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KEYWORDS
ISSN: 0974 - 0376
NSave Nature to Survive
: Special issue, Vol. III:
www.theecoscan.inAN INTERNATIONAL QUARTERLY JOURNAL OF ENVIRONMENTAL SCIENCES
Prof. P. C. Mishra Felicitation Volume
Paper presented in
National Seminar on Ecology, Environment &Development
25 - 27 January, 2013
organised by
Deptt. of Environmental Sciences,
Sambalpur University, Sambalpur
Guest Editors: S. K. Sahu, S. K. Pattanayak and M. R. Mahananda
Susanta Kumar Chakraborty
Mangrove
Biodiversity
Biocomplexity
Environmental Variables
Ecohydrology
Ecorestoration
251 - 265; 2013
INTERACTIONS OF ENVIRONMENTAL VARIABLES DETERMINING
THE BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM OF
WEST BENGAL, INDIA
252
SUSANTA KUMAR CHAKRABORTY
Department of Zoology, Vidyasagar University,
Midnapore - 721 102, West Bengal, INDIA
E-mail: [email protected]
INTRODUCTION
Coastal Zone represents the junction between the land and sea. The extent of
coastal ecosystem is limited to that part of land which is influenced by adjoining
sea and that part of the sea / estuary which is subjected to the impact of adjoining
land. Coastal environment plays a vital role in nation’s economy by virtue of their
resources, potential for ecotourism and fisheries, controlling ability of
meteorological phenomenon, ensuring navigation and water transport system.
India has a coastline of 7517km of which the mainland accounts for 5422km.
Lakshadeep coast extends 132km and Andaman and Nicobar Islands have a
coastline of 1962km. Nearly 250 million people live within a distance of 50km
from the coast (Qasim et al., 1988). Coastal habitats across the world have been
under multi-dimensional threats during last few decades because of high
population and development pressures. Mangroves have been particularly
vulnerable to exploitation because they contain valuable bioresources and
provide significant ecological services.
Mangrove ecosystem, a unique, fragile, highly productive ecosystem in the sea-
land interphase, is the conglomerations of plants, animals and microorganisms
acclimatized in the fluctuating environment of tropical intertidal zone.
Mangrove forests cover wide tropical and subtropical intertidal areas of coastal
environment, and they are very important for their role in maintaining biodiversity,
for sustainable livelihood (e.g., wood and food resources) and for coastal
protection (Robertson and Alongi, 1992; Wolanski, 2006a).
The coastal area of West Bengal extends over 0.82 million hectors and 220km of
NSave Nature to Survive QUARTERLY
Among the 9 maritime states of India with a
coastline of 7500km, West Bengal enjoys a
unique geographical location possessing the
Hoogly-Matla estuarine complex of
Sundarbans shared with neighbouring country
– Bangladesh. The biodiversity of the coastal
area of West Bengal extending over 0.82
million hectare and along 220km of coastal
line shared by two coastal districts viz. South
24 Parganas and Midnapore (East), includes a
good number of mangroves and their associate
plant species, different species of algae, fungi,
lichen, fishes, amphibians, reptiles, birds,
mammals, besides numerous species of
phytoplankton, zooplankton, ichthyoplankton,
benthos, soil inhabiting arthropods and both
soil and vegetation dependent insects. The
settlement of mangrove species and their
simultaneous growth trigger the accretion
process paving the way of deltaic formation in
the coastal West Bengal and simultaneous
settlement of associated floral and faunal
components in tune with interplay of different
eco-bio-physico-chemical factors along
environmental gradients. An ecohydrological
approach towards understanding of mangrove
ecosystem functioning appears to be a
prerequisite for sustainable biodiversity
management and ecosystem restoration. The
present paper highlights the uniqueness of
coastal mangrove ecosystem functioning in
respect of interactions of different
environmental variables with biodiversity in
West Bengal coast. This is also an attempt to
represent how the interplay between
specialized adaptations and extreme trail
plasticity that characterize the mangrove and
intertidal environment giving rise to the bio-
complexity that distinguishes mangrove
ecosystem from others. In such context,
assessment of environmental threats,
biodiversity conservation strategies and
ecorestoration possibilities have been dealt
with in this studied environment.
ABSTRACT
Figure 1: Maps of Sundarban and Midnapore coastal belt
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coastal line. Muddy coast accounts for 180km, of which 90%
are treated as marshy zone having halophytic vegetation and
their associated flora and fauna; only around 40km is
considered as sandy belts. It includes two coastal districts –
The South 24 Parganas, supported by Sundarban Mangrove
Ecosystem and Midnapore Coast having sand flats and
degraded mangrove patch. Indian part of Sundarbans occupies
mangrove area (4262km2) slightly more than that (4109km2)
of highly reclaimed counterpart in Bangladesh (Chaakraborty,
2011). The coastal belt of Midnapore district represents 27%
of West Bengal of coastal tract (60km) extending along the
west bank of Hooghly estuary from New Digha and then
curving around Junput, Dadanpatrabarh, Khejuri and Haldia
on the east to the further north east upto Tamluk or even on
the bank of Rupnarayan (Fig. 1). In West Bengal, three seasons
are very much pronounced (premonsoon, monsoon and
postmonsoon), each with four months duration and are mainly
governed by rainfall and temperature (Chakraborty et al.,
2010).
Seasonal dynamics of mangrove ecosystem
Seasonal change: Mangrove ecosystems are influenced by
seasonal changes in climate, especially seasonal variations in
rainfall as well as sea level. Seasonal changes in sea level
result from a number of factors: (1) variations in wind direction
and speed (often monsoonal) upon the coastal ocean (Ridd et
al., 1988), (2) changes in water temperature that bring about
an expansion in water volume, (3) changes in atmospheric
pressure, and (4) changes in river runoff due to rainfall (Kjerfve,
1990). Such changes in meteorological parameters especially
in view of global climatic change is being thought to impart a
profound impact on mangrove-estuarine biodiversity of West
Bengal (Hazra et al., 2002; Mitra et al., 2004).
Hydrodynamics in mangrove area: Mangrove ecosystems
experience daily inundation and exposure twice a day causing
an interruption of continuous flow throughout the cycle of a
tidal period. Densely vegetated mangrove trees, prop roots,
leaves, pneumatophores and other epiphytic forms, faunal
bioturbatory structures govern the horizontal and vertical
hydrodynamics. Further, since the nature of water flow within
mangrove areas depends on the timescale, it is necessary to
develop different flow models due to tidal flows, groundwater,
and other atmospheric processes, individually. Mangrove
prop roots and pneumatophores are densely intertwined
above the bottom substrate. Because of interaction between
mangrove roots and tidal flows, water turbulence or eddies
occurs primarily in the region of the swamp near the boundary
of the open coast. These turbulent interactions act to mix and
diffuse water and materials, and contribute to form and
maintain the distribution of material within the swamp
(Wolanski, 1995; Furukawa and Wolanski, 1996; Furukawa
et al., 1997). In particular, Wolanski et al. (1998) proposed
that the sedimentation is enhanced by the turbulence around
the vegetation and results in the formation of new land.
Different parts of West Bengal coastal estuarine-deltaic
mangrove settings experience different grades of erosion and
accretion process because of increased velocity of flood inflow
balanced by frictional dissipation of hyper-synchronous
Hooghly estuary (Paul, 2002). Besides, the hydrodynamics
and macrotidal setting, funnel shaped plan of different deltas,
upstream movement of large volumes of riverine sediments
coupled with depositional characteristics have cumulatively
contributed to the depositional landform development (Paul,
2002; Chakraborty et al., 2012)
Material dispersion: The fate of water-born materials in the
mangrove areas is controlled by dispersion processes, which
depend on the unique topography and spatial characteristics
of mangrove vegetation. The fate of cohesive, fine suspended
sediments influences the inflow of sediment-laden waters into
the mangrove forest and triggered the settling of a fraction of
this sediment in the swamp, leading to modify the bathymetry
and expansion of the forest area (Mazda et al., 2009).
Holistic system: The mangrove ecosystem is maintained via
strong feedbacks between many factors as mentioned above.
Each of these factors operates at different timescales. The total
ecosystem is established by nonlinear interactions between
these factors with contrasting timescales. Further, the mangrove
environment should be understood as the ecohydrology,
composed of the river basin, the river, the estuary, and coastal
waters, through which not only water and dissolved materials
but also biotic actions are strongly connected (Wolanski,
2006a).
The four factors, namely, (1) biota (mangrove trees, benthic
fauna such as mad crabs and algae) (2) sediment topography,
(3) water flow such as tidal flow and waves and (4) the
atmospheric processes (wind, rainfall etc), play important roles
individually and in conjunction with others, forming andmaintaining the mangrove ecosystem. Every factor interactsone of other factors (Mazda et al. 2007). The amount of waterthat inundates mangrove swamps depends on vegetationdensity because the vegetation resists water inundation (Mazdaet al., 1999). Further, the tide in mangrove swamps ismeasurably modified from that offshore due to resistance ofmangrove vegetation (Mazda and Kamiyama, 2007). Watson(1928) proposed a simplified classification model to explainthat the growth of mangrove trees and species zonation patternsdepend strongly on the hydrological conditions such as thetides and the elevation of the substrate, as these factors control
the flooding frequency, the duration of inundation and the
depth of inundation (Bunt et al., 1985).
In Sundarbans, the mangrove bioassemblage has been found
to be divided into several ecotonal zones with dominant
characteristic plant species adapted to a set of physico-
chemical variables. Chaudhuri and Chowdhury (1994) divided
the forest into four major forms, based on tidal levels, as, i)
High tide ii) Above general tidal level iii) Frequently inundated
by salt water and iv) Below tide level.
Uniqueness of mangrove ecosystems
Global distribution of Mangroves and associated biodiversity:The term mangroves collectively refers to woody halophyticangiospermic trees inhabiting in the intertidal zone of coastal-estuarine regions in the tropics and subtropics, especiallybetween 25ºN and 25ºS where the winter water temperatureremains not less than 20ºC. Mangrove has a worldwidecircumtropical distribution, the highest concentration beinglocated in the IndoPacific region. The mangroves dominatealmost 1/4th of world’s tropical coastline. The total mangrove
area which spans 30 countries including various island nationsis about 1, 00, 000km2 (Annon, 2003). Mangrove forests have
BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM
254
SUSANTA KUMAR CHAKRABORTY
been estimated to have occupied 75% of the tropical coastsworldwide (Chapman, 1976).
Root causes of loss of mangrove: Anthropogenic pressureshave been found to have reduced the global range of theseforests to less than 50% of the original total cover (Saenger etal., 1983; Spalding et al., 1997). These losses have largelybeen attributed to anthropogenic pressures such as over-harvesting for timber and fuel-wood production (Semesi, 1998),reclamation for aquaculture and saltpond construction(Chakraborty, 1998, Primavera, 1995), mining, pollution anddamming of rivers that alter water salinity levels (Qasim, 1988;Lewis, 1990, Wolanski, 1992) and oil spills (Sengupta andQasim, 1988; Ellison and Farnsworth, 1996; Burns et al.,1994). A major threat to mangrove wetlands is their conversionto areas of aquaculture. After the development of intensiveshrimp farming techniques in Taiwan in the 1970’s, there wasa sudden rush into modern shrimp farming in Southeast Asia(Chakraborty, 1998). In the Indo-Western Pacific region alone,1.2 million hectares of mangroves had been converted toaquaculture ponds by 1991 (Primavera, 1995).
Shrimp farming represents a relatively new form of coastalland use that is becoming a threat in the region. The
construction of shrimp ponds would result in the exposure of
strongly reducing acid-sulphate, soils and a buildup of salinity
levels, such that the subsequent replanting of mangroves in
eventually abandoned ponds is difficult or even impossible
(Stevenson et al., 1999).
More recently mangroves have been managed for integrated
fish culture (Primavera, 1995) and for eco-tourism (Bacon 1987).
Planting mangroves has also been applied for erosion control
in Florida (Teas, 1977) and Nayachar Island, West Bengal,
India (Chakraborty et al., 2012). Beginning with the realization
of ecological roles of mangroves (Odum and Heald, 1975)
and after the enactment of several laws protecting them from
destruction, many small plantings for mitigating environmental
damage have occurred for example in Hawaii, Burma, Fiji(Hamilton and Snedaker, 1984) andJharkhali, Sundarbans,West Bengal, India (Chakraborty et al., 2010).
Role of mangroves: Importance and values: For centuries,mangrove ecosystems have provided goods and services bothon the community, as well as national and global levels(Hamilton and Snedaker, 1984; Stafford-Deitsch, 1996;Dahdouh-Guebas et al., 2000; Kairo and Kivyatu, 2000). Manyof these services are still offered and include collection ofbuilding materials and fuel-wood, gathering of shells toproduce lime and wild honey collection. Mangroves also filterland run-off (Thom, 1967) and control coastal erosion (Davis,1940).
The importance of mangrove ecosystem for its potential forbiological production has received wide acceptance all overthe globe, mainly due to two reasons-
1. Large quantities of energy in the form of mangrove plantscontributed detritus are exported from the mangrove forest
to open water bodies (Odum and Heald, 1975) and positive
correlation exists in between the extent of mangrove and
total bioresource from adjacent water bodies (Macnae,
1974).
2. Profitable regional and international markets for high
quality biological products.
Besides, they are being used as important nursery grounds
and breeding sites for a wide range of faunal components like
birds, mammals, crustaceans, reptiles, finfishes and shellfish.
They act as buffers against coastal erosion and natural disasters
like cyclone, typhoon etc. and provide accumulation sites for
sediment, nutrients, and other elements, including
contaminants.
Adaptability of mangroves and associated fauna and their
response towards ecological changes: Mangrove, being a
unique assemblage living between land and sea display a
number of morphological and ecophysiological adaptations
including viviparous germination, aerial roots
(pneumatophores) and physiological mechanisms to cope with
salinity, inundation and exposure pressure to maintain water
and carbon balance.
Floral adaptations
Adaptations to low oxygen: Red mangroves especially
Rhizophora spp. inhabiting inundated areas, prop themselves
up above the water level with stilt roots and can then take in
air through pores in their bark (lenticels). Black mangroves
like Avicennia spp. living on higher tidal level develop many
pneumatophores having a height of about few meters which
are covered in lenticels. The roots also contain wide
aerenchyma to facilitate oxygen transport within the plant.
Common examples of this type of root are visible in several
species of mangroves like Avicennia spp., Sonneratia spp.,
Heritiera sp., Lumnitzera sp. etc. It is to be noted that Heritiera
fomes (Sundari) shows numerous woody peg like
pneumatophores or blind root suckers (Tomlinson, 1996).
Adaptations for support: Certain mangrove shrubs like
Acanthus sp and climbers like Derris spp., Ipomea sp., grow
on the edges of rivers, saline waterbodies, dunes, marshes
etc. where the anchorage is not very strong. In these cases,
short roots grow obliquely downwards from near the base of
the stem and act like stilts providing additional support as well
as anchorage to the stem (Tomlinson, 1996).
Adaptations to high salinity: Mangrove species have a wide
range of salinity tolerance; as such, mangroves survive and
grow in the frequently tidal inundated saline coastal zones
and estuarine mouths. The soil and water in these coastal and
estuarine zones may interact with mangrove species by three
different ways, viz. by osmotic inhibition of salt water
absorption, by specification effects on nutrition or by causing
toxicity
All mangroves exclude most of the salts in seawater. Thus,
mangroves are endowed with a unique system of ion influx-
efflux regulation by virtue of which they regulate their cellular
ionic contents and have classified mangroves into three
categories (Walter, 1961): salt excluding, salt excreting and
salt accumulating types.
In salt excluding species like Rhizophora mucronata, Bruguiera
gymnorhiza and Ceriops decandra, the root system possess
an ultra-filtration mechanism, which is just like an insurance
of this particular group to dominate in the mangrove
community. Joshi (1975) opined that this characteristic has
enabled these species to dominate other floral components.
The salt excreting species of mangrove community like
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Avicennia alba, Avicennia marina, Avicennia officinalis,
Aegiceros corniculatum, Acanthus ilicifolius etc. regulate their
internal salt levels through foliar glands. However, salt
accumulating species like Sonneratia apetala, Lumnitzera
racemosa, Exoecaria agallocha, Sesuvium portulacastrum,
Sueda maritima etc has the ability to accumulate high
concentration of salts in their cells and tissues, which imparts
succulence. Avicennia spp. can grow best in higher saline
soils and regular tidal inundated areas than in less saline zone.
These species can accumulate sodium ion in its leaf-tissue 10
times higher than potassium ion. Heritiera fomes can also
grow best in less saline soils and are found to accumulate
more of potassium ion than sodium ion (Karmarkar, 1985;
Naskar et al., 2004).
Red mangroves exclude salt by having significantly
impermeable roots which are highly suberised, acting as an
ultra-filtration mechanism to exclude sodium salts from the
rest of the plant. Analysis of water inside mangrove plants has
shown that anywhere from 90% to 97% of salt has been
excluded at the roots. Any salt which does accumulate in the
shoot is concentrated in old leaves which are then shed, as
well as stored away safely in cell vacuoles. White (or grey)
mangroves can secrete salts directly; they have two salt glands
at each leaf base (hence their name-they are covered in white
salt crystals). The most distinctive trichome (appendages which
are epidermal in origin) that develops in certain mangrove
leaves is the structure for secreting certain ions like Na+ and
Cl-. These form a general class of secretory structures referred
to as ‘salt glands’ by Fahn (1979).
Adaptation for limiting water loss: Because of the limited
availability of freshwater in the salty soils of the intertidal zone,
mangrove plants have developed ways of limiting the loss of
water either through transpiration or evaporation that they
lose through their leaves. The orientation of their leaves vary
to avoid the harsh midday sun and so reduce evaporation
from the leaves, and their stomatal openings lie below the
surface of the leaves (shrunken stomata). Mangrove leaves are
almost leathery, coriaceous, thick, fleshy and more or less
translucent with obscure leaf veins, which mean that there,
are no vein sheaths surrounding the veins. Sometimes, the
cuticle is thick and smooth with small hairs, giving the plant a
glossy appearance (Mitra et al., 2004).
Adaptation for nutrient uptake: The mangroves face the
biggest problem in nutrient uptake; thriving in perpetually
waterlogged soil, having little free oxygen. The osmotic
potential of the leaf cells of mangroves is high which is essential
to absorb saline water having higher density with its high
negative water potential.
Thus anaerobic bacteria liberate nitrogen gas, soluble iron,
inorganic phosphates, sulfides, and methane, which make
the soil much less nutritious and contribute to a mangrove’s
pungent odor. Prop root systems allow mangroves to take up
gases directly from the atmosphere, and various other nutrients,
like iron, from the inhospitable soil (Tomlinson, 1996).
Adaptations for increasing survival of offspring: In this harsh
environment, mangroves have evolved a special mechanism
to help their offspring survive. Mangrove seeds are buoyant
and therefore suited to water dispersal. Alternately, viviparous
mode of germination has been developed to ensure the settling
of saplings in the soft soil of mangrove forest floor and thereby
avoid shifting of the propagules by tidal water (Tomlinson,
1996).
Faunal adaptations
Adaptations to wave action: To encounter wave action, soft-
body animals such as bivalves, periwinkles and rock oysters
have hard calcareous shells. Periwinkles and mud snails have
suctorial foot for attachment of rock surface by suction. Crabs
shells are generally flat and round in shape to reduce resistance
to wave action. Crabs also have strong legs for gripping.
Adaptations to salinity stress: Bivalves, rock oysters and
gastropods and mud snails close up their valves or operculum
to prevent the entry of excessively salty water.
Adaptations to desiccation or alternate wetting and drying:
Bivalves, periwinkles, mud snails and rock oysters enclose
themselves in shells and trap a small amount of water within
the shells. Some mobile infauna such as polychaetes,
sipunculans, nemerteans and crabs hide in mud cave to evade
desiccation.
Adaptations for gaseous exchange: Inter-tidal animals enclose
their respiratory organs in a protective cavity to prevent them
from desiccation. Molluscs such as bivalves and rock oysters
have gills in the mantle cavity kept moist and protected by the
shells. Gaseous exchange usually only occurs in the presence
of water trapped in the branchial chamber. In Gastropods
such as mud snails gills are reduced and mantle cavity has
been modified as a lung for aerial respiration.
Adaptations for feeding: Bivalves and rock oysters are filter
feeders. Mud snails possess a radula for browsing the algae
on rock surfaces or plant structures. Fishes like Mudskippers
forage on exposed mudflats when tide water recedes by
hopping about on their limb like fins. Most of these animals
feed during high tides and have their bodies submerged. Crabs
feed at low tides and as detritivores, they feed on any food
washed in by tides.
Response of Mangroves and Mangrove Ecosystems to
Environmental Variables
Mangrove habitats have relatively low levels of species richness
compared with other high biomass tropical habitats like rain
forests and coral reefs (Ricklefs and Latham, 1993). Despite
the relatively low biodiversity, mangrove plants have a broad
range of structural and functional attributes which promote
their survival and propagation in relatively harsh conditions
of the intertidal zone. In this sense, diversity of mangroveplants is not measured in terms of numbers of species, butalso in terms of the ability of each species to cope with thewide range of environmental conditions in utilizing theirindividual, specialized attributes (Duke et al., 1998). Theimportance of such influencing factors is observed wheremangrove species group in distinct forest communityassociations (Bridgewater, 1989), and where species often havedistinct distributional ranges at different geographic scales(Duke, 1992). The distributions of mangroves are constrainedby various physical, environmental and climatic factors.Essentially, all factors must act upon each plant in some way,
and at some, or all, of the various stages of its life, from
propagule development to dispersal, to establishment, to
BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM
256
seedling growth, to reproductive years and maturation, to
advanced age and death. Since mangrove plants are a pool of
individual genetic entities related overall only by their ability
to grow in the intertidal zone, they have understandably
developed different attributes and strategies to live in this
enviroment, including: physical form and structure;
physiological capabilities; productive capacity and growth;
and reproductive development with dispersal of propagules.
Each attribute is then influenced by a range of biotic and
environmental factors which combine to determine the
distributional patterns of each species in combination with
others at global, regional, estuarine and intertidal scales.
The distribution and diversity of mangroves therefore warrants
careful re-consideration in view of the apparently wider
interplay of influencing factors.
Responses to light & temperature: Rates of photosynthesis
drop in mangroves exposed to excessive sunlight, possibly
due to high doses of UV-B radiation. Increasing UV-B produces
biochemical changes like decrease in amino acid and
unsaturated fatty acid production (Kathiresan, et al., 2001).
Excess shading may also negatively affect mangrove plants. In
dense mangrove forests, shaded saplings have lower shoot
biomass than those exposed to the sun.
Response to gases: Mckee (1993) found that flooding and
anoxia reduced the total biomass of Avicennia germinans
seedlings by 20-40% relative to drained controls. High
methane load is associated with anoxia in mangrove
environmants. Mangrove species with pneumatophores may
be best equipped to deal with high methane loads.
Pneumatophore-bearing species release more methane
through their leaves than do those lacking pneumatophores.
Response to gases: CO2: CO
2 stimulates productivity and more
efficient use of water as a result of reduced stomatal
conductance. Elevated CO2 reduced stomatal conductance
and transpiration. However, the effects of increased CO2 may
vary with other physical and chemical conditions.
Response to coastal changes: The mangroves are healthy and
diverse where the land is flat. In shallow basins, poor flushing
and the resultant hypersalinity stunt the mangroves or replace
them with saltmarsh or barren soil devoid of vegetation.
Decreasing rainfall and increasing evaporation also markedly
change mangrove populations. Ellison and Stoddart (1991)
suggested that mangroves are stressed by sea level rises
between 9-12cm. 100y-1 and concluded that faster rates could
seriously threaten mangrove ecosystems. Distribution of
mangroves will be affected by rising sea levels.
Response to tidal gradients and zonation: Zonation can be a
structural feature of mangrove forests in some parts of the
world (Woodroffe, 1992). Contributing factors of zonationinclude plant succession, geomorphology, physiologicaladaptation, propagule size, seed predation and interspecificinteractions (Schwamborn and Saint-Paul, 1996). Interspecificdifferences in tolerance for physiological stress is a cause ofmangrove zonation. Mangrove species respond differently todifferent tidal regimes.
Response to soil conditions: Soil properties have a major
impact on mangrove nutrition and growth. Some of the most
important characteristics are texture, electrical conductivity,
pH and cation exchange capacity. Nutrient fluxes in these
environments are closely tied to plant assimilation and
microbial mineralization (Alongi, 1996). Nutrients availability
may limit growth and production in many mangals. Varying
nutrient concentrations can change competitive balances and
affect species distributions (Twilley and Chen, 1998). In Indian
Sundarbans, texture of soil has been found to show spatial
variations like sandy (18.1 to 50.5), silt (26.6-60.6) and clay
(25.2 to 39.9) (Naskar et al., 2004) and also variations among
different tidal levels like sand (1.3 to 87.8), silt (9.2 to 59.3)
and clay (1 to 47.8) (Chakraborty et al., 2010)
Response to salinity: Salinity, as controlled by climate,
hydrology, topography and tidal flooding, affects the
productivity and growth of mangrove forests (Sylla et al., 1996;
Twilley and Chen, 1998). It can also strongly influence
competitive interactions among species (Ukpong, 1995).
Mangrove vegetation is more luxuriant in lower salinities
(Kathiresan et al., 2001). Chronic high salinity is always
detrimental to the mangroves. True mangroves (e.g. Avicennia
spp. and Rhizophora spp.) tolerate higher salinity than do
non-mangroves, but tolerance also varies among the true
mangroves. The soil and water salinity exhibit three different
mode of interactions in the Sundarbans mangrove estuarine
complex viz by osmotic inhibition of salt water absorption, by
specific effect (Naskar et al., 2004).
Response to sedimentation: Massive land reclamation and
continuous deforestation during last couple of centuries have
resulted detachment of freshwater sources (creeks and
channels of Bidyadhari, Matla estuaries) leading to accelerated
deposition of silts in different deltaic networks of Sundarbans.
Besides, considerable and continuous erosion processes have
led to make the water bodies turbid and to promote unwanted
accretion (Chakraborty, 2011).
Response to metal pollution: The chemical and physical
environment of the mangal may efficiently trap trace metals in
non-bioavailable forms. Disturbances may cause the
mangrove soils to lose their metal-binding capacity, resulting
in mobilization of the metals. The mangal then shift from a
heavy metal sink to a heavy metal source (Lacerda, 1998).
Response to organic and oil pollution: It has been recorded
that mangroves posses high capacity to retain pollutants, which
may be attributed to their presence in anaerobic and reduced
conditions, periodically flooded by tides endowed with high
clay and organic matters. However, the impact of wastewater
to the mangrove ecosystem is a matter of great concern than
the efficiency of wetlands in improving water quality. The
productivity of mangroves may increase due to discharge of
anthropogenic wastes and this process is beneficial particularly
in those areas where nutrient status is low (Wong et al., 1995).
However, excessive discharge of waste may also cause a
negative impact on the positive health of the ecosystem, which
may reduce the efficiency of the system in the process of
biopurification.
Dumping of sewage has become very common in mangrove
systems mainly because of the following factors (Mitra et al.,
2004).
1. The sewage on its way through the mangrove habitat gets
dispersed over vast areas.
SUSANTA KUMAR CHAKRABORTY
257
2. The vegetation automatically filters the nutrients needed
for its growth.
3. The mangrove soil, algae, microbes and physical processes
also absorb large amount of the pollutants (Wong et al.,
1995).
4. The capacity of sequestering the pollutants increases with
the progression of time.
Nutrients (primarily nitrate and phosphates) are often the major
components of sewage load. Researchers have studied the
ability of mangals to absorb nutrients and the effects of the
pollutants on the mangal community and a whole. In general,
mangrove soils have unique capability of trapping wastewater
borne phosphorus, but are less effective in removing nitrogen
(Tam and Wong, 1995). High levels of organic pollutants can
contribute to diseases, death and changes in the species
composition within the mangal (Tattar et al., 1994).
The mangrove soil, algae, microbes, and physical processes
absorb large amounts of pollutants. Nutrients (primarily
nitrogen and phosphorus) are often major components of the
pollution. High levels of organic pollution contribute to disease,
death, and changes in species compositions within the mangal
(Tattar et al., 1994). Oil pollution from oil or gas exploration,
petroleum production and accidental spills severely damages
mangrove ecosystems (Mastaller, 1996). Oiling of mangroves
causes defoliation of the trees. Grant et al. (1993) demonstrated
that sediment oil can inhibit establishment and decrease
survival of mangrove seedlings for several years.
Response to heavy metals: Metal pollution in the estuarine,
harbor and coastal environment is usually caused by land
run-off, mining activities and anthropogenic inputs (Panigrahy
et al, 1999). In coastal West Bengal, the high concentrations
of Zn, Cu, Pb in the sediment and surface waters of Jambu
Island, Frazergaunge and Kakdwip may be related to the
presence of fish landing stations in these areas. A large number
of fishing vessels and trawlers that are engaged in fishing and
landing operations use antifouling paints for their regular
conditioning and protection from biofoulers like barnacle.
Heavy metals like Zn, Cu and Pb being the principal ingredients
of the antifouling paints often contaminate the ambient media
(Goldberg, 1975). Avicennia alba, Avicennia marina, Exoecaria
agallocha and Acanthus ilicifolius have been found to be
unique accumulator of Zn, Cu and Pb. In all the species, the
metal accumulated in the order Zn>Cu>pb>hg in all the
vegetative parts. In a mangrove associated macroalgae
Enteromorpha sp. (Class: Chlorophycea0 this trend has not
been violated (Mitra et al., 2004).
Response to global changes: Because of their location at the
interface between land and sea, mangroves are likely to be
one of the first ecosystems to be affected by global changes.
Most mangrove habitats will experience increasing
temperature, changing hydrologic regimes (e.g., changes in
rainfall, evapotranspiration, runoff and salinity), rising sea level
and increasing tropical storm magnitude and frequency
(Michener et al., 1997). If temperatures exceed 35ºC, root
structures, seedling establishment and photosynthesis will all
be negatively affected. Reduced rainfall and runoff would
produce higher salinity and greater seawater-sulfate
concentrations thus decreasing mangrove production.
Zonation of mangroves: flora and fauna
Mangrove species display distinct zonation in respect of their
adaptability to different environmental stimuli. Small
environmental variations within mangrove forests may lead to
greatly differing methods of coping with the environment.
Therefore, the mix of species at any location within the
intertidal zone is partly determined by the tolerances of
individual species to ecohydrological factors like tidal
inundation exposure and salinity, erosion, but may also be
influenced by other factors such as predation of plant seedlings
by crabs.
The intricate root systems of mangrove plants provide a habitat
for a number of benthic organisms like algae, molluscs,
polychaetes, crabs, bryozoans etc which all require a hard
substratum for anchoring while they filter feed, and help to
impede water flow, thereby enhancing the deposition of
sediment in areas where it is already occurring. Usually, the
fine, anoxic sediments under mangroves act as sinks for a
variety of heavy (trace) metals which are scavenged from the
overlying seawater by colloidal particles in the sediments.
Besides supporting the leaves of a galaxy of pelagic and benthic
fauna of ecological and economic importance, the export of
carbon fixed in mangroves plays very important role in coastal
food webs.
A wide variety of plant species can be found in mangrove
habitat, but of the recognized 110 species, only about 54
species in 20 genera from 16 families constitute the “true
mangroves”, species that occur almost exclusively in mangrove
habitats and rarely elsewhere. Convergent evolution has
resulted in many species of these plants finding similar
solutions to the problems of variable salinity, tidal ranges
(inundation), anaerobic soils and intense sunlight that come
from living in the tropics (Chakraborty et al., 1989).
Why are mangrove ecosystems one of the most productive
ecosystems of the world?
Mangrove ecosystem- the ecosystem dominated by intertidalsalt tolerant halophytic vegetation enjoying the influences oftwo high and two low tides a day, offers a unique environmentfor biodiversity development (Chakraborty, 1995). Duringhigh tide, major parts of the forest subsystem get inundatedand received the major inputs from estuarine water in theform of moisture recharging components of the bottom soildeposition of sediments and nutrients support (macro, microand trace elements) for the forest subsystem. During low tide,the receding water takes away huge amount of mangrove littercontributed detritus to the adjoining aquatic subsystem. Thenutrients released from detritus are utilized by phytoplanktonalong with plenty of water and sunlight available in the openwater system. Mangroves, being a group of perennial evergreenplants, produce huge amount of leaf litter throughout the yearwhich after falling on the moisture rich surface of silt- clayloaded bottom soil are broken down by a galaxy of benthicfauna (crabs, gastropods, microarthropods etc.) into smallerpieces providing more scopes for microbial communities(bacteria, fungi, protozoa) to act upon them for detritusproduction through litter decomposition. The deposit feeders(crabs, molluscs, polychaetes, nematodes etc) through their
feeding activities turn over the surface sediment layer, thereby
BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM
258
exposing new litter surfaces to microbial actions (Chakraborty,
2011).
This detritus based coastal ecosystem is highly productive
having a productivity of about 20 times more than the average
oceanic production (Goudha and Panigrahy, 1996).
Productivity of mangrove ecosystem had been attributed to
four reasons: (1) three types of primary production units (marsh
vegetation, benthic algae and phytoplankton); (2) ebb and
flow of water movements resulting from tidal action; (3)
abundant supplies of nutrients and (4) rapid regeneration and
Floral groups Sundarban Midnapore coast Faunal groups Sundarbans Midnapore coast
True Mangrove 39 16 Zooplankton >100 40
Mangrove associated plant 48 19 Actinarians 07 04
Mesophyte invasive plant 10 6 Polychaetes 59 28
Algae (both benthic & plankton) 150 28 (benthic) Brachyuran crabs 26 14
Molluscs 130 43
Table 1: Diversity of floral and faunal components in mangrove ecosystems of Sundarbans and Midnapore coast, West Bengal
Primary ProducersF
O
R
E
S
T
PhytoplanktonA
Q
U
A
T
I
C
Mangrove
Plants
Benthic algae
Primary Trophic
level
Subsystem
Primary Consumers(Herbivores)
Rotifera , Cladocera ,
Copepods,
Icthyoplankton etc.
Insects, Crabs, Birds,Molluscs , Deer, Wild Boars,Monkeys etc.
Secondary Trophic
level
Secondary Consumers
(Carnivores/Omnivores)
Zooplankton( Chaetognatha )
Subtidal
Benthos(starfish )
and Small Fishes etc
Polychaetes , Globid fishesFishing Cats, Sankes
Birds etc.
.
Tertiary Trophic
Level
Highest Consu mers
(Top Carnivores)
Crocodile
Tiger
Highest Trophic
Level
-----
-----
-----
-----
-----
-----
-----
-----
-----
Primary ProducersF
O
R
E
S
T
PhytoplanktonA
Q
U
A
T
I
C
Mangrove
Plants
Benthic algae
Primary Trophic
level
Subsystem
Primary Consumers(Herbivores)
Rotifera , Cladocera ,
Copepods,
Icthyoplankton etc.
Insects, Crabs, Birds,Molluscs , Deer, Wild Boars,Monkeys etc.
Secondary Trophic
level
Secondary Consumers
(Carnivores/Omnivores)
Zooplankton( Chaetognatha )
Subtidal
Benthos(starfish )
and Small Fishes etc
Polychaetes , Globid fishesFishing Cats, Sankes
Birds etc.
.
Tertiary Trophic
Level
Highest Consumers
(Top Carnivores)
Crocodile
Tiger
Highest Trophic
Level
-----
-----
-----
-----
-----
-----
-----
-----
-----
Figure 2: Trophic relationships in mangrove ecosystem
Figure 3: Diagrammatic representation of food-web in mangrove
ecosystem of Sundarbans, India
conservation of nutrients due to the activity of microorganisms
and filter feeders (Schelake and Odum, 1962) (Fig. 2 and 3).
Biodiversity of Mangrove ecosystems
Global perspective: The genetic as well as species diversity of
mangrove tree species within a given area has been found to
be low compared with other tropical forests and coral reefs.
Research evidence suggests that much greater species richness
is found among fungi, bacteria, protists, viruses, and other
invertebrate phyla (Kathiresan and Bingham, 2001).
As in other ecosystems, species diversity declines as individual
body size increases. Most aquatic invertebrate groups consist
of a few to <50 species within a given forest area (Alongi and
Sasekumar, 1992) with highest diversity most often found
among the crustaceans (Kathiresan and Bingham, 2001).
Insects and birds, although most are only temporary visitors,are highly diverse with species numbers often exceeding 300within a single mangrove estuary. Fish are the most diverseamong vertebrate phyla with species numbers usually rangingfrom 100 to 250 per estuary (Robertson and Blaber, 1992). Ina southeast Asian mangrove estuary, a maximum of 260 fishspecies was recorded (Hong and San, 1993). Such wide rangesof species numbers are a reflection of variable environmentalconditions.
For instance, east Africa has a reduced mangrove crab richness(about 35 species) compared with southeast Asia (>100species; Gillikin and Schubart, 2004), mirroring the diversity
differences between the regions in mangrove flora. At the local
scale, metazoan diversity is, on average, higher on the tree
(encrusting or epibiont assemblages) or on the forest floor
surface and in tidal waters than within the forest floor (Alongi,
1989).
Like other forests, the fauna and flora inhabiting the mangrove
canopy are important in structuring food webs and in
influencing the species composition of mangroves, but this
SUSANTA KUMAR CHAKRABORTY
259
was not recognized until quite recently (Ellison and Farnsworth,
2001; Kathiresan and Bingham, 2001). Insects ordinarily
consume mangrove material equivalent to only approximately
5% of net primary production (Robertson, 1991), but recent
findings point to the importance of insects in affecting the
establishment and growth of seedlings (Minchinton and Dalby-
Ball, 2001; Burrows, 2003; Sousa et al., 2003) and as
pollinators (Ellison and Farnsworth, 2001).
Birds and mammals either temporarily or permanently reside
in mangrove forests, using the forest as shelter and to find
food. The works (Lefebvre et al., 1992, 1994; Lefebvre and
Poulin, 1996, 1997) have established the importance of
mangroves as a home for many species of birds, with some
forests containing up to 315 species and feeding extensively
on invertebrates on the trees, on the forest floor, and in tidal
water.
Scenario in West Bengal: Sundarbans and Midnapore Coast.
Sundarbans Mangrove Ecosystem
Sundarbans mangrove ecosystem harbours various floral
species viz. 34 true mangrove species, more than 50 mangrove
associate species, 163 species of fungi, 150 species of algae,
32 species of lichens and 40 species of mangrove associate.
Among true mangrove plant species, special mention may be
made of Rhizophora apiculata, Sonneratia apetala, Avicennia
marina, Excoecaria agallocha, Bruguiera cylindrica, Acanthus
ilicifolius etc. The mangrove associated plants are represented
by species such as Sarcolobus carinatus, Suaeda maritima,
Pandanus tectorius etc. Some examples of the mesophytic
bioinvasive plants occurring in the three study sites are
Casuarina equisetifolia, Alternanthera sp. etc. Important
phytoplankton species include Nitzschia sp., Peridinium sp.
Ceratium sp (Chakraborty and Choudhury, 1994; Chakraborty
et al., 2010).
The faunal biodiversity of Sundarbans mangrove ecosystem
includes 215 species of fishes, 7 species of amphibia, 59
species of reptiles, more than 100 species of birds, 39 speciesof mammals, besides numerous species of phytoplankton,zooplankton, ichthyoplankton, benthos, soil inhabiting andmangrove plants dependant insects. The faunistic compositionof zooplankton in this system includes copepods as principalcomponent of the total zooplankton (67.8% to 90%). A totalof 36 copepod species belonging to 19 families and 21 generahave been recorded. Other zooplanktonic forms aremysidacea, sergestidae, amphipoda, cladocera, ostracoda,
cumacea, chaetognatha, hydromedusea etc (holoplankters)
and polychaete larvae, nauplius, zoea, megalopa, fish eggs
and larvae, echinoderm larvae etc (meroplankters) (Annon,
2003; Chakraborty et al., 2010).
Nektonic fauna include hundred of species under 29 families.
Important ichthyoplanktons mainly belong to the families
Clupeidae, Engraulidae, Megalopidae etc.
A hoard of benthic fauna, both infauna (sessile, semisessile
and burrowing) and epifauna (crawlers and creepers) are the
happy residents of these habitats. Benthic fauna are divisible
into three broad groups based on their body sizes-macrobenthos, meiobenthos and microbenthos. Among the
intertidal macrobenthic fauna, 26 species of brachyuran crabs
belonging to 15 genera and 5 families have been recorded
from the deltaic Sundarbans estuarine complex. A total of 69
species of polychaetes under 45 genera and 25 families have
been documented from this ecosystem. 110 species of benthic
mollusca classified under class gastropoda (59 species) and
bivalvia (40 species) and a rich abundance of benthic insects
have also been identified. 44 species of microarthopods
belonging to six major taxonomic groups viz. Acarina,
Collembola, Coleoptera, Diptera, Isoptera, and Hymenoptera
have been identified. Actiniarian, sipunculans, echiurans,
hemichordates, globid fishes etc are other important benthic
faunal groups. 80 species of nematodes belonging 26 families
have been documented which are the major meiofaunal groups
(Chakraborty et al., 2011).
Midnapore coastal tract
The floral diversity includes 32 families of mangroves, 28
species of benthic algae under 4 families and 8 phytoplankton
species under 3 families have been recorded in Midnapore
coastal belt.
The faunal components of Midnapore coast remain in the
state of pelagic and benthic forms. Seventeen species of
zooplankton mainly comprising of copepoda, chaetognatha,
rotifera and some considerable number of nauplius larvae
have been recorded. Seventeen species of zooplankton mainly
comprising of copepoda, chaetognatha, rotifera and some
considerable number of nauplius larvae have been recorded.
A total number of 48 molluscan species belonging to 3 classes,
15 orders and 36 families have been reported from intertidal
habitats.A total number of 22 polychaete species belonging
to 10 families have been documented. A total number of 12
actinarian species belonging to 2 classes, 3 orders and 6 families
have been observed in different study sites. Besides, sea
cucumbers (Holothuroida), sea pen (Cnidaria), Lingula sp
(Brachyopoda), were found to occur in mudflats of Talsari,
Shankarpur, Junput, and Nayachar Islands. Out of 68
arthropod species recorded from this coast, 13 species
brachyuran crabs, 13 species of prawns and shrimps, 21
insects belonging to 33 families represent the major groups of
fauna . A total number of 51 soil microarthropods belonging
to insects orders viz. Collembola, Hymenoptera, Diptera and
Isoptera have been recorded from different parts of this coast.
Both the species of horse- shoe crabs viz. Carcinocorpius
rotundicauda and Taphypleus gigas have been also observed
in Digha-Talsary intertidal flats. A total of 51 fish species under
2 classes, 9 orders and 25 families have been documented
from different fish markets and landing centers (Chakraborty
et al., 2010).
Functional contribution of macrobenthic fauna – Crabs and
Microarthropods
Different intertidal brachyuran crabs especially those belonging
to the families grapsid and ocypodid are the most important
faunal components influencing the structure and function of
many tropical mangrove forests, after bacteria and the trees
(Lee, 1998)). Mangrove crabs used to adjust to significant
temperature and salinity fluctuations, which they do by
adopting nocturnal foraging behavior, retreating into burrows
in the day, decreased urine production etcThrough their life
activities, they exert extraordinary influence on the structure
and function of mangrove ecosystem. Through their
BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM
260
consumption of mangrove leaf litter, they significantly reduce
the amount of detritus available for export, thus enhancingretention and recycling of nutrients and organic matterinternally; their wastes can support coprophagous organismsfurther ensuring conservation of materials within the forest,and their selective consumption of mangrove propagulesaffects forest structure by reducing the recruitment and relativeabundance of tree species whose propagules are preferentiallyconsumed (Lee, 1999; Kristensen, 2008). Bioturbation by crabsalso results in changes in soil texture and chemistry, surfacetopography, degree of anoxia, and abundance of meiofaunawhile stimulating microbial production (Alongi, 2009). Thepresence of crab burrows enhances the flow of tidal waterthrough the forest floor, speeding up the flow of water andassociated dissolved and particulate material between forestand adjacent waterway (Ridd, 1996; Chatterjee et al., 2008).
Recent work has focused on clarifying the trophic role of crabs,especially positive feedback loops and interactions with treesand other flora and fauna in relation to food availability(Ashton, 2002; Kristensen and Alongi, 2006), and theirreproductive and life history strategies in relation to treecomposition and environmental factors (Lee and Kwok, 2002;Koch et al., 2005; Moser et al., 2005). In mesocosmexperiments, Kristensen and Alongi (2006) found that the
presence of the fiddler crab, Uca vocans, stimulated the growth
and development of Avicennia marina saplings but depressed
the abundance and productivity of microalgal mats at the soil
surface. Smith et al. (1991) found that the absence of crabs
increased the concentration of ammonium and sulfide in soils,
but reduced plant stipule and propagule production. The
presence of crabs therefore facilitates plant growth by aeratingthe soil to limit the buildup of toxic metabolites. The presenceof tree species may also influence crab productivity by way ofaltering tidal height, modification of soil texture and nutrientavailability (Lee and Kwok, 2002). Regardless of themechanisms involved, positive interactions between trees,crabs, and microbes make ecological sense in that the overallstability of mangrove ecosystems is enhanced (Alongi, 2002).
Digging by crabs, in conjunction with other benthic fauna likenematodes, polychaetes, and mudskippers can also have aprofound effect on nutrient cycling and the physical andchemical environment of the mangal (Lee, 1998). Burrowsenhance aeration, facilitate drainage of the soils, and promotenutrient exchange between the sediments and the overlayingtidal waters. One characteristic features of burrowing by crabsis the formation of varied structures by bio-turbation.Bioturbation is the stirring or mixing of sediment layers bybiological activities, mobility, feeding, burrowing etc. of benthicfaunal components. Bioturbation affects the geochemistry ofsediments and their interstitial water by mixing the soil of upperlayer with the lower one, pumping water and oxygen into thedeep layers of sediment providing different niche for the growthand propagation of microorganisms. It influences the microbialdegradation rate of sediment particulate organic matters byway of affecting the standing crop, community structure andphysiological state of microbial, micro and meiobenthiccommunity. A study from Midnapore Coastal Belt revealedthat different brachyuran crabs construct different categoriesof burrows of different depths, shapes, and diameters in orderto ensure courtship, breeding, feeding and escaping from
predators which show distinct variations in different seasons,
mudflats, sandflats and tidal levels. The various bioturbatory
structures formed by brachyuran crabs in the coastal belt of
West Bengal include Pseudo pellets, Sand Ball, Mud balls,
San Pyramids, Semi domes, Chimneys, Hoods etc (Chatterjee
et al., 2008).
Microarthropods: Soil microarthropods are considered to be
among the strongest determinants of plant litter decomposition
in warm, humid sites. These faunal components not only
enrich the biodiversity wealth of the system but also play crucial
role in nutrient cycling by the way of mangrove litter
decomposition. Microarthropod population peak coincide
with the maximum nutrient status of decomposing litters (Dey
et al., 2010). It has also been proposed that climate change
may increase the effects of elevation on soil microarthropod
litter processing (Wang et al., 2009).
Seasonal variation of physico-chemical factors, soil, water
and their interrelationships
Different physicochemical parameters displayed a wide range
of temporal and spatial variation. Water temperature, salinity,
pH, conductivity, turbidity, dissolved oxygen (DO), and
biochemical oxygen demand (BOD) were found to be higher
during pre-monsoon, while, silicate, phosphate phosphorous,
nitrite nitrogen, ammonical nitrogen, and nitrate nitrogen were
maximum during monsoon. The post-monsoon season was
characterized in having lowest temperature, moderate salinity,
and other parameters. Soil temperature, salinity, organic
carbon, and sand content were found to be higher during pre-
monsoon, while available potassium, available nitrogen, and
available phosphorous were maximum during monsoon. The
post-monsoon season was characterized in having lowest
temperature, available phosphorous, available potassium,
available nitrogen, and moderate level of other parameters.
CCA involving 12 environmental parameters of water viz.
temperature, pH, salinity, turbidity, conductivity, dissolved
oxygen, biochemical oxygen demand, silicate, phosphate
phosphorous (W PO4), nitrate nitrogen (W NO3), ammonical
nitrogen (W NH3), nitrite nitrogen (W NO2), and 10
environmental parameters of soil viz. temperature, pH, organic
carbon, salinity, available potassium, available phosphorous,
available nitrogen, and textural components (sand, silt, and
clay) revealed interrelationships among different
macrozoobenthic species in terms of their different ecological
parameters on one hand and also recorded cumulative
influence of a group of ecological parameters on the
abundance of macrozoobenthic population on the other
(Chakraborty et al., 2010). In Midnapore coast, different
physico-chemical parameters also exhibited seasonal
variations like water temperature (20.8°C-32.8°C), soil
temperature (20.1°C-34.8°C), salinity of water (8.6%-26%),
salinity of soil (10%-33.4%), D.O. (3.24 mg/L) to 5.47 mg/L,
pH of water (7.15-8.17) and pH of soil (7.68-8.72). Besides,
texture of sediment displays variations among different parts
and tidal levels of this coastal belt (Khalua, 2008).
Management of Mangrove coastal belts of West Bengal
Several natural and anthropogenic threats have been inflicting
their impact on biodiversity of West Bengal Coast in several
ways which are mentioned below:
SUSANTA KUMAR CHAKRABORTY
261
• Non judicious exploitation of mangrove bioresources to
meet the demand of steadily increasing local human
population.
• Considerable changes of land use pattern for the
development of aquaculture, fisheries and agriculture
promoted large scale reclaimation of land of virgin deltaic
islands leading to deforestation (Hazra et al., 2002;
Chakraborty et al., 2010).
• Large scale destruction of juveniles of hundred of species
of different fishes, and shell fishes for the collection of
shrimp juveniles, especially juveniles of Peneaus monodon
to be used in semi intensive aquaculture.An estimate shows
for the collection of one juvenile of Peneaus monodan,
juveniles of other aquatic fauna having a range of 27.41 to
31.77 at different sites of Sundarbans were destroyed
(Annon, 2003).
• Fishermen camps often lead to disturbances to the coastal
ecosystem functioning because of the release of different
waste materials as well as operation of increased number
of nylon nets having small mesh size to the death of marine
turtles, migratory birds and threatened fish species
(Chakravorty et al., 2004).
• Changing flow pattern of Ganga River during last centuries
because of faulty neotectonic movements has influenced
the hydrology of deltaic region and modified the
sedimentation patterns and reduction of fresh water inflow
leading to salinity invasion. (Gopal and Chauhan, 2006;
Hazra et al., 2002).
• Indiscriminate use of agrochemicals (fertilizers and
pesticides) in the catchments of Ganga and Brahamaputra
rivers, their numerous tribularis as well as in agriculture
fields close to the mangroves, pollute both the water ways
and the land mass and thereby affect the vegetation and
fauna directly.
• From seaward side, major pollution occurs through oil
spills that cause damage especially to aquatic fauna and
seabirds (Qasim et al., 1988). Besides, thousand of
mechanized boats for carrying passengers and fishing, are
the major source of oil pollution (Paul, 2002).
• Construction of embankment and dredging of riverbeds
hamper the water circulation, distabilise bottom sediments,
increase turbidity and affects settlement of flora and fauna
(Paul, 2002).
• Ecotourism to different parts of Sundarbans (Sagar Island,
Bakhali, Sajnekhali etc) contribute profusely for the
ecodegradation of the West Bengal coast.
• Growing Industrialization of the area around Kolkata and
Haldia industrial complex and industries situated on the
wastern side of the Hoogli estuary, contribute significantly
to the pollution load and hence to the degradation of
mangroves. (Chakravorty et al, 2004).
• Errosion and accretion pattern in Sundarbans: A time series
analysis of the change in the shape, size and geomorphic
features of the island over the part 32 years (1969-2001)
show some important changes like degradation of
mangrove swamps and mudflats, increase in salinisation
and development of saline banks within mangrove swamp.
• Global warming: A steady rate of increase of water
temperature (0.050C/year) in this coastal environment over
the past 27 years has had a profound impact on other
physical parameters of estuarine water and enhanced the
salinity (~6 psu over last 30 years), evaporation, free CO2,
precipitation, fresh water runoff and intrusion of sea water
and decreased pH (0.015 per decade) (Mitra et al., 2009).
Management strategies of the Mangrove coastal areas include
In- situ conservation of Wildlife
An integrated approach for the conservation of Sundarban
Mangrove estuarine complex and Midnapore coastal belt in
general and the flora and fauna in particular is very much
needed.
• To conserve diversity and integrity of plants, animals and
micro- organisms;
• To promote research on ecological conservation and other
environmental aspects;
• To provide facilities for education, awareness and training
for effective participation of the people living around
biosphere reserves.
Participatory management of bioresources
The stability of ecosystem is individually insignificant but
collectively determined cumulative ecological effects are not
attributable to any one source or action and cannot be
regulated in isolation. This requires cross sectional approaches
to natural resource management which should take into
account the participatory management practices in order to
integrate the goals of conservation into mainstream of
economic development (Patra et al., 2005; Mishra et al., 2009).
Integrated coastal zone management
An integrated management scheme for the judicious utilization
of coastal resources and also to minimize eco- degradation
involves monitoring changes and the handling of much
information. These tasks have been aided in recent years by
the application of remote sensing techniques and Geographic
Information system (GIS) (Paul, 2009).
The Ministry of Environment and Forests enacted the CRZ
notification in 1991 under the Environment Protection Act to
protect coastal areas from over- development and
industrialization. The CRZ areas of the country is defined as
coastal stretches of sea, bays estuaries, creeks, rivers, back
waters, all influenced by tidal action on the landward margins.
However, based on ecological sensitivity, geomorphic features
and demographic distribution, the CRZ is categorized into
four significant areas as:
CRZ-I: (ecologically sensitive and tidally influenced upto 500m
from HTL)
CRZ-II: (Urban built-up areas or densely developed areas along
the shoreline)
CRZ-III: (rural coastal dwelling units or developed areas of the
coast).
CRZ-IV: (islands surrounded by water bodies and isolated from
the main land).
Much of Purba-Medinipur coastal district falls within CRZ-I
and CRZ-III. Areas of CRZ-III are further sub-divided. The area
BIODIVERSITY OF COASTAL-MANGROVE ECOSYSTEM
262
upto 200 meters HTL is no construction zone where only
repairs of existing authorized constructions are permitted or
allowed.
Restoration
In terms of ecology, restoration will seldom mean returning an
ecosystem to its initial state but will more often mean bringing
it back to a state of effectiveness. A practical definition of
restoration is given by Morrison (1990) ‘Restoration is the re-
introduction and reestablishment of community-like groupings
of native species to sites which can reasonably be expected to
sustain them with the resultant vegetation demonstrating
aesthetic and dynamic characteristics of the natural
communities on which they are based.’ Field (1998)
distinguished between rehabilitation - ‘the partial or full
replacement of the ecosystem’s structural and functional
characteristics’ and restoration - ‘the act of bringing an
ecosystem back to its original condition’.
Restoration provides an opportunity to improve or enhance
the landscape and increase environmental quality (Kairo et
al., 2001).
Mangrove resilience factors that are a prerequisite for
mangrove restoration
1. Association with drainage systems including permanent
rivers and creeks that provides freshwater inputs and
sediment supply.
2. Sediment rich-macrotidal environments to facilitate
sediment redistribution and accretion
3. Mangroves backed by low-lying retreat areas (for
example, salt flats, marshes, coastal plains) which may
provide suitable habitat for colonization and landward
movement of mangroves in view of rising sea level.
4. Mangroves in areas where abandoned alternate land
use provides opportunities for restoration, for example,
flooded villages, tsunami-prone land, unproductive
pond.
5. Areas with a large tidal range may be better able to adjust
to increases in sea level due to stress tolerance
6. Permanent strong currents to redistribute sediment and
maintain open channels
7. Diverse species assemblage and clear zonation over
range of elevation (intertidal to dry land)
8. Tidal creek and channel banks consolidated by
continuous dense mangrove forest (which will keep
these channels open)
9. Access to healthy supply of propagules, either internally
or from adjacent mangrove areas
10. Close proximity and connectivity to neighboring stands
of healthy mangroves
11. Access to sediment and freshwater
12. Limited anthropogenic stress
13. Integrated Coastal Management Plan or Protected Area
Management Plan implemented
DISCUSSION
Mangrove ecosystem is a highly valued ecosystem in terms of
economy, environment and ecology. Its uniqueness is in therandom and rapid fluctuations in varied physico-chemicalparameters, and the surprising degree of adaptation displayedby the resident biota in response to such fluctuations (Chandraet al., 2003; Mishra et al., 2009).
A review of the available literature on mangrove plantationestablishments shows mixed successes of restoration efforts(Ellison 2000), even though it has been said that mangrovewetlands are easy to restore and create (e.g. FAO, 1994).Whereas the lost mangrove plant species can be returned(Kairo, 1995a), a restored forest may or may not function asthe original pre-disturbed system (McKee and Faulkner, 2000,Bosire, 1999). If a mangrove forest is disturbed by logging it isunlikely that the forest will regenerate to function as the pre-disturbed state, since the species mix, soil type, stocking ratesand numbers of animals will certainly have changed.
When contemplating mangrove rehabilitation, specialattention must be paid to soil stability and flooding regime(Pulver, 1976), site elevation (Hoffman et al., 1985), salinityand fresh water runoff (Jeminez, 1990), tidal and wave energy(Field, 1996), propagule availability (Loyche 1989; Kairo,1995a, 2001), propagule predation (Dahdouh-Guebas et al.,1997), spacing and thinning of mangroves (FAO, 1985; Kairo,2001), weed eradication (Saenger and Siddique, 1993),nursery techniques (Siddique et al., 1993), monitoring (Lewis,1990), community participation (Kairo, 1995b) and total costof restoration measures (Field, 1998).
CONCLUSION
Although, significant measures have been taken up for the
conservation of biodiversity of Sundarbans and Midnapore
coast, an integrated action plan is required incorporating the
outcome and recommendations of multidimensional
researches undertaken during the last four decades. Based on
these, proper guidelines are to be framed for future researchers
on this globally important environmental sector so that TimeSeries Analysis and Long Trend Analysis on the natural and
anthropogenic stress factors become possible. These will
facilitate the process of pointing out problems more distinctly
and remedial measures more effectively.
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