Sunderbans:
The
Sunderbans comprise the outer portion of the Ganges/Brahmaputra delta, a vast
region of tidal channels and islands covering about one million hectares and including
one of the largest contiguous blocks of mangrove forest in the world. Geographically
the delta is situated between 21º32' - 22º 20' N latitude and 88º 05' - 89 º 05'
E longitudes; form the Hooghly-Matlah estuary east to the Bangladesh border, 100
km southeast of Calcutta. Almost 60% of this extensive area lies in Bangladesh.
The remainder, including some 2000- 3000 sq.km of mangrove forest, is in Indian
Territory. The Indian portion includes the deltaic region of the Hooghly-Matlah
estuarine system, and the area bordering the Hooghly, Muriganga, Saptamukhi, Thakuran,
Goshaba, Vidya, Matlah, and Hasinbhanga estuaries. In the western part of this
section, large areas have been bunded for human settlement and cultivation, and
very little mangrove forest now remains. The Indian Sunderban lies at the western
edge of the ancient delta of the Ganges and suffer from an extreme scarcity of
freshwater. Morphogenetic and tectonic uplift of this western margin has shifted
the course of the Ganges eastwards, and the inflow of fresh water is now almost
exclusively confined to the monsoon period between May and November. As a consequence,
the Bengali Sunderban do not receive the volume of fertile alluvium brought by
desalinating monsoon floods as they did in the past, and this is reflected in
the pronounced changes in the floristic composition of the forest which have taken
place over the past 600 years. Partly to remedy this situation but mainly to alleviate
rapid siltation in the Port of Calcutta, the Indian government commissioned the
building of the Farakka Barrage across the Ganges in 1971 to bring fresh water
southwards into the region.
The Sunderbans lie at the apex of this funnel-shaped
Bay of Bengal and receive the full force of between four and eight intense cyclonic
storms per year between August and November. Storm surges with waves of five to
eight meters in height present a considerable marine invasion, creating tidal
amplitudes far in excess of the 4.16 m average for spring tides. Under normal
conditions, the tidal amplitude in the Hooghly Estuary ranges from 1.6 m to 4.3
m and the tidal influence extend's almost 290 km inland. Between 40% and 60% of
Sunderban soil is composed of silt; the amount of coarse sand does not exceed
2%, and the amount of organic matter remains constant at between 1% and 1.5%.
The pH ranges from 5.5 to 8. Salinities range from 3.6 ppt to 23.5 ppt; at the
beginning of the dry season the salinity is high, but by the end of the dry season
it is excessive as to be inimical to the development or regeneration of all but
the most halophilous plant species. The Sundarbans delta has a variety of ecologically
important features such as mangroves, mud flat, sand bar etc. The area of important
wetland classes in Sunderbans has been described in Table 1.
Sunderbans:
A View
Mangroves
Climatic conditions:
Tropical monsoon climate with an annual
rainfall of 1500 - 2500 mm. January is the coolest month with a mean temperature
of 20º C and minimum temperature of 10º C. There is a pronounced dry season from
Jan to April / May.
Table
1. Wetland classes
and its areal extent of Sunderbans
| Sl
No | Wetland
classes | Area
in km² |
| 1 |
Dens Mangrove |
1768.85 |
| 2 |
Space Mangrove |
127.84 |
| 3 |
Mud Flat |
78.74 |
| 4 |
Salt Marsh |
2.94 |
| 5 |
Aquaculture
Ponds | 19.91 |
| 6 |
Salt pan |
0.02 |
| 7 |
Habitation
with Vegetation | 107.97 |
Fig: Ecologically
Important Area in West Bengal : Sunderbans
Flora:
The
mangrove forests of the Indian Sunderban are now almost confined to a number of
islands to the east of the Matlah River.
Five principal formations
have been distinguished in the Sunderban:
(a) mangrove scrub or back-mangals
(b) saltwater mangrove forest
(c) a phoenix paludosa belt
(d) moderately
saline, mixed forest dominated by heritiera fomes
(e) brackishwater, mixed
forest dominated by Heritiera
forest.
However, only the first three of these
formations are widespread in the Indian portion of the Sunderban. The back-mangals
are bushy, discontinuous thickets of halophytes, often situated at the equinoctial
high-tidal level. Species of Suaeda,
Sesuvium,
and Heliotropium
are common.The back-mangals form the inland edge of the mangrove
forests and were once widespread in the northern part of the delta. However, land
reclamation has been very acute in this zone and, as a result; back-mangals are
now found only in small isolated patches. The saltwater mangrove forest is the
most widespread formation in the Indian Sunderban. These forests cover a large
number of islands and islets, 3-5 m above sea level.The plant cover is very dense,
averaging 12 trees (10-18 m in height), 440 shrubs (4-8 m high), and 2200 understorey
shrubs (less then 4 m high) per hectare. The floral composition varies considerably
from one island to another. Excoecaria
agallocha or Sonneratia
apetala may be the dominant species,ofter in pure stands
or interspersed with woody thickets of Ceriops
decandra and Aegiceras
corniculatum.
Other common species include Avicennia
spp., Cruguier gymnorhiza, Xylocarpus granatum, X. moluccensis,
and Rhizophora
mucronata.
The palm Phoenix paludosa occurs throughout the delta but always between the water's
edge and the mangrove proper. It may extend up to 60 km inland, along channels
having a heavy accumulation of salts during the dry season.The two other mangrove
formations-moderately saline, mixed forest and brackishwater mixed forest- are
now almost confined to the Bangladeshi portion of the Sunderban where salinities
are low.. Some 5000 years ago, the arborescent vegetation of the Calcutta region
was dominated by Heritiera
fomes, which
flourished on recent alluvial deposits with relatively low salinity. However,
since the Gangetic floods no longer flush through the western Sunderban, it has
been largely replaced by species of Rhizophoraceae. The
grass Porteresia coarctata is very abundant in the northern part of the delta,
where it fixes the recent alluvial deposits. . The palm
Nypa fructicans is now almost extinct
in the West Bengal; the few remaining small stands occur along creeks with a good
supply of fresh water.
Fauna:
The Sunderban
contain some 300 tigers (Panthera
tigris), the largest single population of the species. The spotted
deer (Axis
axis), wild bear (Sus
scrofa), andrhesus macaque (Macaca
mulatta) are particularly common. Marine mammals include the
cetaceans
Sousa plumbea, Delphinus delphis, Orxaella
brevirostris
and Neophocaena
phocanoides. The
Ganges river dolphin (Platanista
gangeitca) lives all along the estuaries. The mangrove forests
support a rich and relatively undisturbed avifauna.
There are six species of kingfisher
Alcedinidae, including Pelargopisis amauroptera. Pelecanus philippensis
reportedly breeds and some 5000 Anastomus
oscitans inhabit the kulik forest. Small numbers of
Leptoptilos javanicus
are regularly seen and presumably breed. The coastal mudflats and
estuaries are of great importance as staging and wintering areas for migratory
shorebirds, gulls, and the endangered river terrapin (Batagur
baska) may
in habit the area. The marine turtle Lepidochelys
olivacea and Erenmochelys
imbricate
breed along the seaward edge of the forest and the endangered river terrapin
(Batagur
baska) may inhabit the area.There is a small, but growing, population
of the estuarine crocodile (Crocodylus
porosus),supplemented by captive-reared individuals. Snakes
incude the python (Python
molurus), some more then five meters in length. The Bengal monitor
(Varanus
bengalensis) is also found.
Environmental
Status click
here
