CLIMATE CHANGE AND IMPACT ON MANGROVES BY DR. ABHIJIT MITRA, CALCUTTA UNIVERSITY

1. 1 23
Biodiversity and Conservation
ISSN 0960-3115
Biodivers Conserv
DOI 10.1007/s10531-012-0260-z
Climate change impacts on Indian
Sunderbans: a time series analysis (1924–
2008)
Atanu Raha, Susmita Das, Kakoli
Banerjee & Abhijit Mitra

2. 1 23
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3. ORIGINAL PAPER
Climate change impacts on Indian Sunderbans: a time
series analysis (1924–2008)
Atanu Raha • Susmita Das • Kakoli Banerjee • Abhijit Mitra
Received: 14 April 2011 / Accepted: 17 February 2012
Ó Springer Science+Business Media B.V. 2012
Abstract Climate change induced sea level rise (SLR) added with anthropogenically
altered environment leads to rapid land dynamics in terms of erosion and accretion; and
alteration in species diversity and productivity, more pronouncedly in sensitive ecosystems
such as river deltas. Here, we tried to analyze the historical records to understand the SLR
with respect to hydrological conditions, sedimentation and morphological processes. We
analyzed the land transformation of few islands in Indian Sunderbans using maps and
satellite images in increasing order of temporal frequency between 1924 and 2008, which
revealed that both the erosion and accretion processes go hand in hand. Increase of
downstream salinity due obstruction in upstream has led to decrease in transparency of
water causing decrease in phytoplankton and fish, density and diversity in the central sector
of Indian Sunderbans. Analysis of the above ground biomass of three dominant mangrove
species (Sonneratia apetala, Avicennia alba and Excoecaria agallocha) revealed better
growth in the western sector compared to the central sector. The study reveals the
cumulative effect of climate change and anthropogenic disturbance on the diversity and
productivity in World’s largest ecosystem; and advocates mangrove plantation and
effective management of freshwater resources for conservation of the most vulnerable
and sensitive ecosystem.
Electronic supplementary material The online version of this article (doi:10.1007/s10531-012-0260-z)
contains supplementary material, which is available to authorized users.
A. Raha Á S. Das
Office of the Principal Chief Conservator of Forests, Block LA-10A, Aranya Bhawan, Salt Lake,
Kolkata, West Bengal 700098, India
K. Banerjee (&)
School of Biodiversity and Conservation of Natural Resources, Central University of Orissa,
Landiguda, Koraput, Orissa 764020, India
e-mail: banerjee.kakoli@yahoo.com
A. Mitra
Department of Marine Science, University of Calcutta, 35 B.C. Road, Kolkata 700019, India
123
Biodivers Conserv
DOI 10.1007/s10531-012-0260-z
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4. Keywords Climate change Á Indian Sunderbans Á Mangrove Á Phytoplankton Á
Land dynamics
Abbreviations
AD Anno domini
AGB Above ground biomass
ANOVA Analysis of variance
AWiFS Advanced wide field sensor
DLRO Directorate of Land Records and Surveys
FCC False colour composite
GIS Geographic information system
IPCC International Panel for Climate Change
IRS Indian remote sensing satellite
LISS Linear imaging self-scanning sensors
t ha-1
Tons per hectare
UNEP United Nations Environmental Programme
UNESCO United Nations Education Scientific and Cultural Organisation
WWF World Wide Fund for Nature
Introduction
The Sunderbans represent the largest contiguous mangrove ecosystem in the world; consist
of hundreds of islands crisscrossed by a maze of tidal rivers, estuaries and creeks. It is
located in the north–east coast region of India at the apex of Bay of Bengal. Declared as the
World Heritage Site by UNESCO in 1987 and Global Biosphere Reserve in 1989, the
Government of India endorsed the deltaic complex as a Biosphere Reserve to ensure
protection to this unique gene pool of the planet Earth that spreads over 102 islands. The
Indian Sunderbans biodiversity includes about 100 species of vascular plants, 250 species
of fishes, 300 species of birds and a variety of reptiles, amphibians and mammals besides
numerous species of benthic invertebrates (like arthropods, molluscs etc.), phytoplankton,
zooplankton, bacteria, fungi etc. (Gopal and Chauhan 2006). Mangrove forests provide
critical ecosystem services, fulfill important socio–economic and environmental functions
and support coastal livelihoods. Their unique root systems create a great deal of physical
roughness, thus capturing and storing vast quantities of sediment from upland and oceanic
origin. Sea-level rise (SLR) is expected to decrease the geographic distribution and species
diversity of mangroves on small islands with micro-tidal sediment-limited environments.
Mangroves with access to allochthonous sediments, such as riverine mangroves, are more
likely to survive SLR than those with low external inputs.
Past climate
There have been at least 17 major glacial advances (glaciations) in the last 1.6 million
years alone, of which the most recent and the last glacial one reached its peak some
20,000–18,000 years ago and came to an end about 10,000 years ago (Goudie 1983).
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5. Glaciations are followed by ‘interglacial’ periods and presently we are passing through one
of those phases wherein, sea level rise (SLR) becomes extremely prominent as evidenced
in case of Ganga–Brahmaputra basin. Evidently, the Sunderbans delta at the mouth of the
rivers of Ganga and Brahmaputra existed at all times at any level of the ocean and any
climatic variations in the water flow and sediment runoff, while the delta coastline was
always confined to the ocean level (Table 1). It became more than 100 m higher in the last
18,000 years (Goodbred and Kuehl 2000). The exact location (i.e. at which present depth
of the ocean) of the Ganga–Brahmaputra delta at low levels of the ocean during the glacial
epoch and the form of that delta are still unknown. However, the fact that such delta had
been available at that time is evident (Mikhailov and Dotsenko 2007). The development of
the modern Holocene delta began, when the ocean level reached the elevations of the top of
Pleistocene deposits, which represented the surface of the old delta formed in the Bengal
basin during the previous interglacial epoch (Mikhailov and Dotsenko 2007). According to
Goodbred and Kuehl (2000) the elevations of this surface were 50–70 m lower than the
present ocean level. Partial submergence of this surface, sedimentation and formation of
the modern delta began 11,000 years ago. The ensuring years featured the formation of a
Table 1 Periods in variation of the ocean level and hydrological and morphological processes in the Ganga
and Brahmaputra mouth area
Period
1,000 years
ago
Ocean level
relative to
the present
level (in m)
Ocean level rise
(m) (cm year-1
)
Hydrological conditions Sedimentation and
morphological processes
Before 18 -120 – Maximum of glacial ocean
regression, high slopes of
water surface and current
velocities, evacuation of
coarse sediments
(boulders, pebble, and
gravel)
Formation of deep down-
cut channel
18–11 -120/-55 65 (0.93) Beginning of post glacial
transgression,
propagation of backup
into the lower part of
erosional downcutting
Shifting of zone of river
sediment accumulation in
the landward direction
11–7
7–4
-55/-10
-10/-5
45 (1.12)
5 (0.17)
Increased river water flow
and sediment runoff,
considerable rise of ocean
level, propagation of
backup into the basin
Slowing down of ocean
level rise, decrease in
backup
Large scale accumulation
of sandy and silty
sediment of the basin
surface, intense
development of Holocene
delta, accretion of branch
channels
Further sedimentation on
the basin surface and
delta development,
accretion of branch
channels, shelf formation
4–0 -5/0 5 (0.12) Stabilization of ocean level Formation of modern
hydrographic system of
delta
Source Mikhailov and Dotsenko (2007) (the figures on the left in 2nd column means at the beginning of the
period, while at the right represents the end of the period; dash here means lack of information)
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6. thick layer of deposits (exceeding 50–60 m, on the average) and a dynamic channel
network of the modern delta.
According to Nichols and Goodbred (2004), the considerable sediment runoff of the
Ganga and Brahmaputra rivers compensates for the SLR and reduces the impact of this rise
within the range of about 30 m at the rate of rise up to 1 cm year-1
. In addition to this, the
considerable sediment runoff of these rivers compensates the impact of the tectonic sinking
and land subsidence in the delta; it also reduces the impact of dynamic factors (waves and
even intense tidal currents) on the nearshore zone (Mikhailov and Dotsenko 2007). During
the maximum last glaciation period (about 18,000 years ago), the ocean level was nearly
120 m lower than the present day level (Table 1). The land and shelf surface was no less
than 60 m lower than the present surface. Most likely, water and sediments of both rivers
entered the ocean in the form of a combined flow through the erosional channel inherited
by the present Swatch of No Ground Canyon (Goodbred and Kuehl 1999; Goodbred and
Kuehl 2000). About 11,000 years ago, the ocean level rose to the elevation of 55 m and the
backup began its propagation to the Bengal basin. Simultaneously, the accumulation of
deposits and the Holocene delta formation began on its surface (Table 1). Over the period
of 11,000–7,000 years back, the deposited sediment layer on the Bengal basin surface
exceeded 50 m; and in the last 7,000 years, the layer thickness was more than 15 m
(Goodbred and Kuehl 1999, 2000).
Sea-level rise
India has been identified as one amongst 27 countries, which are most vulnerable to the
impact of global warming related accelerated SLR (UNEP 1989). The dynamics of any
delta and coastline is mainly controlled by three major factors, namely (1) compaction and
tectonic subsidence, (2) relative SLR and wave action, and (3) sediment supply from the
rivers. The Northern Indian Ocean, which includes the Bay of Bengal, is experiencing a
relatively high rate of SLR compared to other oceans globally. The Sunderbans is expe-
riencing one of the most pronounced effects of climate change resulting in the form of SLR
at an average rate of 3.14 mm per year (Hazra et al. 2002; WWF 2010). The floral and
faunal communities of the Sunderbans are well adapted to the diurnal rise and fall of water
level by 10–15 feet, twice a day. Earlier studies, without the use of satellite imageries,
indicate that the island has been subjected to erosion by various processes (Bandyopadhyay
1997, 2000; Sanyal et al. 2000; Ghosh et al. 2002). The biotic communities in the deltaic
system have also been affected particularly in terms of species composition and biomass of
mangrove trees. Excess salt in estuarine water poses a retarding effect on mangrove growth
(Mitra et al. 2004).
The Indian Sunderbans is presently facing erosional features in the western sector,
which may be attributed to sediment run-off, water flow and current pattern regulated
mostly by Farakka barrage. The central Indian Sunderbans is experiencing high salinity
trend and more sedimentation due to complete blockage of the Bidyadhari channel since
the fifteenth century (Chaudhuri and Choudhury 1994). Gradual disappearance of sweet
water loving mangrove floral species like Heritiera fomes (locally called Sundari) and
Nypa fruticans (locally referred to as Golpata) is a confirmatory test of such salinity
variation between western and central Indian Sunderbans. The biomass of mangroves,
species composition of phytoplankton and fishes are also being influenced by salinity
fluctuation. Here, we analyze the diversity pattern of phytoplankton between western
and central Sunderbans over last two decades. We also present some initial results of
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7. above-ground biomass (AGB) analysis of three selected dominant mangrove species, along
with their seasonal variation.
Materials and methods
Sites
The deltaic complex of Sunderbans encompassing an area of 1 million ha formed by the
depositional activities of the rivers Ganga, Brahmaputra and Meghna is shared between
Bangladesh (62%) and India (38%). The Indian Sunderbans (between 21°130
N–22°400
N
latitude and 88°030
E–89°070
E longitude) is bordered by Bangladesh in the east, the
Hooghly river (a continuation of the Ganges river) in the west, the Dampier and Hodges
line in the north and the Bay of Bengal in the south. The important morphotypes of deltaic
Sunderbans include beaches, mudflats, coastal dunes, sand flats, estuaries, creeks, inlets
and mangrove swamps (Chaudhuri and Choudhury 1994). The average tidal amplitude is
around 3.5 m.
Changes in shoreline configuration
Various data has been acquired for three vulnerable islands of Indian Sunderbans (Sagar,
Jambu and Thakuran Island). The coastal zone mapping has been done based on the
DLRO’s maps of 24 Parganas district for 1924, topographical map of 1954, Landsat MSS
data for the year 1975 and 1989, IRS 1D LISS III data for the years 1999 and 2002 and IRS
P6 AWiFs data for the years 2005 and 2008 in order to evaluate the changes in shoreline
configuration. Time series analysis was carried out for the interval period of 30, 20, 15, 10
and 3-years before present (Fig. 2a–c).
Habitat selection for diversity and biomass enumeration
Two sampling zones (comprising of five stations each) were selected in the western and
central sectors of the Indian Sunderbans (Fig. 1). The stations in the western zone are
situated in the Hooghly (continuation of Ganga–Bhagirathi system) estuarine stretch. This
zone receives the snowmelt water of Himalayan glaciers after being regulated through
several dams (e.g. Farakka barrage) on the way. The central zone on the other hand, is fully
deprived from fresh water supply due to heavy siltation and clogging of the Bidyadhari
channel in the late fifteenth century (Chaudhuri and Choudhury 1994).
Phytoplankton samples were collected seasonally through a vertical tow of a plankton
net (20 lm effective mesh size) at each stations in the high tide condition during
1990–2010. The plankton net was approximately 50 cm long, with a 26 cm diameter
mouth and a 10 cm diameter opening at the cod end, which was tied to a 125 ml TARSON
collection bottle. The samples collected were preserved by using 1 ml of 37% formalde-
hyde (*2% final concentration) to identify and enumerate the phytoplankton species.
Centrifugation was done to concentrate the sample. The final volume of plankton con-
centration was recorded to achieve the result of plankton density in terms of cells 9 105
/
m3
. The total number of phytoplankton (standing crop) present in a litre of water sample
was calculated using the formula: N = nv/V, where N = total number plankton cells per
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8. litre of water filtered, n = average number of plankton cells in 1 ml of plankton sample,
v = volume of plankton concentrate (ml) and V = volume of total water filtered (l).
For each station, species diversity index was computed as per the expression of Shannon
Weiner species diversity index (1949)
ð "HÞ ¼ À
XS
i¼1
ni
N
loge
ni
N
where, ni = importance probability for each species, N = total of importance values.
The mean results of diversity and density of each sector during 1990–2010 were finally
presented.
Secondary data of fish catch data were recorded from two sectors (western and central)
from the local fishermen association from 1990 to 2010. The data presented are the mean
of selected stations in both the sectors (Fig. 3a). Species diversity was computed using
Shannon–Weiner index considering 100 kg of the catch.
Fig. 1 Map showing location of Sagar and Jambu Island in the western Sunderban and Thakuran island in
central Sunderban. Sampling locations marked in black flag shows the stations selected in both the sectors
for diversity and biomass estimation
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9. Mangrove biomass estimation
In both the sectors, selected forest patches were *12 years old. Dominant species were
selected on the basis of mean relative abundance of the species in the sample plots. 15
sample plots (10 9 10 m2
) were laid (in the river bank) through random sampling in the
various qualitatively classified biomass levels for each zone. Seasonal sampling in both the
sectors was carried out during the low tide period in the year 2009 and 2010. Sonneratia
apetala, Avicennia alba and Excoecaria agallocha are abundant in the mangrove forests at
the seaward end of the Sunderbans estuary, where all mangrove sampling took place.
AGB in these species refers to the sum of total stem, branch and leaf biomass that are
exposed above the soil. AGB of individual trees of three dominant species in each plot was
estimated and the average values of 15 plots from each region were finally converted into
biomass (t ha-1
) in the study area. The stem volume of five individuals from each species
in each of the 15 plots per station (n = 5 individuals 9 15 plots = 75 trees/species/
station) was estimated using the Newton’s formula (Husch et al. 1982). Dry weight of
branches from each species was recorded separately using the equation of Chidumaya
(1990). The leaf biomass of each tree was calculated by multiplying the average biomass of
the leaves per branch with the number of branches in that tree. This exercise was per-
formed for all the stations in both the sector and the results were analyzed.
Results
Change in shoreline configuration
The deltaic complex of Indian Sunderbans is extremely dynamic and the process of erosion
and accretion occur almost simultaneously in different pockets of the deltaic lobe. The
satellite imageries reveal that the islands of western Indian Sunderbans are gradually
eroding (Fig. 2a, b). On contrary, the islands of the central Indian Sunderbans are showing
expansion owing to accretion (Fig. 2c). The sea-facing islands like Jambu Island and the
southern part of Sagar Island are also eroding due to wave action from the Bay of Bengal.
Density and diversity
Phytoplankton density ranged from 23.86 cells 9 105
/m3
during 1990 to 71.96 cells 9
105
/m3
during 2000 in the western sector and 69.32 cells 9 105
/m3
during 1990 to 159.10
during 2010 in the central sector. The diversity values ranged from 3.4622 during 2005 to
3.6270 during 2000 in the western sector and 1.2489 during 1990 to 2.9894 during 2010 in
the central sector (Fig. 3a, b). From the ANOVA results, it is observed that there is
significant difference in phytoplankton density between the western and central sectors
(p 0.05). Although there is a consistency in phytoplankton diversity in the western
sector, but a gradual rise in the central sector may be attributed to intrusion of stenohaline
species (e.g. Cymbella marina, Asterionella formosa, Dityllum brightwelli, Triceratium
jentacrinus, Pleurosigma salinarum, Fragillaria oceanica etc.) in the high saline tide fed
estuaries of central Indian Sunderbans. This has caused significant difference in phyto-
plankton species diversity between the two sectors (p 0.05). The overall result reflects
that salinity plays a crucial role in regulating the phytoplankton density and diversity of
Indian Sunderbans.
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10. Fig.2aShorelinechangesinSagarIslandduring1924–2008.bShorelinechangesinJambuIslandduring1924–2008.cShorelinechangesinThakuranIsland
during1924–2008
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11. Fig.2continued
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12. Fig.2continued
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13. Fish
The fish catch data reflects a shift or orientation in the fish community in response to
ecological conditions. The difference in salinity in the aquatic subsystem has caused a
compositional variation in commercially important and trash fish community as evidenced
from the current study during the period 1990–2010. The catch composition segregated
between commercially important fishes and trash fishes reveals more trash fish (Stole-
phorus sp., Thryssa sp., Harpodon nehereus, Trichiurus sp. etc.) diversity in the central
sector than the western sector (p 0.05). The trend of fish diversity index (bar H) values
shows gradual increase in catch of commercially important fishes (Tenualosa ilisha,
Polynemus paradiseus, Sillaginopsis panijus, Pama pama, Arius jella, Osteogeneiosus
militaris etc.) in the western sector compared to central sector (Fig. 3a). This significant
spatial difference (p 0.05) is due to increased dilution factor in the western sector due to
barrage discharge.
0
0.5
1
1.5
2
2.5
3
3.5
4
1990 1995 2000 2005 2010
Biodiversityindex(barH)
Western phytoplankton Central phytoplankton
Western Commercially important fishes Western trash fishes
Central Commrcially important fishes Central trash fishes
0
20
40
60
80
100
120
140
160
180
1990 1995 2000 2005 2010
cellsx105
/m3
Western
1990 1995 2000 2005 2010
Western phytoplankton Central phytoplankton
Western Commercially important fishes Western trash fishes
Central Commrcially important fishes Central trash fishes
0
0
0
0
0
0
0
0
0
0
Western
Central
a
b
Fig. 3 a Temporal variation of fish and phytoplankton diversity index (H) in the western and central sectors
of Indian Sunderbans. b Temporal variation of phytoplankton standing stock (N) in the western and central
sectors of Indian Sunderbans
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14. Above ground biomass (AGB)
The AGB of the mangrove species was relatively higher in the stations of the western
sector (stations 1–5) compared to the central sector (stations 6–10). It is observed that the
average AGB of the three dominant species in the stations of western sector are 71.99 and
82.88 t ha-1
during pre-monsoon; 83.31 and 93.81 t ha-1
during monsoon and 95.12 and
102.85 t ha-1
during post-monsoon in 2009 and 2010 respectively. In the stations of
central sector the values are 58.11 and 67.72 t ha-1
during pre-monsoon; 67.87 and
79.92 t ha-1
during monsoon and 82.73 and 90.09 t ha-1
during post-monsoon in 2009
and 2010 respectively (Fig. 4).
Discussion
SLR and coastline changes
In the Indian coast past observations on the mean sea level indicates a long-term rising
trend of about 1.0 mm year-1
on an annual mean basis (Unnikrishnan et al. 2006).
However, the recent data suggests a rising trend of 2.5 mm year-1
in SLR along Indian
coastline (Bhattacharya 2007). The east coast of India is more vulnerable to SLR in
comparison to that of the west coast (Shetye et al. 1990). The rate of relative SLR is
presently approaching 3.14 mm per year near Sagar island (88°030
06.1700
longitude and
21°380
54.3700
latitude), the largest island in the western sector of Indian Sunderbans and
this could increase to 3.5 mm per year over the next few decades due to global warming,
including the other global and local factors (Hazra et al. 2002). The exact reason for SLR is
not pinpointed in case of Indian Sunderbans (Mitra et al. 2009a), but for Bangladesh
Sunderbans the dominant factors are the monsoonal rains and land subsidence (Singh
2002). Slow tectonic sinking of the entire Bengal basin and rather intense land subsidence
(more than 15 mm/year in some areas of the delta) caused by compaction of loose deltaic
deposits often results in the depletion of the deposited sediment height. The joint impact of
the eustatic sea level and more intense subsidence of deltaic deposits results in the so-
called relative SLR, which reaches 10–20 mm year-1
in the seaward part of the delta of
the Ganga and Brahmaputra rivers (Allison 1998; Coleman 1969). The relative SLR in
deltaic Sunderbans is more intense than in some other large deltas of the world
(1–5 mm year-1
in the deltas of the Nile Delta, and up to 10 mm year-1
in the Mississippi
Delta) (Dowell and Rickards 1993), which may be largely due to land subsidence. Sedi-
ment transport rates show that net transport is towards northern side (Kumar et al. 2006)
due to high south-ward winds and interference to free passage of longshore sediment
transport. Continuous and long term data of land subsidence is however lacking for Bengal
basin and hence, its direct correlation with relative SLR is difficult to ascertain.
Neotectonic movements in the Bengal basin between the twelfth and fifteenth century
AD resulted in an easterly tilt (Morgan and McIntire 1959) of the deltaic complex. During
the sixteenth century, the river Ganga changed its course to shift eastwards and join the
Brahmaputra (Deb 1956; Blasco 1975; Snedaker 1991). Later, in the mid eighteenth
century, the combined Ganga (now called Padma) and Brahmaputra again tilted eastwards
to empty into the River Meghna (Snedaker 1991). This continuing tectonic activity greatly
influenced the hydrology of the deltaic region because of changes in the sedimentation
patterns and the reduction in freshwater inflows. Most rivers (distributaries) other than the
Hooghly, that contributed to the formation of the Ganga Delta (from west to east:
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15. Muriganga, Saptamukhi, Thakuran, Matla, Gosaba and Bidya), have lost original con-
nections with the Ganga because of siltation and their estuarine character is now main-
tained by the monsoonal runoff (Cole and Vaidyaraman 1966) and tidal actions (Mitra
et al. 2009b, 2011).
70
60
2009prm
50
ha)
2010 prm
30
40
B(t/
mon2009
mon2010
20
AG
pom2009
10
pom2010
0
1 2 3 4 5 6 7 8 9 10
60
70
50
)
2009prm
2010 prm
40
B(t/ha
mon2009
20
30
AG
mon2010
pom2009
10
pom2010
0
60
70
50
40
2009prm
2010 prm
mon2009
30
mon2010
pom2009
20
pom2010
10
0
1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10
)B(t/haAG
Fig. 4 Seasonal variation of AGB of a Sonneratia apetala, b Avicennia alba and c Excoecaria agallocha in
the selected stations during 2009–2010; x-axis depicts the number of stations and y-axis the ABG value up to
70 t ha-1
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16. The construction of dikes had profound impact on the processes of river sediment
accumulation on the delta surface. The construction of earth-full protection dikes began in
the middle of the eighteenth century (Allison 1998) and large-scale diking began only in
the 1960s. Diking resulted in a considerable decrease in the submergence of the protected
delta areas and, as a consequence, in cessation of input of sediments onto these lands and
vertical accretion of the delta. Man-made diking of channels of the deltaic watercourses
often aggravates the hazard of floods. This is because the water levels in a diked channel
(particularly, after two-sided diking) rise triggering the phenomenon of flooding the
islands. Such processes were previously recorded in the deltas of the Amudarya and
Huanghe rivers (Mikhailov 1998; Mikhailov et al. 2004). In the present study area about
3,500 km embankment exists as insurance to protect the low lying islands. This inhibits the
natural flow of tidal waters in the islands resulting in the deposition of sediment on the
river bed. Finally the relative water level tends to rise due to apparent rise of the river bed.
The rise of water level in the estuaries of deltaic Sunderbans coupled with anthropo-
genic factors has altered the salinity profile of the deltaic complex (Mitra et al. 2009a) the
pulse of which has been be transmitted in the domain of mangrove biotic community by
way of mangrove growth rate, species diversity alteration etc.
Salinity effect
The impact of salinity in the deltaic Sunderbans is significant since it controls the distri-
bution of species and productivity of the forest considerably (Das and Siddiqi 1985). Due
to increase in salinity, H. fomes (Sundari) and N. fruticans (Golpata) are declining rapidly
from the Indian Sunderban region (Gopal and Chauhan 2006). The primary cause for top-
dying of the species is believed to be the increasing level of salinity (Balmforth 1985;
Chaffey et al. 1985; Shafi 1982). Salinity, therefore, is a key player in regulating the
distribution, growth and productivity of mangroves (Das and Siddiqi 1985).
Height and growth of different species in the Sunderbans are related with the salinity.
Salinity in the Sunderbans is highly dependent on the volume of freshwater coming from
the upstream. The variation is subject to the nature of tide in the area. Annual pattern of
salinity changes inside the Sunderbans is also related with the changes of freshwater flow
from upstream rivers. The peak salinity was found to be about 26 ppt in 2001 and 2002 and
the minimum salinity during post monsoon was found to be about 5 ppt (IWM 2003). The
adverse effects of increased salinity on the ecosystem of the Sunderbans are manifested in
the dying of tops of Sundari trees, retrogression of forest types, slow forest growth, and
reduced productivity of forest sites (MPO 1986).
The present study reveals that the growth of dominant mangrove flora is more in the
western sector of Indian Sunderbans compared to the central sector. The reduced fresh-
water flows in central region of the Sunderbans have resulted in increased salinity of the
river water and has made the rivers shallow (particularly Matla) over the years. This caused
significant effect on the biomass of the selected species thriving along these hyper-saline
river banks. Interestingly, the effects are species-specific. Increased salinity caused reduced
growth in S. apetala whereas salinity could hardly influence the growth of A. alba and
E. agallocha. Such differential adaptability of mangrove species to salinity was also
reported from Bangladesh Sunderbans (Cintron et al. 1978). The basic cause of such
variation may be attributed to anatomical and physiological adaptations, which are species-
specific. Species like A. alba and E. agallocha have the capacity to excrete salts through
roots and salt glands in leaves. However, S. apetala, which is a salt accumulating species
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17. lose salt through cuticular transpiration. This has imparted the species a low tolerance to
hyper-saline condition of central Indian Sunderbans.
Effect on phytoplankton community
Our knowledge on the impacts of climate change on phytoplankton populations is poor due
to lack of continuous time series data. Here we observe that process like erosion and
sedimentation, along with subsequent churning action increases the load of suspended
solids. This results in the decrease of transparency, which affects the growth and survival
of phytoplankton in the region mostly in the western part of the delta (Fig. 3b).
ANOVA results of phytoplankton species diversity since 1990 exhibits significant
spatial variation. This may be due to influx of few stenohaline species (e.g. C. marina,
A. formosa, D. brightwelli, T. jentacrinus, P. salinarum, F. oceanica etc.) from the Bay of
Bengal in the hyper-saline central sector. The number of phytoplankton species (standing
stock) also shows similar trend with relatively higher values in the central sector. In the
western Indian Sunderbans uniformity in taxonomic variability is revealed that may be
attributed to relatively stable ambient environment.
Effect on fishery
The impact of climate change on marine fisheries stems from the fact that global warming
may change the salinity level of the estuarine water that fish inhabit, the amount of oxygen
in the water, pollution level and turbidity levels due to increased frequency of erosion
caused by increased tidal amplitude. Direct effects act on physiology and behaviour and
alter growth, reproductive capacity, mortality and distribution of fishes. Indirect effects
alter the productivity, structure and composition of the marine ecosystem on which fish
depend for food. In mangrove dominated deltaic complex of Indian Sunderbans, the
aquatic subsystem has significantly altered in terms of salinity, nutrient load, productivity,
planktonic composition and heavy metal concentration over a period of 30 years (Mitra
et al. 2009a, b, 2011; Mitra and Banerjee 2011). The present study clearly indicates distinct
dissimilarity between the western and central sectors in terms of fish diversity. The
diversity of commercially important fish species has not altered significantly over years in
western Indian Sunderbans, but in the central sector the diversity has reduced due to hyper-
saline condition. The trash fish diversity, however, has increased which are opportunistic in
nature and can adapt even in stressed condition.
Effect on shoreline configuration
Global warming is accelerating the process of erosion in coastal and estuarine zones either
through increased summer flow from the glaciers or by increased tide penetration due to
SLR. It is evident that in Indian Sunderbans region erosion and accretion almost occur
simultaneously. The western Indian Sunderbans exhibits more erosion compared to
deposition (Fig. 2a, b), which is reverse in case of central Indian Sunderbans (Fig. 2c). The
net result, however, is inclined towards erosion as the total area eroded is almost
283.58 km2
, whereas the total area of accretion is 83.97 km2
(Ganguly et al. 2006). The
phenomena of erosion and accretion are largely regulated by littoral current pattern and
sediment influx from different rivers and adjacent Bay of Bengal. However, anthropogenic
causes like dam construction and water discharge from the upstream regions are also
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18. important factors. There are two major dams on the River Ganga. One at Haridwar which
diverts much of the Himalayan snow melt into the upper Ganga canal, built by the British
in 1854 to irrigate the surrounding land. This caused severe alteration to the water flow in
the Ganga. The other dam is at Farakka, close to the point where the main flow of the river
enters Bangladesh, and the tributary Hooghly (also known as Ganga–Bhagirathi) which
continues in West Bengal through Calcutta. This barrage feeds the Hooghly branch of the
river by a 26-mile (42 km) long feeder canal. Construction of dams and barrages in the
upstream has not only affected the quantum of sediment load but also altered the salinity
profile (Mitra et al. 2009b). The velocity of water has also increased in the Hooghly
channel which is a powerful agent of erosion. The quantum of fresh water discharge often
exceeds the normal level in monsoon as seen during 10–15th August, 2011. An average
135 mm rainfall for these 5 days resulted in the release of 80,000, 5,000 and
1,10,000 cusec water per day from Panchet dam, Mython dam and Durgapur barrage
respectively all of which drains in the main Ganga–Bhagirathi–Hooghly channel in the
western sector of Indian Sundarbans. Such flow through Hooghly channel is responsible for
erosion of the northern portion of Sagar Island. Severe bank erosion is observed in southern
tips of Sagar Island and Jambu Island (in the western sector of Indian Sundarbans facing
towards Bay of Bengal). This is due to high flood velocity and meandering nature of the
river course. The siltation and clogging of the Bidyadhari River results in negligible fresh
water flow in the central sector. The sediments carried during high tide from the Bay of
Bengal deposit due to absence of fresh water flow pressure from the upstream and causes
accretion. The gradual increase of Thakuran char in the central Indian Sundarbans confirms
the hypothesis.
Western versus central Indian Sunderbans
The results generated from our studies clearly represent contrasting outcome in two distinct
sectors in Indian Sunderbans: western and central. It was found that in the western sector
(Sagar and Jambu Island) island area has decreased compared to the central sector (Tha-
kuran Island). The aquatic salinity is gradually decreasing in the former sector, while the
later sector exhibits a rise in salinity. The geo-physical phenomena in this deltaic system
are the roots of such variation. During early fifteenth century, the River Ganga changed its
main course from the Bhagirathi. The eastward change of the course of the main flow of
the River Ganga brought metamorphic changes in the deltaic lobe. A number of distrib-
utaries and tributaries were cut-off from the upland flow that signaled the end of those
channels. Human interference (particularly in and around the city of Kolkata) further
accelerated the decay of the Bidyadhari river thereby choking the system with silt and
sewage. The central sector thus became isolated from the western Indian Sunderbans and
the freshwater supply to the rivers like Matla, Saptamukhi, Thakuran (in the central sector)
stopped. These rivers survive today through tidal inflow from the Bay of Bengal. Hence,
SLR and subsequent increase in salinity is more acute in the central sector compared to the
western part.
The phytoplankton community has shown compositional changes in the tide-fed rivers
of central sector with dominancy of stenohaline species. In the fishery sector, many species
(like T. ilisha) that prefer freshwater for breeding has changed its course from central to
western Indian Sunderbans. More trash fishes which can survive and reproduce in stressful
saline condition have become dominant in the central sector and their diversity has
increased over time. The stunted growth of mangroves in the central Indian Sunderbans is
an outcome of hyper-saline condition.
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19. The most important direct physical effect of SLR is coastal erosion, which is more
visible in the western sector compared to central. This may be attributed to absence of
head-on discharge from the upstream zone in the rivers of central sector due to complete
decay of the Bidyadhari River. The sediment brought by tidal currents therefore settles in
and around the islands as seen in case of Thakuran char. The exact cause of SLR in Indian
Sunderbans is yet not clear. It may be attributed to absence of head-on discharge (of fresh
water), siltation on the river bed, land subsidence or the synergistic effects of all the
factors, the impact of which is more in central Indian Sunderbans, compared to the western
sector.
Conclusions
The discharge from the Farakka dam along with siltation in the Bidyadhari river basin has
created a marked difference in water chemistry (particularly salinity) between the western
and central Indian Sunderbans. It was found that the mangrove growth in Sunderban areas
is the most severely affected biotic component by salinity alteration. As a result the growth
of freshwater loving species would be severely affected. The AGB of dominant mangroves
(S. apetala, A. alba and E. agallocha) exhibit significant spatial variation. The AGB
values are more in the western sector compared to central sector. Salinity seems to be the
key player for such variation. Significant difference in phytoplankton community structure
is observed between western and central Indian Sunderbans. Few stenohaline species are
recorded during the study period which reflects the intrusion of seawater (from the Bay of
Bengal) in the central Indian Sunderbans. Spatial variation in fish community is revealed
from the catch statistics. Commercially important fish species is more relative to low
priced trash fishes in the catch basket of the western Indian Sunderbans. In the central
sector the picture is reverse. The erosion and accretion phenomena are regulated by littoral
current pattern, and sediment influx from different rivers and adjacent Bay of Bengal along
with anthropogenic factors like dam construction and barrage discharge. Under ideal plant
succession conditions, species might migrate inland in response to advancing salinity. In
addition, more than half a million people, dependent on forest products in the Sunderbans,
would also be exposed to economic uncertainties.
The extremely high population pressure in and around the Indian Sunderbans is a major
threat to the delta. The embankments constructed to ensure safety (from tidal surges and
wave actions) to island dwellers have not only hindered the natural flow of tidal water, but
at the same time enhanced the process of sediment deposition on the adjacent river basin.
The observed change in the biotic community of Indian Sunderbans has little linkage to
climate change as it is difficult to segregate the noise. We recommend different strategies
for two sectors of Indian Sunderbans for addressing the gaps in understanding the physical
processes, water chemistry, living resources and island dwellers: a coordinated programme
of long-term research linking monitoring, process studies and numerical modeling. The
scope of these issues facing the mangrove dominated deltaic system requires that the
recommended program reflects a diverse, inter-disciplinary, multi-institution approach and
strong institutional network between researchers and decision makers.
Acknowledgments The authors acknowledge the Global Land Cover Facility (GLCF) website (http://glcf.
umiacs.umd.edu/aboutUs/) for providing the Landsat MSS and TM data for the year 1975 and 1989.
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20. References
Allison MA (1998) Historical changes in the Ganges–Brahmaputra delta. J Coast Res 14(4):1269–1275
Balmforth EB (1985) Observations on Sundri top dying-descriptive sampling in the Sundarbans Reserved
Forest. Draft Working Paper of UNDP/FAO Project BGD/79/017, pp 32
Bandyopadhyay S (1997) Natural environmental hazards and their management: a case study of Sagar
Island, India. Singap J Trop Geogr 18:20–45
Bandyopadhyay S (2000) Coastal changes in the perspective of long term evolution of an estuary: Hugli,
West Bengal, India. In: Rajamanickam V, Tooley MJ (eds) Quaternary sea level variation, shoreline
displacement and coastal environments. New Academic Publishers, New Delhi, pp 103–115
Bhattacharya S (2007) Lessons learnt for vulnerability and adaptation assessment from India’s first national
communication. Paper 7 of BASIC Project, Winrock International, pp 11
Blasco F (1975) The mangroves of India. Institut Franc¸ais Pondichery Travaux Section Scientifique et
Technologie 14:1–175
Chaffey DR, Miller FR, Sandom JH (1985) A Forest Inventory Project, Bangladesh: Main Report. Overseas
Development Administration, England, p 196
Chaudhuri AB, Choudhury A (1994) Mangroves of the Sundarbans, Vol 1: India, IUCN-The World Con-
servation Union
Chidumaya EN (1990) Above ground woody biomass structure and productivity in a Zambezian woodland.
For Ecol Manage 36:33–46
Cintron G, Lugo AE, Pool DJ, Morris G (1978) Mangroves of arid environments in Puerto Rico and adjacent
islands. Biotropica 10:110–121
Cole CP, Vaidyaraman PP (1966) Salinity distribution and effect of fresh water flows in the Hooghly River.
In: Proceedings of Tenth Conference on Coastal Engineering, Tokyo, Japan, September, pp 1312–1434
Coleman JM (1969) Brahmaputra River: Channel processes and sedimentation. Sed Geol 3:129–239
Das S, Siddiqi NA (1985) The mangroves and mangrove forest of Bangladesh. Mangrove Silviculture
Division, Bulletin No. 2, Bangladesh Forest Research Institute, Chittagong, Mangrove plantations of
the coastal afforestation project. UNDP Project BGD/85/085, Dhaka. Field document No. 2, pp 69
Deb SC (1956) Paleoclimatology and geophysics of the Gangetic delta. Geogr Rev India 18:11–18
Dowell SL, Rickards LJ (1993) Recent developments in sea level networks and data centres. In: Sea level
changes and their consequences for hydrology and water management. Proceedings of an International
UNESCO Workshop held at Noordwijkerhout, Netherlands, 19–23 April 1993. Bundesanstalt fur
Gewasserkunde, Koblenz, pp 39–50
Ganguly D, Mukhopadhyay A, Pandey RK, Mitra D (2006) Geomorphological study of Sundarban deltaic
estuary. J Indian Soc Remote Sens 34(4):431–435
Ghosh T, Bhandari G, Hazra S (2002) Coastal erosion and flooding in Ghoramara and adjoining islands of
Sundarban deltaic system. In: Proceedings of an International Conference on Water Related Disasters,
Analysis and Practice of Water Resource Engineering for Disaster Mitigation, New Age International
Pvt Ltd., New Delhi, pp 38–41
Goodbred SL, Kuehl SA (1999) Holocene and modern sediment budgets for the Ganges–Brahmaputra River
system: evidence for high stand dispersal to flood-plain, shelf, and deep-sea depocenters. Geology
27(6):559–562
Goodbred SL, Kuehl SA (2000) Enormous Ganges–Brahmaputra sediment discharge during strengthened
early Holocene monsoon. Geology 28(12):1083–1086
Gopal B, Chauhan M (2006) Biodiversity and its conservation in the Sundarban mangrove ecosystem. Aquat
Sci 68:338–354
Goudie A (1983) Environmental change, 2nd edn. Clarendon Press, Oxford, p 258
Hazra S, Ghosh T, Dasgupta R, Gautam S (2002) Sea level and associated changes in the Sundarbans. Sci
Cult 68(9–12):309–321
Husch B, Miller CJ, Beers TW (1982) Forest mensuration. Ronald Press, New York
Institute of Water Modeling (IWM) (2003) Sundarban Biodiversity Conservation Project. Surface Water
Modeling Final Report. Dhaka Bangladesh
Kumar VS, Pathak KC, Pednekar P, Raju NSN, Gowthaman R (2006) Coastal processes along the Indian
coastline. Curr Sci 91(4):530–536
Master Plan Organisation (1986) National Water Plan, vol I. MPO, Dhaka
Mikhailov VN (1998) Gidrologiya ust’ev rek (River Mouth Hydrology). Moscow Mosk Gos Univ
Mikhailov VN, Dotsenko MA (2007) Processes of delta formation in the mouth area of the Ganges and
Brahmaputra Rivers. Water Resour 34(4):385–400
Mikhailov VN, Kravtsova VI, Magritskii DV et al (2004) Deltas of Caspian Rivers and their response to sea
level variations. Vestn Kaspiya 6:60–104
Biodivers Conserv
123
Author's personal copy

21. Mitra A, Banerjee K (2011) Trace elements in edible shellfish species from the lower Gangetic delta.
Ecotoxicol Environ Saf 74(6):1512–1517
Mitra A, Banerjee K, Bhattacharyya DP (2004) The other face of mangroves. Department of Environment,
Government of West Bengal Press, India
Mitra A, Banerjee K, Sengupta K, Gangopadhyay A (2009a) Pulse of climate change in Indian Sundarbans:
a myth or reality? Natl Acad Sci Lett 32:1–2
Mitra A, Gangopadhyay A, Dube A, Schmidt ACK, Banerjee K (2009b) Observed changes in water mass
properties in the Indian Sunderbans (northwestern Bay of Bengal) during 1980–2007. Curr Sci
97(10):1445–1452
Mitra A, Mondal K, Banerjee K (2011) Spatial and tidal variations of physico–chemical parameters in the
lower Gangetic Delta Region, West Bengal, India. J Spatial Hydrol, American Spatial Hydrology
Union Spring 11(1):52–69
Morgan JP, McIntire WG (1959) Quaternary geology of the Bengal Basin, East Pakistan and Burma. Bull
Geol Soc Am 70:319–342
Nichols RJ, Goodbred SL (2004) Towards integrated assessment of the Ganges–Brahmaputra delta. In:
Proceedings of 5th International Conference on Asian Marine Geology, 13–18 Jan 2004
Sanyal T, Chatterjee AK, Mandal GC (2000) Erosion-deposition in Hooghly estuary. Defense Sci J
50:335–339
Shafi M (1982) Adverse effects of Farakka on the forests of southwest region of Bangladesh (Sundarbans).
Proceeding of second national conference on forestry 12–26 Jan, Dhaka, pp 30–57
Shannon CE, Weiner V (1949) The mathematical theory of communication. University of Illionis Press,
Urbana, p 117
Shetye SR, Gouveia AD, Pathak MC (1990) Vulnerability of the Indian coastal region to damage from sea
level rise. Curr Sci 59(3):152–156
Singh OP (2002) Spatial variation of sea level trend along the Bangladesh coast. Mar Geol 25:205–212
Snedaker C (1991) Notes on the Sundarbans with emphasis on geology, hydrology and forestry. In:
Seidensticker J, Kurin R, Townsend AK (eds) The commons in south Asia: societal pressures
and environmental integrity in the Sundarbans. The International Center, Smithsonian Institution,
Washington, DC
UNEP (1989) Criteria for assessing vulnerability to sea level rise: a global inventory to high risk area. Delft
Hydraulics, Delft, The Netherlands, p 51
Unnikrishnan AS, Kumar KR, Fernandes SE, Michael GS, Patwardhan SK (2006) Sea level changes along
the Indian coast: observations and projections. Curr Sci 90(3):362–368
WWF-India (2010) Sunderbans: future imperfect, Climate Adaptation Report
Biodivers Conserv
123
Author's personal copy