CLIMATE CHANGE AND IMPACT ON MANGROVES

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Biodiversity and Conservation ISSN 0960-3115 Biodivers ConservDOI 10.1007/s10531-012-0260-z

Climate change impacts on IndianSunderbans: a time series analysis (1924–2008)

Atanu Raha, Susmita Das, KakoliBanerjee & Abhijit Mitra

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ORI GIN AL PA PER

Climate change impacts on Indian Sunderbans: a timeseries analysis (1924–2008)

Atanu Raha • Susmita Das • Kakoli Banerjee • Abhijit Mitra

Received: 14 April 2011 / Accepted: 17 February 2012� 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 � S. DasOffice 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, Indiae-mail: [email protected]

A. MitraDepartment of Marine Science, University of Calcutta, 35 B.C. Road, Kolkata 700019, India

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Biodivers ConservDOI 10.1007/s10531-012-0260-z

Author's personal copy

http://dx.doi.org/10.1007/s10531-012-0260-z

Keywords Climate change � Indian Sunderbans � Mangrove � Phytoplankton �Land dynamics

AbbreviationsAD 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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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 Gangaand Brahmaputra mouth area

Period1,000 yearsago

Ocean levelrelative tothe presentlevel (in m)

Ocean level rise(m) (cm year-1)

Hydrological conditions Sedimentation andmorphological processes

Before 18 -120 – Maximum of glacial oceanregression, high slopes ofwater surface and currentvelocities, evacuation ofcoarse sediments(boulders, pebble, andgravel)

Formation of deep down-cut channel

18–11 -120/-55 65 (0.93) Beginning of post glacialtransgression,propagation of backupinto the lower part oferosional downcutting

Shifting of zone of riversediment accumulation inthe landward direction

11–77–4

-55/-10-10/-5

45 (1.12)5 (0.17)

Increased river water flowand sediment runoff,considerable rise of oceanlevel, propagation ofbackup into the basin

Slowing down of oceanlevel rise, decrease inbackup

Large scale accumulationof sandy and siltysediment of the basinsurface, intensedevelopment of Holocenedelta, accretion of branchchannels

Further sedimentation onthe basin surface anddelta development,accretion of branchchannels, shelf formation

4–0 -5/0 5 (0.12) Stabilization of ocean level Formation of modernhydrographic system ofdelta

Source Mikhailov and Dotsenko (2007) (the figures on the left in 2nd column means at the beginning of theperiod, while at the right represents the end of the period; dash here means lack of information)

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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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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�130N–22�400Nlatitude and 88�030E–89�070E 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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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 �XS

i¼1

ni

Nloge

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 incentral Sunderban. Sampling locations marked in black flag shows the stations selected in both the sectorsfor diversity and biomass estimation

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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. Sonneratiaapetala, 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, Triceratiumjentacrinus, 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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Fig

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Fig

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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, Osteogeneiosusmilitaris 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

Bio

div

ersi

ty in

dex

(b

ar H

)

Western phytoplankton Central phytoplanktonWestern Commercially important fishes Western trash fishesCentral Commrcially important fishes Central trash fishes

0

20

40

60

80

100

120

140

160

180

1990 1995 2000 2005 2010

cells

x105 /

m3

Western

1990 1995 2000 2005 2010

Western phytoplankton Central phytoplanktonWestern Commercially important fishes Western trash fishesCentral Commrcially important fishes Central trash fishes

0

0

0

0

0

0

0

0

0

0Western

Central

a

b

Fig. 3 a Temporal variation of fish and phytoplankton diversity index (H) in the western and central sectorsof Indian Sunderbans. b Temporal variation of phytoplankton standing stock (N) in the western and centralsectors of Indian Sunderbans

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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�03006.1700 longitude and

21�38054.3700 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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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

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2010 prm

30

40

B (

t/

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01 2 3 4 5 6 7 8 9 10

60

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2009prm

2010 prm

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B (

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30

mon2010pom2009

20

pom2010

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1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

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(t/

ha

AG

Fig. 4 Seasonal variation of AGB of a Sonneratia apetala, b Avicennia alba and c Excoecaria agallocha inthe selected stations during 2009–2010; x-axis depicts the number of stations and y-axis the ABG value up to70 t ha-1

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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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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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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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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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http://glcf.umiacs.umd.edu/aboutUs/

http://glcf.umiacs.umd.edu/aboutUs/

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CLIMATE CHANGE AND IMPACT ON MANGROVES

Environment

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