Introduction

Mangroves and seagrasses, two significant coastal communities, are located between coral reefs
and rainforests, two of the world's most famous tropical habitats. The protected intertidal margin
area has mangroves that are only a few feet above mean sea level. Depending on the transparency
of the water column, seagrasses can be found in environments from the intertidal to deeper levels.
Each of these organized ecosystems, like coral reefs, contributes significantly to coastal processes
through highly developed links and interconnectedness. Each of these relationships is essential to
the other's survival. Both plant communities perform important ecosystem services such as
sediment stabilization, nutrient processing, shoreline protection, providing habitat and nursery. the
diversity of organisms that reside in, and use, mangroves and seagrasses can be quite high. Many
fish and shellfish, including important commercial, recreational and artisanal fisheries, spend all
or part of their life cycle in mangroves and/or seagrasses. Other fishery species offshore depend
on the abundant supply of food resources coming from these highly productive shoreline
ecosystems. Mangroves and seagrass beds are now two of the most important tools to fight against
climate change and global warming.

Mangroves
The halophytic plant communities along the tropical and subtropical coastlines are termed as man-
groves (Biswas et al. 2012). Respiratory or knee roots (pneumatophores) are characteristic of many
species; they project above the mud and have small openings (lenticels) through which air enters,
passing through the soft spongy tissue to the roots beneath the mud. Mangrove forest is one of the
primary features of the coastlines throughout the tropics and sub-tropics of the world. Being the
source of a variety of renewable resources, mangroves are playing a significant role in the local
coastal economy and livelihood of the people, in the national economic development and also in
the regional environmental balance. The ecological significance of mangrove forests has now been
universally recognized. Mangroves are characterized by a higher fisheries biodiversity as well as
higher standing stock (Christensen, 1982; Chong et al., 1990; Morton, 1990; Robertson and
Blaber,1992; Hong and San, 1993; Sasekumar et al.,1994).

Figure. Mangrove tree(left) and Mangrove forest scape(right).

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Environmental requirements of mangroves
(1) Climate: Mangroves are mainly tropical species. They can also occur in the sub-tropical
areas. But mangroves cannot tolerate freezing conditions. Their latitudinal limits vary
depending on air and water temperature.
(2) Salinity: a mangrove species can withstand salinity, but not necessarily needs high level
of salt to survive (Saenger 2002). However, they do not develop in a strictly freshwater
habitat.
(3) Tidal inundation: Tidal inundation is an indirect requirement for mangrove development
because inundation of saltwater helps exclude most competitively superior vascular plants.
It also helps transport sediment, nutrient, and clean saltwater to the mangrove environment.
Usually, mangroves reach their greatest development in low-lying areas withlarge tidal
range (Tomlinson 1986).
(4) Sediment and wave energy: Mangroves grow best in low-wave environment. This is
because high waves prevent propagule establishment, expose the shallow root system, and
prevent sediment accumulation (Tomlinson 1986).

Distribution of mangrove forests

With this new insight, we can now say that there are 147,000km2 of mangroves remaining
worldwide, an area about the size of Bangladesh (Source: The Global Mangrove Alliance). A total
of 121 countries and areas were identified as containing one or more species of true mangroves.
the most extensive area of mangroves is found in Asia, followed by Africa and South America.
Four countries (Indonesia, Brazil, Nigeria and Australia) account for about 41 percent of all
mangroves and 60 percent of the total mangrove area is found in just ten countries. 4% of worlds
mangrove cover is located in Bangladesh.

Figure. Mangrove extent of the world.

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Biodiversity of mangroves
Many species still being discovered, the exact number of flora and fauna supported by mangroves
can’t be determined easily. Mangroves are packed with staggering number of flora and fauna. Most
of the major groups of terrestrial animals are significantly represented in mangroves.

Table: Number of species of associated biota recorded from mangroves in the various geographic
regions of the world. Source: Saenger et al. (1983)

Taxonomic group No. of species Taxonomic group No. of species

Flowering plants 238 Amphibians 4

Palms 143 Reptiles 28

Protozoans 21 Birds 559

Echinoderms 63 Mammals 48

Insects 572 Fish 765

Crustaceans 607 Molluscs 629

Ecosystem services of mangrove forests

Mangrove forest is one of the few pristine ecosystems in the world (Spalding et al. 2010), and it
offers a large number of ecological, economic, and protective functions and services (Biswas et al.
2009), including (i) habitat for flora and fauna; (ii) timber, pole, fuel wood, and fiber production;
(iii) diversified non-timber forest products, e.g., tannin, honey, wax etc.; (iv) breeding and nursery
grounds for fish, crustaceans, mollusks, and a wide range of aquatic and terrestrial species; (v)
positive effect on microclimate; (vi) protection from wave erosion; (vii) enhancement of sediment
deposition/land accretion; (viii) input of organic detritus into the coastal zone to support the
productivity of these waters; (ix) amelioration of the environment by acting as a carbon sink; and
(ix) combating natural calamities such as tsunamis, cyclones, and tidal surges. Mangroves are also
among the most carbon-rich forests in the tropics (Donato et al. 2011).

Ecology of mangroves
Mangroves are extremely productive ecosystems, frequently matching the rates of primary
production of tropical terrestrial forests. The mangrove trees themselves, algae that grows on tree
roots and on the forest floor, and phytoplankton in the water column are the three main contributors

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to this primary production. Mangroves might also get nutrients from outside sources. Each of these
sources helps to increase the secondary fisheries production that mangroves are known to sustain.

Figure. Interrelations and food chains in mangrove ecosystem.

Role of mangroves in fisheries

Two key mechanisms, the availability of food and shelter, allow mangroves to increase fish output.
Mangrove forests are extremely fruitful, with mean primary production values that are comparable
to those of tropical terrestrial forests. Their woody debris (detritus) and leaves are an important
component of the marine food webs that sustain fisheries. Additionally, mangroves frequently gain
from nutrient inflow from rivers and other nearby environments. They may also export live
biomass, such as planktonic larvae, mature fish, and invertebrates, as well as nutrients in the form
of dissolved and particulate organic carbon. It is not only the high productivity of the mangroves
that creates value for fisheries, but also their physical characteristics. Mangrove roots and trunks
provide shelter and support to many organisms. driven by the nutritional and physical benefits,
many species use mangroves as nursery grounds. These include species that spend time in
mangroves as juveniles before moving to offshore habitats such as coral reefs. Thus fisheries in
these offshore habitats benefit from stock replacement from mangroves. Around 210 million

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people reside in low-lying regions less than 10 kilometers from mangroves, and many of them
profit from fisheries related to the mangroves. Because there are so many distinct fisheries,
markets, and degrees of exploitation, the economic worth of fisheries related with mangroves vary
greatly. Along with economic benefits, fisheries related to mangroves supply millions of people
with food and employment.

Mangroves against climate change

Climate change is the biggest threat our world faces. Already, we are seeing astonishing ice melt
in the Arctic, sea level rise threatening coastal communities, and more extreme weather events
including forest fires, winter storms, and tropical cyclones worldwide. Mangroves are regularly
referred to as a “nature-based solution”, a term often used in reference to tackling the climate crisis.
A nature-based solution leverages the strengths that already exist in nature to mitigate or adapt to
the impacts of change. One of mangroves’ biggest strengths lies in their ability to capture and store
carbon. The muddy soil that mangroves live in is extremely carbon-rich and over time the
mangroves help to not only add to this store of soil by capturing sediment but hold it and the carbon
in place. They are the largest reservoir of world’s blue carbon, which is defined as the organic car-
bon stored, sequestered, and released from coastal and marine ecosystems (Pendleton et al. 2012).
It is estimated that global mangroves store 4–20 billion tons of blue carbon (Donato et al. 2011).

Threats against mangrove forests

Multiple challenges make mangrove ecosystems susceptible. Globally, regionally, and locally
distinct risks exist. Multiple hazards can have an impact on mangroves at once, or over time as
land use patterns change. There are some naturally occurring threats, like typhoons and shoreline
erosion, but most of them are caused by humans, like overfishing, conversion of mangrove habitats
for habitation and agriculture, aquaculture, a decline in freshwater supplies, silt deposition, and
heavy metal pollution. There will be additional challenges to mangrove ecosystems in the future,
in addition to the expected climate change, which includes global warming, sea level rise, and
extreme weather events. For mangroves, sea level rise is the biggest climate-related threat, with
some tree species unable to tolerate the influx of saltwater or escape the surging tides. Coastal
wetlands, including mangrove forests, absorb a significant amount of greenhouse gas emissions.
When these forests are cleared, we compound the climate change problem by releasing even more
carbon into the atmosphere.

Recommendations for mangrove management and protection

1. Avoiding mangrove loss through regulation protection and/or the development of strong
local or community level ownership. Controlling ex situ threats can be another critical
activity. Protected areas are another widely used tool, and over a quarter of the world’s
mangroves are located in areas designated for nature conservation.

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 2. Restoring mangroves by active mangrove plantation or restoring other systems of
mangrove like hydrological conditions, sediment supply, freshwater flow etc. In many
place mangroves recover naturally given the opportunity. Before any efforts are made
towards restoration it is critical to understand both the cause of original loss and the current
ownership and regulatory regime.
3. Managing fisheries by local resource ownership and use, gear or harvest regulations,
declaring no take zones, access agreements and certifications. Regular monitoring of
existing arrangements between stakeholders is very important.
4. Awareness, communication and engagement raising among local people to protect
mangrove forests.

Seagrass beds
Seagrasses are submerged blooming plants that may be found in bays, lagoons, and along the
continental shelf in shallow marine waters. While the taxonomic diversity of seagrass is low, its
acreage typically extends to hundreds of thousands of kilometers of the coastline (Short et al. 2007;
Orth et al. 2006). Seagrasses can form dense underwater meadows, some of which are large enough
to be seen from space. These underwater grasslands are known as seagrass beds. Although they
are sometimes mistaken for seaweed, they are really more closely linked to blooming land plants.
Seagrasses produce flowers and seeds in addition to having roots, stems, and leaves. There are
over 72 distinct species of seagrass now, having emerged about 100 million years ago. A
staggeringly wide range of creatures, including small invertebrates, huge fish, crabs, turtles, marine
mammals, and birds, find shelter and nourishment among seagrasses. Additionally, seagrasses
offer many valuable benefits to humans, but due to human activity, many seagrass meadows have
been destroyed.

Figure. A seagrass bed in the Mediterranean Sea.

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Special features of seagrasses

• Seagrasses belong to a group of plants called monocotyledons that include grasses, lilies
and palms. They have leaves, roots and veins, and produce flowers and seeds.
• Lacunae, or tiny air pockets, are found in seagrasses and assist the plant's leaves stay
buoyant and exchange oxygen and carbon dioxide.
• They lack stomata, in contrast to blooming plants on land. Instead, they have a thin cuticle
layer, which allows gasses and nutrients to diffuse directly into and out of the leaves from
the water.
• Seagrasses use their roots and rhizomes (thicker horizontal stems) to penetrate the bottom
sediment and anchor themselves to the soil and to absorb and store nutrients.
• In order to absorb sunshine and nutrients from the water and sediment, seagrasses grow
both vertically and horizontally. Their blades extend upward, while their roots extend
downward and sideways.
• They reproduce sexually and by asexual clonal development, which is how they spread.

Requirements for development of seagrass beds

Seagrasses grow in the following circumstances (Spalding et al.2003)

• Salt or brackish water.

• Enough light for photosynthesis. They need an excess of 11% of the incident light in the
surface water. They therefore grow in shallow (less than 20 meters) regions along the
coasts. The average depth is a few meters, but they are recorded down to 90 meters depth
in clear water.
• Clear water. The plants die when the water becomes turbid by sediment suspension.
• A soft substrate such as mud or sand, but some species can also grow on rocky sediments
and corals.
• A gently sloping coast, with little or no tidal currents or strong waves, is preferred.

Distribution of seagrass beds

Typically, temperate and tropical coastal locations sheltered shallow seas are where seagrasses
may be found. Seagrass can be patchy, but more often it forms large swaths of vegetation,
sometimes over 10,000 km2 in size. The most extensive areas are found in the tropics. The total
area covered by seagrass meadows globally is estimated to be on the order of 160,000 km2, but
possibly about 100,000 km2 more. About 70 seagrass species have been reported. A few species
occur in colder regions. Four closely related species are native of European waters. There are
several distinct areas of seagrass meadows. These areas are the Indo-Pacific region, the seas around

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Japan and Australia, the central Western Atlantic region, the north East Atlantic region and the
Baltic and Mediterranean seas.

Figure. Global distribution of seagrass beds.

Biodiversity of seagrass beds

Numerous different types of creatures, including hydroids, sponges, bryozoa, and seaweed etc. can
be found on the vast seagrass meadows. Most of these creatures stick to older leaves. Loss of
seagrasses would result in the extinction of many invertebrate species or a sharp decline in their
population. Because they are too rough, most seagrasses are not directly devoured by herbivores.
Sea urchins, sea turtles, dugongs, manatees, several fish species, and ducks are among the animals
that feed on the seagrasses. The sediment is full of filter feeders. Scallops, jackknife, clams, and
sea cucumbers are a few examples. practically 60% of all sea turtles, 80% of dugongs and
manatees, 13% of dolphins and porpoises, 9% of sharks and rays depend on them for their habitat.

Ecosystem services provided by seagrass beds

Seagrass meadows provide many ecosystem services; some major services are:

1. Fisheries. By providing fish, bivalve, and crustacean species with nursery environments,
seagrasses aid in world fisheries. They offer protection from predators (particularly for
young invertebrates) and a supply of essential nutrients in the form of palatable blades,
debris, and epiphytic algae.
2. Climate regulation. Seagrass meadows store large amounts of carbon in the biomass and
sediment below.
3. Biodiversity. Rich marine biodiversity may be found in seagrass meadows, including
endangered and well-known species like dugongs, sea turtles, sharks, and seahorses.

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 4. Ocean acidification buffer. Seagrass meadows regulate the chemical composition of
seawater by releasing oxygen and removing carbon dioxide during daylight, oxygenating
water and buffering ocean acidification.
5. Water filtration. Seagrasses are natural filters that trap sediments and excessive nutrients
out of the water.
6. Coastal protection. Seagrass meadows stabilize the substrate, enhance sedimentation and
dampen wave activity, thereby helping to mitigate coastal erosion and protect against
flooding and storm surges.
7. Disease prevention. Seagrasses may clean the water of microbiological impurities,
lowering the risk of fish, people, and invertebrates coming into contact with bacterial
infections. They create secondary metabolites that have antifungal and antibacterial
properties.
8. Tourism. Seagrass meadows provide cultural services such as sense of identity for local
communities and opportunities for recreational activities (e.g. birdwatching, diving,
fishing).

Fisheries in relation with seagrass beds

On generally featureless sediment bottoms, seagrasses add physical structure, increasing
community diversity, biomass, and primary and secondary production. The leaves offer a substrate
for epiphytic microalgal development that supports food webs as well as a haven for invertebrates
and fish that may be found in much higher concentrations than in bare benthic environments. This
combined productivity of seagrasses and associated algae ranks seagrass beds among the most
productive ecosystems on earth (Duarte & Cebrián 1996). Seagrasses act as a nursery habitat for
various species of fish. Some of these species have commercial importance when they become
adults. Other species are the prey for species that are commercially important (Heck et al., 2003).
crustaceans are eaten by red snapper for example. However, distinct correlations between seagrass
beds and commercially captured species have been discovered for various regions of the world,
which enables more accurate economic estimations of the seagrass' true value. Additionally, the
seagrass nursery serves as a home (and occasionally a feeding site) for marine animals that migrate
during the day between coral reefs and the nearby seagrass meadows.

Blue Carbon and seagrass beds

Until very recently, the role of seagrass ecosystems in carbon sequestration was not documented
on a global scale. Seagrasses are capable of capturing and storing a large amount of carbon from
the atmosphere. Similar to how trees take carbon from the air to build their trunks, seagrasses take
carbon from the water to build their leaves and roots. As parts of the seagrass plants and associated
organisms die and decay, they can collect on the seafloor and become buried, trapped in the
sediment. It has been estimated that in this way the world's seagrass meadows can capture up to
83 million metric tons of carbon each year. The carbon stored in sediments from coastal

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ecosystems including seagrass meadows, mangrove forests and salt marshes is known as "blue
carbon" because it is stored in the sea. While seagrasses occupy only 0.1 percent of the total ocean
floor, they are estimated to be responsible for up to 11 percent of the organic carbon buried in the
ocean. One acre of seagrass can sequester 740 pounds of carbon per year. Alongi (2020) estimated
the average rate of C sequestration by seagrasses to be about 221 g C m-2yr-1 and the average
sequestered stock to be 16.3 kg C m-2.

Figure. Carbon sequestration process by seagrass beds.

Existing threats to seagrass beds

Recent studies have reported a perpetual worldwide decline in seagrass abundance (Orth et al.,
2006). The causes of these de-clines vary spatially and temporally. Heavy dredging from marine
construction is a well-documented negative impact activity on seagrass beds. Shallow seagrass
beds are especially prone to scouring from vessel grounding and scarring from the propellers of
motorized boats. These injuries not only remove the aboveground biomass, but excavate the
rhizomes and sediment sometimes creating blowholes. Marine fauna can then create further
damage by excoriating the adjacent rhizome thus causing neighboring beds to. Near shore seagrass
beds are also vulnerable to allocthonous nutrient inputs as effluent from human activities or from
groundwater. These nutrient increases can result in an ecological shift to faster growing micro and
macroalgae both of which outcompete seagrasses for light, and are physiologically better equipped
to proliferate in a high nutrient environment. Overfishing can also spur cascading effects that have
negative effects on seagrasses in a couple of ways. Firstly, the removal of large predators releases
the consumer pressure on smaller predators who feed on epibenthic fauna in seagrass ecosystems.
Epibenthic fauna feed on epiphytic algae that accumulate on the blades of seagrasses. When
epibenthic fauna is removed from the system, the accumulation of epiphytes on seagrass leaves
can prevent seagrasses from accessing much needed light for photosynthetic activity. Secondly,

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the removal of large predators allows herbivores to feed unimpeded on seagrass beds (Myers et
al., 2007).

Management of Seagrass beds

Most management that protects seagrasses focuses on maintaining their biodiversity and the
services these habitats provide for humans and ecosystems. There is no international legislation
for seagrasses, and so protection typically occurs by local and regional agencies. Actions taken to
help seagrasses include limiting damaging practices such as excessive trawling and dredging,
runoff pollution and harmful fishing practices (such as dynamite or cyanide fishing). There are
also attempts to rebuild and restore seagrass beds, often by planting seeds or seedlings grown in
aquaria, or transplanting adult seagrasses from other healthy meadows. Some of the most
successful restoration stories come from the Chesapeake Bay and coastal Virginia.

Conclusion
Today, approximately 3 billion people about half of the world's population live within 200
kilometers of a coastline. Mangroves and seagrass beds are essential for maintaining the highly
productive, ever changing and most vulnerable ecosystem of the coast. Without a healthy
ecosystem and their key components like mangroves and seagrass beds, our coasts will surely fail
to support such a population living on coasts as their livelihood is connected to it through fisheries,
agriculture, aquaculture etc. The most important service they provide is that they are continuously
cleaning our environments by trapping more and more carbons. All studies conducted recently
have shown that mangroves and seagrass beds are facing all kinds of threats from natural to
anthropogenic. If we become determined to save these important ecosystems, they will keep
providing for us and all other dependent marine and coastal animals in years to come.

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