SEGH Articles

# The Indian Sundarban Mangrove wetland: an ecological perspective

01 February 2012
Dr Sarkar has carried out research on the changes in the ecological and pollution status of the Indian Sundarban mangrove wetlands, as a result of intense anthropological pressures affecting the biotic and abiotic compartments of this fragile ecosystem.

Dr Santosh Sarkar is a professor at the Department of Marine Science, University of Calcutta in India. Over the past 30 years he has carried out research on the changes in the ecological and pollution status of the Sundarban mangrove wetland, as a result of intense anthropological pressures affecting the biotic and abiotic compartments of this fragile ecosystem.

The Indian Sundarban is located in North East India, in close proximity to Kolkatta. It is the largest delta in the estuarine phase of the river Ganges, and is situated in the low-lying, meso-macrotidal, humid and tropical belt at the estuarine phase of the Ganges River and Bay of Bengal. The Sundarban hosts the world's largest mangrove forest together with associated flora and fauna.

There have been remarkable ecological changes due to multiple human activities. such as; excavation of sand from the sand dunes, dredging and intensive boating, deforestation, collection of prawn seed, immersion of idols in the river etc., thus affecting sediment and water quality as well as biodiversity.

Dr Sarkar has quantified the adverse impacts due to collection of tiger prawn seeds for aquacultural farms and molluscan shells for poultry feed and edible lime. Indiscriminate exploitation of these resources leads to a heavy reduction of the species concerned and other associated marine communities. The impacts of biodiversity loss and their after-effects on the ecobalance of the coastal system have become a matter of great concern to ecologists to maintain security and sustainability.

Collection of  tiger prawn seed (Penaeus monodon) from Sundarban coastal regions

Dr Sarkar first worked on the occurrence, distribution and sources of several persistent organic pollutants (POPs) (PAHs, PCBs, DDTs, PBDEs, HCHs, HCB) measured in  sediments from the Sundarban wetlands, obtaining a dataset with more than 2000 analyses. The POPs belong to a group of pollutants that are semivolatile, toxic and  bioaccumulative in nature and resist photolytic, chemical and biological degradation. The concentration of four isomers of  HCH exhibit a heterogenic distribution. Among the isomers and metabolites of HCH, DDT and PCB, alpha-HCH, pp ′-DDT and PCB 101, PCB 118,  PCB 153 and PCB 138  were found to be dominant. High ratios of metabolites of DDT to ∑ DDTs reveal recent use of DDT in this coastal environment. PBDE, an important group of brominated flame retardants (BFR), showed moderate to low contamination closely in uniformity to other Asian aquatic environments. The PAH diagnostic ratios indicated that the PAHs in sediment were of pyrolytic origin, contaminated by local vehicle combustion, biomass burning and domestic an industrial coal combustion.

Dr Sarkar performed the first screening ecotoxicological risk evaluation of the persistent organic pollutants (POPs) in the Sundarban wetland. The pollutant effects were assessed by the use of three different sediment quality guidelines (SQGs) previously developed in the literature to evaluate toxicity induced in sediment-dwelling organisms. The three different approaches chosen for risk assessment of the Sundarban were: (1) the consensus SQGs obtained by TEC (threshold effect concentration); (2) PEC (probable effect concentration; and (3) EEC (extreme effect concentration), the threshold/ probable effect level (TEL/PEL) approach and, finally, the ERL-ERM guidelines, including the m-ERM-Q (mean ERM quotient). The evaluation of the toxicity induced by a mixture of the target pollutants indicated the importance of gama-HCH contamination in the Sundarban sediments despite the very low concentrations measured in sediments. A different sensitivity for toxicity assessment due to quality guidelines was obtained, as the consensus SQGs based on TEC were less conservative and protective than the TEL and ERL approaches, while the use of m-ERM-Q seems to be the most powerful tool to predict the toxicity related to a contaminant mixture.

Collaborative research work with Michael Watts of the British Geological Survey, provided initial findings for arsenic speciation in four soft-bottom benthic polychaetes (Perenereis cultifera, Ganganereis sootai, Lumbrinereis notocirrata and Dendronereis arborifera) along with host sediments from the Sundarban mangrove wetland.  Arsenic concentrations in polychaete body tissues varied greatly, suggesting species-specific characteristics and inherent peculiarities in arsenic metabolism. Arsenic was generally present in polychaetes as arsenate (AsV) or arsenite (AsIII) (30 to 53 % as inorganic As) and dimethylarsinic acid (DMAV; <1 to 25 %). Arsenobetaine (AB; <16%), and PO4-arsenoriboside (8 to 48%) were also detected as minor constituents, whilst monomethylarsonic acid (MAV) was not detected in any of the polychaetes. The highest total As (14.7 mg kg-1 dry wt) was observed in the polychaete D. arborifera collected from the vicinity of a sewage outfall in which the majority of As was present as an uncharacterized compound (10.3 mg kg-1 dry wt) eluted prior to AB. Host sediments ranged from 2.5 to 10.4 mg kg-1 total As. This work supports the importance of speciation analysis of As, because of the ubiquitous occurrence of this metalloid in the environment, and its variable toxicity depending on chemical form. Follow up work is being carried out on further samples collected with the support of Royal Society funding.  This will enable consideration of a range of polychaete species in terms of diverse habitat and food preferences to assess the arsenic uptake pathways and to determine the influence of ecological factors on total As concentrations and species proportion in this wetland ecosystem.

Gradually a full picture of the growing impact of human activity on the pristine environment of the Sundarban mangrove wetlands is being developed.  This is gradually being achieved through multiple international collaborations and will provide vital information for the planning and use of land and waterways in the wetlands.

Professor Santosh Sarkar, Department of Marine Science, University of Kolkatta, Indiasarkar.santosh@gmail.com

References

Biogeochemistry of mercury and methylmercury in sediment cores from Sundarban mangrove wetland, India - a UNESCO World Heritage Site - Mousumi Chatterjee, João Canário, Santosh Kumar Sarkar, Vasco Branco, Nallamuthu Godhantaraman, Bhaskar Deb Bhattacharya, Asokkumar Bhattacharya  - Environmental Monitoring and Assessment, Springer. DOI:10.1007/s10661-011-2336-8 2011. http://www.springerlink.com/content/v16047jhk2027416/

Quantification and source identification of polycyclic aromatic hydrocarbons in core sediments from Sundarban Mangrove Wetland, India  - C. Domínguez; S. K. Sarkar, A Bhattacharya, M Chatterjee, B D Bhattacharya, E Jover,  J Albaigés, J M Bayona, Md. A Alam and K K Satpathy. Archives of Environmental Contamination and Toxicology, Springer Publishers, 59(1): 49 - 61, 2010.           http://www.springerlink.com/content/l32803q28210256t/

Metal concentrations in water and sediments from tourist beaches of   Acapulco, Mexico  - M.P. Jonathan, P.D. Roy, N. Thangadurai, S. Srinivasalu, P.F. Rodríguez Espinosa, S.K.Sarkar, C. Lakshumanan, M. Navarrete-López and N.P. Muñoz-Sevilla - Marine Pollution Bulletin, Springer, 62, 845-850, 2011. http://www.sciencedirect.com/science/article/pii/S0025326X11001135

Baseline of organotin contamination in sediments of Sunderban mangrove wetland and adjacent coastal regions, India - P . Anderson, S K Sarkar, B D Bhattacharya, M Chatterjee, K K Satpathy, T Peshkur and B Antizar-Ladislao. - Ecotoxicology,Springer, 20 (8), 1975-1983, 2011. DOI 10.1007/s10646-011-0739-5. http://www.springerlink.com/content/52j87u2658171821/

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