SCIENCE REPORTER, FEBRUARY 2013 44

DIPANJAN GHOSH AND SREEPARNA GHOSH

Feat

ure

Art

icle

Sunderban, the largest mangrove ecosystem in the world, is facing serious threats to its survival. Also threatened are a large number of endangered and globally threatened faunal species and local inhabitants who earn their livelihood.

SUNDERBAN is the largest mangrove ecosystem in the world, covering an area

of about one million hectares, of which 60% is located in Bangladesh and the remaining in India. Located in the southern part of the state of West Bengal, it extends over the area from Dampier-Hodges Line in the north to the Bay of Bengal and from the western embankment of the rivers Ichhamati-Kalindi-Raymangal in the east to the river Hooghly in the west. The area included in the UNESCO World Heritage list in 1987 is a part of the famous Ganges-Brahmaputra Delta.

The name ‘Sunderban’ has probably been derived from the Sundari trees (Heritiera fomes) that once abundantly populated the Sunderban. Alternatively, it has been proposed that the name is a corruption of Samudraban meaning ‘Sea Forest’.

Sunderban is made up of 102 small islands and is crisscrossed by several rivers like Hooghly, Ichhamati, Bdyadhari, Thakuran, Matla, Gosaba, Haribhanga, etc., along with their rivulets and creeks. Out of the total area of the Indian Sunderban of

about 410,000 hectares, 212,500 hectares is occupied by mangrove forest and 178,100 hectares is water body. The entire forested area constitutes the Biosphere Reserve (declared on 29 March 1989). The present Sunderban National Park, with three sanctuaries, is an integral part of this biosphere.

The Sunderban creates a unique ecological environment that is composed of rich assemblages of both floral and faunal wealth, along with dense human habitation surrounding the forest core area. Human settlements in Sunderban are believed to have started at least four hundred years ago.

It has a funnel-shaped landscape with numerous linear tidal mud flats and a network of tidal channels. The Sunderban mud flats are found at the estuary and on the deltaic islands where low velocity of river and tidal current occurs. The flats are exposed during the low tides and submerged in the high tides. The tidal action deposits silt back on the channels thus raising the bed and forming new islands and creeks contributing to an uncertain geomorphology. Due to the gentle slope of the coast and large tidal amplitude, tidal water penetrates to an average distance of 110 kilometres inland from the shore line and in some areas, the effect of the tides is felt over 300 kilometres inland.

The Sunderban region has a warm humid climate. The average maximum and minimum temperature of this region is 40° C and 18° C respectively, with an annual rainfall of about 1600 to 1800 millimetres. Sunderban receives rainfall during the southwest monsoon season (June to October), with occasional rainfall throughout the year. Rainfall is negligible in the winter months. Occasionally, during May and October, violent cyclonic storms accompanied with high sea waves and tides devastate the coastal Sunderban area.

Soil in the SunderbanThe active delta of the Sunderban contains different types of soil such as silty clay loams, sandy clay and loams, soil with organic and peaty deposits, swampy and marshy soil, also called as ‘mangrove soil’. The soil is oxygen deficient and nitrogen content is also low. Soil pH is more or less acidic, ranging from 5.4 to 7.8.

Although the soil contains adequate water, plants cannot avail of the water because of high concentration of salts (mostly chloride, sulphate and bicarbonate salts of sodium, magnesium and calcium). The salinity of the surface soil is governed by the quantity of freshwater flow and monsoon rainfall, being highest in the middle of the summers and lowest during the rainy

Challenges to Survival

SCIENCE REPORTER, FEBRUARY 201345

Feature Article

months. The Ganges and Brahmaputra, two snow-fed rivers and their large number of tributaries and rivulets, supply a huge amount of freshwater to the Sunderban mangroves throughout the year. Because of this, salinity of the water body within the mangrove forest as well as near the sea is lower than that of the sea water even during the summer months.

Mangrove Vegetation Mangroves are flowering plants comprising at least three types of floral components – true mangroves, back mangroves and mangrove associates.

True mangroves are salt-tolerant halophytic plants, growing on tidal swamp habitats. The true mangrove species thrive in high salinity, daily submergence, fine clayey oxygen deficient soil, tidal currents, fresh water flow and strong wind as well. To adapt to the situation, they have stilt roots and supporting cable-like shallow holding roots that support the plant in tidal currents. True mangroves also possess pneumatophores to access atmospheric oxygen. Most of these plants possess salt-secreting glands in their leaves.

Back mangroves are a bushy, discontinuous type of vegetation. They are not subjected to the same degree of tidal inundations as experienced by true mangroves that grow near mangrove stands towards the landward side. Though able to withstand the high salinity and low nutrient soil associated with coastal areas, these plants generally are not found in the intertidal areas colonized by true mangrove plants.

Mangrove associates are not true mangrove plants. They are common mesophytic plants occurring near the human habitation. These plants can grow in nutrient-deficient soil but cannot withstand environmental conditions as encountered by the true mangroves.

The freshwater river discharge coupled with high rainfall, the tropical humid climate and the less number of dry months all support plant growth. Hence, the species variety as well as biomass is obviously high in this region, in comparison to the rest of the mangrove wetlands in India.

The interior parts of the mudflats are home to luxuriant mangroves. According to an estimate by the West Bengal Forest Department, there are 84 species of true mangroves, mangrove-associates and back mangroves found in the Sunderban mangrove forest. However, the distribution of species is not uniform and primarily controlled by the level of salinity and not by the tidal inundation.

The true estuarine zone is dominated with species with tolerance to high salinity

Swamp Forest

Creek

Swamp Thicket

Creek

Herbaceous Swamp High Tide

No Current Max. CurrentNo Current

Mid Tide

Mud Plates Sandy Mud

Sand Flats

Low Tide

Above: Indian Sunderban comprises more than hundred small islands (Photo: Ayan Mondal)

Above right: Human settlements in Sunderban are believed to have started long ago (Photo: Ayan Mondal)

Right: Fishing in brackish water is a very familiar occurrence in Sunderban (Photo: Ayan Mondal)

Sectional view of mudflats and sandflats in Sunderban delta (Courtesy: A Banerjee, Environmental, Population and Human Settlements of Sunderban Delta, 1st Ed, 1998, Concept Publishing Company, New Delhi)

Sunderban mudflats are exposed in low tides and submerged in high tides, thus being changed morphologically even in a single

tidal cycle.

SCIENCE REPORTER, FEBRUARY 2013 46

Feature Article

and submergence, including Jatbaen (Avicennia officinalis), Pairabaen (A. alba), Kalabaen (A. marina), Garia (Kandelia candel), Kripa (Lumnitzera racemosa), Tora (Aegialitis rotundifolia), etc. The middle estuarine zone where the salinity is lower but the tidal current passing through the narrow creeks and channels is high is dominated by Garjan (Rhizophora apiculata), Goran (Ceriops decandra), Mathgoran (C. tagal), Bakul (Bruguiera

cylindrica), Keora (Sonneratia apetala), etc. The inner estuarine zone comprising of elevated areas with less saline soil and more fresh water flow is dominated by Genwa (Excoecaria agallocha), Kankra (Bruguiera gymnorrhiza), Khalsi (Aegiceras corniculatum), Ora (Sonneratia caseolaris) and Hental (Phoenix paludosa).

However, some mangrove plant species are rare, endemic and restricted to specific salinity. The distribution of

trees like Amur (Agalia domestica), Dhundul (Xylocarpus granatum), Passur (X. mekongensis), Sundari (Heritiera fomes) and Golpata (Nypa fruticans) palm-swamps are extremely limited. Among these species, Sundari and Golpata are very rarely sighted in the wild. Besides, plants like Amur, Passur, Tora, etc. are endemic to the Sunderban region only.

Freshwater inflow in the western part of Sunderban has been reduced by geotectonic and fluvial reasons. This has caused the forest cover in this area to be less dense and discontinuous. The mangroves in this zone are dwarf as well as bushy types, like Hargoza (Acanthus ilicifolius), Ban Lebu (Merope angulata) and Lata Sundari (Brownlowia tersa).

Survival Challenges Since the last few decades, certain natural and man-made problems are posing a serious threat to the natural environmental balance and species diversity of the Sunderban region. These have disturbed the complex food chain and the wildlife population, enhanced coastal erosion, diminished soil fertility and now also provide less resistance to the ravages of gales and storms over the land. Besides, several important fish species, some other faunal species and mangroves have been declining dramatically in the region.

Left: A vast area of the mangrove forest has been flooded due to tidal flow (Photo: Ayan Mondal)Right: Pneumatophores or breathing roots help mangroves in gaseous exchange directly from the air (Photo: Ayan Mondal)

Above: Stilt roots support the mangrove plant to stand on feculent soil and in tidal currents (Photo: Ayan Mondal)

It is high time that well thought out environmental protection measures are implemented before this unique biosphere is lost forever.

Mangroves are flowering plants comprising at least three types of floral components – true mangroves, back mangroves and mangrove associates. True mangroves are salt-tolerant halophytic plants, growing on tidal swamp habitats.

SCIENCE REPORTER, FEBRUARY 201347

Extensive human intervention is considered the main reason behind the rapid degradation of mangrove vegetation in Sunderban. Many mangrove trees are of great economic value as building materials for rural houses, furniture, for making

country boats and as fuel wood. However, indiscriminate exploitation of mangrove forest produce, mainly for timbers and firewood, is causing a massive loss of biodiversity. During the last two centuries, more than 5000 square kilometers of the

mangrove forests were reclaimed by people for agriculture and human settlement.

Today, a large area of the forest is being taken over each year for establishment of shrimp ponds, a common practice in Sunderban. Moreover, there is also a proposal for setting up a National Waterway through Sunderban; such infrastructural development would adversely affect the mangrove ecosystem by destroying its rich assemblage of species.

Mangroves act as natural sewage treatment plants and absorb pollutants both from air and water. They possess high percentage of tannins in their bark and leaves that can neutralize some of the industrial pollutants. Mangroves are also an important carbon dioxide sink; among mangrove plants found in Sunderban, Keora is the most efficient species followed by Baen and Genwa. According to a recent estimate based on a study conducted by researchers of the Department of Marine Science, Calcutta University, the annual assimilation of carbon dioxide by Sunderban mangroves amounts to 3.8 million tonnes. Presently there is about 6.62 billion tonnes of carbon dioxide captured within the mangrove plants of Sunderban.

Though the mangroves have an enormous capacity to absorb industrial effluents and other forms of pollutants, dumping of superfluous pollutants and greenhouse gases may affect the mangrove ecosystem adversely. Besides, oil spills from old and defective launches and boats are also a potent threat that causes immense damage to the aquatic fauna and the mangrove vegetation.

Golpata palms (Nypa fruticans) are restricted only to some areas in Sunderban with low soil-salinity (Photo: Ayan Mondal)

Propagules are ready for viviparous germination (Photo: Dipanjan Ghosh); above: Sundri tree (Heritiera fomes) is a rare sight in Indian Sunderban and flowering is also rather rare (right)(Photo: Ayan Mondal)

Human settlements in Sunderban are believed to have started at least four hundred years ago.

SCIENCE REPORTER, FEBRUARY 2013 48

The mangrove forest is an important living, feeding and breeding ground for many brackish water faunal species of great economic importance. Local inhabitants of Sunderban collect tiger prawn seeds (known as meen) by dragging fine-mesh nylon nets along the river banks. In this process, mangrove seedlings and many species of fish are destroyed, along with the possibility of regeneration of mangrove along the river banks. Construction of irrigation and drainage canals as well as raising embankments along the major river systems against insurgence of saline water also interferes with the natural gradient. Further, fresh water flow has been thwarted due to the setting of fisheries in rivers, creeks and estuaries.

In the Sunderban, low saline tolerant species like Heritiera fomes and Nypa fruticans are gradually disappearing and species like Avicennia marina that can

tolerate a high and broad range of salinity are becoming dominant. The main reason for such floristic changes is reduction in the periodicity and quantity of fresh water reaching the mangrove environment.

Geologists are of the opinion that the Bengal basin is slowly tilting towards the east as a result of neo-tectonic movement. Due to the raising of the western region of the Sunderban, the ancient branches of the river Ganges have separated from the area, which today comprises the Indian part of the Sunderban. Now the highest level of fresh water run-off is restricted to the Bangladesh Sunderban. Hence, the salinity of Bangladesh Sunderban is much lower than that of the Indian Sunderban.

The mangrove habitat along the coastline acts as a natural buffer from damage by tides and surges, and also traps sediments thus protecting the coastal region from erosion. However, excessive

sedimentation due to tidal action suffocates the mangrove plants. Daily submersion in saline water for a long duration may also cause trees to die. As soon as the mangrove dies, the pneumatophores decompose rapidly which promotes soil erosion. The tidal action also washes away a considerable amount of the organic matter produced by the trees and leaf litters do not accumulate. All this may cause a lack of adequate nutritive elements in the mangrove ecosystem.

The Sunderban mangrove functions as a wind-breaking barrier and minimizes the intensity of cyclonic storms. However, disasters sometimes may strike, for instance the devastating cyclonic storm Aila that struck Sunderban on 25 May 2009. The Sunderban and its dependants have not still been able to overcome the drastic blow of Aila.

The mangrove vegetation on the active delta of Sunderban is part of a unique biosphere where the sea meets the land. This biosphere sustains a large number of endangered and globally threatened faunal species. All constituting parts of this mangrove ecosystem depend upon each other for existence, including the local inhabitants who earn their livelihood from it.

It is high time that well thought out environmental protection measures are implemented before this unique biosphere is lost forever.

Mr Dipanjan Ghosh is a postgraduate teacher in Biology in a Govt. Sponsored Higher Secondary (+2 level) School situated at the Burdwan Township in West Bengal. Address: Chotonilpur Pirtala, PO Sripally, Dist. Bardhaman -713103 West Bengal; Email: dpanjanghosh@gmail.com. Ms Sreeparna Ghosh is associated with an NGO that is engaged with environmental problems and children’s awareness through education. Email: zeenaghosh@gmail.com

Above: Estuarine crocodile (Crocodylus porosus) is a menacing aquatic animal of the Sunderban forest (Photo: Ayan Mondal)Left: ikes along the major river system in Sunderban combat against the insurgence of saline water in agricultural fields (Photo: Ayan Mondal)

It has a funnel-shaped landscape with numerous linear tidal mud flats and a network of tidal channels. The average temperature of this region is between 40o C and 18o C respectively, with an annual rainfall of about 1600 to 1800 millimetres.

Occasionally, during May and October, violent cyclonic storms accompanied with high sea waves and tides devastate the coastal Sunderban area.

Feature Article