Sundarban mangroves


Threats to Sundarban mangroves

Human pressure remains the largest threat to the Sundarbans like any other mangroves across the world. Though Sundarbans is claimed to be pristine in state, more than 50% of the north-western parts and intertidal lands of Sundarbans delta have been continuously cleared for human habitation and settlement, agriculture and brackish water fisheries. Landuse change, pollution from industrialization and economic activities e.g. shipping port close to the forest, intensive aquaculture and over-extraction of resources remain the major threat to the forest.

Sundarabans lies in the Khulna administrative zone – a major industrial hub for the country, offering sound strategic location for shipping yards, paper mills and port. Unchecked pollution from these sources, intensification of economic activities and landuse change in recent years have further aggravated the decline of its ecosystems despite the fact that, Sundarbans is a proven natural coastal barrier saving the forest and inland. Surely as cyclones become more frequent with global warming, significance of such natural protection increases exponentially.

Barrage construction

Dry season freshwater flow into the mangrove dominated western Sundarabns is reduced from the construction of Farakka barrage on the distributries of teh Ganges river since 1975. Freshwater starvation, of both natural and anthropogenic, led the major macrofauna (arboreal primates, gavial, large cats) of the system threatened. Even more alarmingly, it alter the natural salinity of the forest system allowing salinity intrusion. Major mangrove species of the Sundarbans is Heriteris fomes, more suited to lower salinity level. Salinity intrusion into the western Sundarbans in India already has eliminated this low salinity tolerant species from the zone. Assumably, a similar fate looms for the species in Bangladesh as well as freshwater flow gets increasingly squeezed due to the barrage.


Waste discharges from the trawlers, fishing boats and cargoes, unchecked sludge disposal from the industries add to already contaminated water in the rivers of the Sundarbans delta. A major oil-spill from a capsized cargo in Shela river in the Sundarbans in 2014, draw huge public attention. The disaster exposed zero-preparedness in disaster management. World alarmingly observed its slap on the biodiversity and its ecosystem as locals started oil-recovery under a health-hazardous condition. Little has improved since then. Neither the cargo-shipping through the rivers of the Sundarbans have ceased nor has the disaster management seen any improvement.

Furthermore the government in Bangladesh has undertaken a coal-based thermal power plant at Rampal, close to the Sundarbans. Despite the government assuring environmental impact assessment of the project, experts are doubtful and concerned about the future of the forest.

Nutrient enrichment

Nutrient enrichment is a major threat to marine ecosystems. Although mangroves have been proposed to protect the marine environment from land-derived nutrient pollution, nutrient enrichment can have negative consequences for mangrove forests and their capacity for retention of nutrients may be limited(Reef et al., 2010).

Non-point source pollution from agricultural fields and yet more alarmingly, the booming aquaculture industry in and around the Sundarbans enrich river water with nutrients. Consequence of such nutrient enrichment e.g. hypoxia , yet unobserved in the area, cannot be invalidated. In fact, Manna et al. (2010) already reported eutrophication and its possible consequences on the sea from in Indian part of Sundarbans.


Reef, R., Feller, I. C., & Lovelock, C. E. (2010). Nutrition of mangroves. Tree Physiology30(9), 1148-1160.


Like elsewhere in many coastal water, a bimodal pattern of plankton distribution in Sundarban estuaries is observed in Sundarbans as well. It means dense plankton population once in premonsoon (May) and again in postmonsoon (November) .

Dominant phytoplankton species in Sundarbans during are  Ditylum, Ceratium, Biddulphia, Chaetoceros, Coscinodiscus, Thalassiothrix, Rhizosolenia and Thalassionema. In post-monsoon phytoplankton species of Bacteriastrum, Biddulphia, Protoperidinium were most dominant. During monsoon, dominant species are Skeletonema, Fragillaria, blue green and euglenoids.

Zooplankton variation is insignificant across the Sundarban estuary unlike phytoplankton. The eastern part of the Sundarban estuary (India) is dominated by phytoplankton species like Biddulphia diatoms and green and blue green algae, while the central part is dominated by a variety of diatom species viz, Chaetoceros, Coscinodiscus, Bacterioastrum, Cyclotella, Ditylum, Skeletonema, Thallassiothrix, Thalassionema and Triceratium. In contrary, the western region is dominantly represented by species of Fragillaria, Gyrosigma, Nitzschlia and Bacillaria.

Plankton assemblage and distribution however is affected by both biotic factors and abiotic factors which includes pH, temperature, salinity etc. Since temperature variation is low in this tropical climate of Sundarbans, salinity and nutrient are considered as the most important biological factors governing phytoplankton composition and abundance. In fact, phytoplankton  and the biotic and abiotic factors (pH, alkalinity, and salinity) of the estuary are positively correlated i.e. rise of the factors will encourage a phytoplankton bloom.

Methane emission in Sundarbans

Tropical coastal mangroves being one of the most productive ecosystems of the world1 higher leaf litter deposition together with faster tidal sedimentation gives litter little chance for aerobic decomposition, resulting organic carbon rich anoxic condition in the subsurface sediment layer. Mangroves sediments rich in clay content further reduces the porosity of the sediment and induce anoxic condition.

Such anoxic condition encourages microbial processes in the subsurface mangrove sediments like denitrification, sulphate reduction, methanogenesis, and other redox reactions, which may contribute significantly to nutrient turnover2 and production of gases like methane, nitrous oxide, and hydrogen sulphide.

Research findings on Methane emission in Sundarban mangroves:

  • Sundarbans is a sink for methane during pre and post monsoon months; but during the monsoon it becomes a source. The annual contribution of this subtropical mangrove as a source of methane was calculated to be10.9 Gg.3
  • The onset of methanogenesis in this mangrove environment primarily occurs at a shallow depth (20–25 cm) of the sediment. 3
  • The produced methane undergoes vertical diffusive transport resulting methane emission (7.056 mg m−2d−1) from sediment surface to the atmosphere. 3
  • Horizontal transport as pore water dissolved methane enriching estuarine water column dissolved methane, which partially escapes to the atmosphere through air–water exchange (0.157 mg m−2d−1) and the remaining conveyed to the coastal water. 3


  1. E. Odum & Heald, E.J. The detritus-based food web of an estuarine mangrove community. L.E. Cronin (Ed.), Estuarine Research, Academic Press, New York, pp. 265–286 (1975).
  2. M. Alongi & Christoffersen, P. Benthic infauna and organism–sediment relations in a shallow, tropical coastal area: influence of outwelled mangrove detritus and physical disturbance. Marine Ecol. Prog. Ser., 81, pp. 229–245 (1992).
  3. Dutta, M. K., Chowdhury, C., Jana, T. K. & Mukhopadhyay, S. K. Dynamics and exchange fluxes of methane in the estuarine mangrove environment of the Sundarbans, NE coast of India.  Environ. 77, 631–639 (2013).

Sundarban mangrove forest

Sundarbans is the world’s largest contiguous mangrove forest and also is world’s largest tiger reserve. It spreads over an area of about 1000 sq. km in southwest Bangladesh and southeastern portion of the state of West Bengal in India. The forest is located  in the estuary of the Ganges River delta.  Major part of the forest lies in Bangladesh with an area of 6017 km2  that consists of forest, wildlife sanctuaries, sand bars, rivers, creeks and canals (Siddiqi, 1994).


Administrative boundary and Physiography

The Sundarbans forest constitutes about 51% of the forest area of Bangladesh. It spreads over the districts of Khulna, Bagerhat and Satkhira. The total land area of Sundarbans is 401,600 hectares of which 395,500 hectares are non-forested area. The area covered by rivers and khals (or canals) is 175,600 hectares (Katebi, 2001). Its physiography consist of a large number of fluvial and tidal landscapes, features created by the Ganges, Brahmaputra, and the Meghna Rivers.

Administrative zones (ranges) in Sundarbans, BangladeshAdministrative zones (ranges) in Sundarbans, Bangladesh


Katebi, M.N.A. 2001. Sundarbans and Forestry , In: Haider (ed.), Cyclone ´ 91– an environmental and Perceptional study, BACS, Dhaka, pp 79-100.
Siddiqi, N.A., (ed.) 1994. The Importance of Mangroves to the People in the Coastal Areas of Bangladesh, Proceedings of VII Pacific Science International Congress, International Society for Mangrove Ecosystems, Tokyo, Japan.



Global distribution of Mangroves

Mangroves consist of trees, shrubs, and Nypa palm that grow in tropical and subtropical tidal zones—the largest concentration straddling the equator between 5ºN and S—and they have adapted to extremely harsh environmental conditions, thriving in regions of high salinity, scorching temperatures, extreme tides and flourishing in muddy, sediment-laden waters with oxygen-free soils.

  • A total of 15.2 million hectares of mangroves are exist worldwide. Mangroves dominate the majority of the world’s tropical and subtropical coastline accounting for 0.7% of the tropical forest area (Giri et al. 2010).
  • About one-third of the world’s mangroves are found in Asia (42%), followed by Africa (21%) and North and Central America (15%) (FAO, 2007), Oceania (12%) and South America (11%).
  • While the world’s remaining mangrove forests are spread across 118 countries and territories, three-quarters of the mangroves reside in just 15 countries and less than seven percent are protected.
  • The world map below shows where mangrove forests are located.

Global distribution of mangroves , largely lying between latitudes 5ºN and 5ºS (Giri et al. 2010)

Global distribution of mangroves , largely lying between latitudes 5ºN and 5ºS (Giri et al. 2010)
Global distribution of mangroves

An international assessment on global mangroves extent confirms that the vast majority of mangroves exist between 30º N and S (with a few exceptions just outside of this range in Japan, Bermuda, Australia, New Zealand, and South Africa). Mannrove area decreases with increasing latitude except for Sundarbans—the world’s largest tract of mangroves located in the Ganges delta (20–25º N)— which remarkably has remained undiminished despite the world’s highest population density in its immediate vicinity.

Although World Resource Institute observes a lower loss of mangrove canopy in Sundarbans compared to accelerated deforestation in Asia where mangrove loss rate was the highest of all regions, development and expansion of economic zones right into the forest in Bangladesh is deemed to reverse the situation. As the threat lurks on the fate of the forest,  lack of background data is already posing serious problem for the scientific  community to make future projection emphasizing on interdisciplinary scientific research.




This blog will be a compilation of what I learn about mangroves from recent researches. The name tells you that the blog will focus on sundarban mangroves in south-east Asia. To keep it simple at the beginning, I will try to bring together scientific knowledge we have on mangroves in general as easily as possible.

Happy mangroving!

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