms are the dominant
primary producers and may contribute more than 90% of the total algal biomass
(Poulin 1990). The seasonal development of the sea ice algae is mainly controlled by
abiotic parameters. The onset of algal growth in spring is triggered by an increase
in available light intensities after the dark polar winter. Sea ice, and especially its
snow cover, reduces the incoming radiation by more than 90% due to high albedo.
Therefore, ice algae are already adapted to start growing under extremely low light
intensities (2-10 |rm ol m~2 s_1; Horner & Schrader 1982). The biomass built up by
sea ice algae during the Arctic summer varies between 1 and 100 mg Chi a m~2 (fig­
ure 1). Highest concentrations have been observed in fast ice areas of the Canadian
shelf (Clasby et al. 1973; Smith et al. 1989), while the concentrations within the
multiyear ice floes of the transpolar drift system are one or two orders of magnitude
lower (Gradinger, unpublished data).
Large fluctuations in tem perature, and therefore brine salinity, restrict life within
the Arctic ice floes to the lowermost decimetres, and so-called sea ice bottom com-
Phil. Trans. R. Soc. Lond. A (1995)
 Climate change and biological oceanography o f the Arctic Ocean 279

sal. (psu) brine volume (%)

100 -

01 chlorophyll in ice chlorophyll in brine

5° \ 50 i
§ ioo
S 150 i •3 150
1*200 -j
250 i
300 ■-= 0 10 20 30 40 50 60
mg Chi a nr ^ ice mg Chi a m "3 brine

Figure 2. Temperature, salinity, brine volume and algal biomass in an Arctic multiyear ice floe
(Gradinger, unpublished data).

munities are formed (Horner 1985). Figure 2 shows an example of the chlorophyll
distribution in an Arctic multiyear ice floe, sampled in the East Greenland Current
in August 1994 (Gradinger, unpublished data). Low salinities and relative high tem­
peratures are idiosyncratic for Arctic summer sea ice. The calculated brine volume
based on the equations by Frankenstein & Garner (1967) varies between 10 and
30% of the total ice volume. The chlorophyll profile clearly shows a well developed
bottom community with concentrations above 50 mg Chi a m -3 ice in the lowermost
centimetres. The actual algal concentration within the brine channel system is even
higher exceeding values of 400 mg Chi a m~3 brine. This high algal biomass serves as
the food source for a variety of proto- and metazoans (figure 3), which are mostly
smaller than 1 mm. In shallow sea areas, nematoda and crustaceans are the domi­
nating organism groups (Carey & Montagna 1982; Cross 1982; Kern and Carey 1983;
Grainger etal. 1985), while a distinct community inhabits multiyear ice floes, with
ciliates and turbellarians as most abundant taxa (Gradinger et al. 1991).
The high algal biomass inside Arctic ice floes is used by pelagic and benthic or­
ganisms during parts of their life cycle. Carey & Montagna (1982) observed larvae of
benthic polychaetes and molluscs inside Arctic sea ice, and Kurbjeweit et al. (1993)
made a similar observation for the Antarctic pelagic copepod, Stephos longipes. For
these organisms, ice floes serve as a kind of ‘kindergarten’ to the juveniles, providing
both food and shelter against possible predators.

3. T h e u n d er-ice realm
The boundary-layer between Arctic ice floes and the water column forms the
habitat for a specific community of organisms. Diatoms, mainly the species Melosira
arctica, may grow to long, macroscopic visible bands, reaching lengths of more than
15 m and widths of 1-2 m, hanging down from the underside of the floes into the
water column (Melnikov & Bondarchuk 1987). Amphipods of the genera ,
Apherusa and Onisimus (Lpnne SzGulliksen 1991) are perman
boundary between ice floes and the pelagic realm in densities of up to 60 individuals
per m2 of ice (Carey 1985). These organisms, which are partially endemic to the
Phil. Trans. R. Soc. Lond. A (1995)
280 R. Gradin

100 -

$
80 -
1 60 -
iiCiliates
C □ Nematoda
•S G3Turbellaria
> 40 - ■ Rotatoria
^32 20 □ Nauplii
-
□ Others

0 1
Pond Stefanson Beaufort Frobisher Greenland
Inlet Sound Sea Bay Sea
Figure 3. Relative composition of sea ice meiofauna in various parts of the Arctic Ocean. Data
for Pond Inlet from Cross (1982); Stefanson Sound from Carey & Montagna (1982); Beaufort
Sea from Kern & Carey (1983); Frobisher Bay from Grainger al. (1985) and Greenland Sea
from Gradinger etal. (1991).

Arctic Ocean, use the high algal biomass formed both directly at the underside and
by the bottom community as a food source (Carey & Boudrias 1987). Besides the
availability of food, they use the ice underside as a refuge to find shelter in the
three-dimensional structure of, for example, pressure ridges.
Beside the autochthonous under-ice fauna, pelagic zooplankton, like the copepod
species Calanus aglcis nd seudocalnspp. (specially P.
P
ascend from deeper water layers to the underside of the ice floes to feed on ice algae
(Runge et al. 1991). The under-ice fauna forms the link between the ice based primary
production and the pelagic animals. These feed on ice algae and are im portant prey
organisms for higher trophic levels like the polar cod ( ; Bradstreet
& Cross 1982).

4. T h e p ela g ic realm
The biomass of pelagic organisms in the permanently ice-covered central regions
of the Arctic Ocean is extremely low. The permanent ice cover reduces the incoming
radiation, significantly suppressing algal growth to a degree already recognized by
the early studies of Braarud (1935) and Steemann-Nielsen (1935). Concentrations of
inorganic nutrients are relatively high throughout the year, and oxygen concentra­
tions are in near equilibrium with the atmosphere, in agreement with the general idea
of very low primary productivity in the central Arctic regions (Jones et al. 1990).
The low algal biomass under the permanent pack ice is formed by small flagellates
(Braarud 1935; Horner &: Schrader 1982) in contrast to the diatom-dominated ice
algal community. Investigations in the permanently ice-covered western part of the
Greenland Sea (Gradinger & Baumann 1991) revealed an average algal biomass of
7 mg Chi m-2 in the upper 40 m of the water column under dense pack ice (figure 4), a
value similar to the biomass observed inside the ice brine channel system. Thus, algal
biomass has almost the same total value in sea ice and in the water column below, but
the ambient concentrations (sea ice brine channels: greater than 400 mg Chi a m -3 ;
euphotic zone: less than 0.2 mg Chi a m~3) are extremely different.
High phytoplankton concentrations, with integrated chlorophyll concentrations
above 40 mg Chi m -2 , are restricted to marginal ice zones (miz ) and polynyas. The
Phil. Trans. R. Soc. Lond. A (1995)
 Climate change and biological oceanography of the Arctic Ocean 281

, Chi a
i (mg/m2)l
1 h40
1 -30 /
/
-20
ice cover -10

(°W) longitude (°E)

Figure 4. Distribution of algal biomass in the upper 40 m of the Greenland Sea (modified after
Gradinger and Baumann (1991)).

significance of miz as regions of enhanced pelagic productivity was. first shown for
Arctic shelf areas (Rey Sz Loeng 1985; Alexander &; Niebauer 1981), where melting of
ice floes leads (i) to an enhanced water column stratification and (ii) to increasing ra­
diation. These conditions allow an even earlier onset of the phytoplankton growth in
the miz than in the adjacent open water. Plankton blooms in miz are mainly formed
by Phaeocystis pouchetii and pelagic diatom species (Gradinger & Baumann 1991).
During the Arctic summer, nutrients become depleted in the upper layers of the
water column (Spiess et al. 1988; K attner & Becker 1991). Mesoscale processes like
eddies and local wind-induced upwelling events (Buckley et 1979; Johannessen et
al. 1983) lead to spatial patchiness in nutrient and algal concentrations and permit
a prolongation of the algal growth period throughout the Arctic summer until the
months September/October (Heimdal 1983).
Other areas of enhanced primary productivity in the Arctic Ocean are polynyas.
The North East Water polynya, as one example, opens each year on the Green­
land shelf, starting in late spring (May-June), and reaching its maximum extent
of 44000 km2 in late summer (Wadhams 1981). Investigations in the polynya re­
vealed similar biological characteristics to those described for marginal ice zones,
since improved light availability and water column stratification enhance phytoplank­
ton growth (Gradinger Sz Baumann 1991). The gradual increase in algal biomass is
related to a decrease in nutrient concentrations until nitrate becomes depleted in the
surface layer (Lara et al. 1994).
The life cycles of the Arctic zooplankton species are strongly adapted to the ex­
treme seasonality and patchiness of food availability. During the short Arctic summer,
Phil. Trans. R. Soc. Loud. A (1995)
282 R. Gradinger
Arctic mesozooplankton, mainly consisting of copepods ( , Calanus
hyperboreus, and Metridia )longa feed and grow as young stages in t
and accumulate energy-storage products, especially lipids, to survive the long star­
vation periods. They overwinter using a diapause-like strategy in deep waters, and
again ascend to the euphotic layer in early or late spring (Smith & Schnack-Schiel
1990). The high algal biomass in polynyas and miz is used by the herbivorous zoo­
plankton to sustain themselves in the Arctic Ocean. While the mesozooplankton may
only have a minor impact on the algal production in the polynya and the marginal
ice zone (Barthel 1986; Hirche et al. 1994) these regions are of special importance as
areas of successful reproduction for the Arctic zooplankton (Hirche al. 1991).
Due to the availability of food, marginal ice zones and polynyas are of major
importance to the higher trophic levels of the Arctic marine ecosystem, as mesozoo­
plankton species ( alnussp p.) are central to the pelagic food web (Bradstre
C
Cross 1982). Various species of birds and marine mammals use marginal ice zones as
migration routes due to the reliable availability of food (Ainley & DeMaster 1990).
The breeding success of Arctic seabirds is dependent on the development of marginal
ice zones at an accessible distance from the breeding grounds (Bradstreet 1988). Bird
densities in marginal ice zones may be one to three orders of magnitude higher than
in the adjacent ice-covered or open water area (Divoky 1979). Polynyas are for the
same reasons attractors for both predators and their prey (Dunbar 1981). Large
sea bird rookeries in the Canadian Arctic are located in the bird’s flight range to
a recurring polynya (Brown Nettleship 1981). The distribution of marine mammals
is to a large extent determined by the position of polynyas as well (Stirling et al.
1981). Changes in the extent and distribution of polynyas, marginal ice zones and
permanent ice cover will consequently directly influence recruitment success, migra­
tion behaviour and, in the long term, life cycle strategy of Arctic marine birds and
mammals.

5. E ffects o f clim a te change on th e A r ctic m a rin e s y ste m

The atmospheric CO 2 concentration has increased from a pre-industrial level of
280 ppm to a current level of approximately 360 ppm with an annual rate of about
1.5%. The concentration of methane, which is an even more effective greenhouse gas
than CO 2 , is increasing at a similar rate. These changes in the composition of the
Earth s atmosphere have the potential to affect the global climate and increase the
surface temperature. Predictions for the global climate in the next century using
general circulation models indicate an enhanced warming of Arctic areas relative to
lower latitudes, making the Arctic one of the most sensitive areas in the world.
Evidence of warming in high latitudes has already been gathered. Temperature
in certain parts of northern Alaska has increased over the last decade, and will
largely affect the conditions for terrestrial ecosystems (Oechel & Vourlitis 1994).
The expected warming of the Arctic atmosphere will cause the permanent pack ice
to shrink or even disappear in the next century.
A reduction in the extent of the permanent ice cover and a shift to a more seasonal
ice regime will greatly affect the structure of the Arctic marine ecosystem (figure 5).
Polynyas and marginal ice zones will occur in regions which until now have been
characterized by a permanent ice cover and an extremely low biological productivity
(Manak & Mysak 1989). Today, there is still uncertainty about the role of the biolog­
ical C 0 2 pump in relation to physical processes in general (Longhurst 1991) and in
Phil. Trans. R. Soc. Lond. A (1995)
 Climate change and biological oceanography of the Arctic Ocean 283

(a) Present situation

ice cover

under-ice fauna - ■ i i high

sedimentation
zooplankton ^ Chlorophyll concentration
in sea ice and pelagial
— mixed layer depth

(b) Future scenario with reduced ice cover

ice cover polynya MIZ

Figure 5. Structure of the marine ecosystem of the Arctic Ocean: (a) present state; ( ) changes
due to a reduction in the ice cover

polar oceans in particular, due to the scarcity of information (Legendre et al. 1992).
Nevertheless, Anderson et al. (1990) have stated th at the Arctic Ocean will be an ac­
tive part of the biological pump transferring atmospheric CO 2 into the biogenic food
web. An increase in the extent of polynyas and marginal ice zones further north will
increase the biological productivity of the Arctic Ocean and the transfer of carbon
from the atmosphere to the sea floor. Thus, the Arctic Ocean, despite its relatively
small contribution to the world’s ocean surface area, may play an important role
in the global carbon cycle through enhancement of biological carbon fixation and
subsequent sedimentation.
Besides the effects on total biological productivity, a reduction of the sea ice cover
and changes in the location of polynyas and the marginal ice zones will have se­
vere impact on several Arctic animals. Endemic ice-related species like the under-ice
amphipod Gammarus ,w
ilktz or sea ice meiofauna species which are restricte
Phil. Trans. R. Soc. Loud. A (1995)
284 R. Gradinger
in their distribution to the. permanently ice-covered regions, will be diminished. Sea
bird rookeries, located at present in the vicinity of polynyas and marginal ice zones,
will either follow the receding ice extent, or the breeding success will decrease due
to a higher energy consumption of the adults as a result of longer flight distances
between feeding source and breeding area.
Endemic pelagic species like Calanus glacialis or Calanus hyperboreus will come
into interspecific competition with sub-Arctic species like Calanus ,
and the distribution boundaries of high-Arctic species may shift northward as sea-
surface warming occurs. These changes in the composition of communities in the
various habitats of the Arctic marine environment can be expected at timescales of
years to decades and will largely depend on variations in the hydrographical regime,
like, for example, the inflow of warm water from the North Atlantic.
The expected warming of the Arctic Ocean will change the structure of the marine
communities into a more productive scenario. Harmful effects may be restricted to the
flora and fauna living in close association to the Arctic multiyear ice floes. Greater
danger to the Arctic marine environment on shorter timescales must be expected
through pollution by oil, chlorinated hydrocarbons and radioactive waste, already
introduced into the Arctic environment through human activity (Sakshaug & Skjodal
1989).

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