Dynamics of Sundarban estuarine ecosystem: eutrophication induced threat to mangroves

Abstract

BackgroundSundarbans is the largest chunk of mangrove forest and only tiger mangrove land in the world. Compared to the rich species diversity and uniqueness, very few studies have so far been conducted here, mainly due to its inaccessibility. This study explores water quality, density of biomass, species diversity, phytoplankton abundance and bacterial population of a tidal creek in Sunderban estuary during the post and pre monsoon period of 2008-09.ResultsPhytoplankton community was observed to be dominated by diatoms (Biacillariophyceae) followed by Pyrrophyceae (Dinoflagellates) and Chlorophyceae. A total of 46 taxa belonging to 6 groups were recorded. Other algal groups were Cyanophyceae, Euglenophyceae and Chrysophyceae. Species diversity was highest in summer (March) and lowest in winter season (November) in all the sample stations indicating its close correlation with ambient temperature. Species evenness was fairly high in all five stations throughout the study period. Present study indicated that dissolved oxygen, nutrients and turbidity are the limiting factors for the phytoplankton biomass. The estuary was in eutrophic condition (Chlorophyll-a ≥10 μg/L) in winter. During the month of May phytoplankton biomass declined and at high salinity level (21.2PSU) new phytoplankton species take over, which are definitely better resilient to the high saline environment. Bio-indicator species like Polykrikos schwartzil, Dinophysis norvegica and Prorocentrum concavum points to moderately polluted water quality of the estuary.ConclusionEutrophication as well as presence of toxic Dinoflagellates and Cyanophyceae in the tidal creek of Sundarban estuary definitely revealed the deteriorated status of the water quality. The structure and function of the mangrove food web is unique, driven by both marine and terrestrial components. But little attention has been paid so far to the adaptive responses of mangrove biota to the various disturbances, and now our work unfolds the fact that marine status of Sundarban estuary is highly threatened which in turn will affect the ecology of the mangrove. This study indicates that ecosystem dynamics of the world heritage site Sundarban may facilitate bioinvasion putting a question mark on the sustainability of mangroves.

Interplay of Physical, Chemical and Biological Components in Estuarine Ecosystem with Special Reference to Sundarbans, India

“…Noticeable variation of phytoplankton forms was also observed in seasons and sampling locations due to variations of water quality parameters, like pH, salinity, TSS and nutrients and DO. Phytoplankton community was observed to be dominated by diatoms (Biacillariophyceae) followed by Pyrrophyceae (Dinoflagellates) and Chlorophyceae and higest abundance was noticed in postmonsoon (Biswas et al 2010;Manna et al 2010) Centric Diatoms predominated in winter months and Pennates in summer whereas Chlorophyceae, Cyanohyceae and Euglenoids dominated the estuary in monsoon. During premonsoon the dominant phytoplankton were species of Ditylum, Ceratium, Biddulphia, Chaetoceros, Coscinodiscus, Thalassiothrix, Rhizosolenia Nitzschia and Thalassionema.…”

“…However, during postmonsoon phytoplankton species of Bacteriastrum, Biddulphia, Protoperidinium and Ceratium were most dominant (Fig 6a) But in monsoon species of Skeletonema, Fragillaria and some blue green algae, green algae and also euglenoids are quite common. The average phytoplankton load is higher mostly in postmonsoon (Manna et al 2010). The eastern part of the estuary 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 Sundarban estuary bacterial population ranged from 1.62X10 3 -3. 18X10 6 , lowest in January (3.68 × 10 6 CFU L -1 ) and the highest in May (8.9 × 10 8 CFU L -1 ) (Manna et. al., 2010).…”

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“…Manna et al (2010) found positive correlation of chlorophyll-a with the nitrogen, silicate and phosphate in the Indian Sundarbans Estuary. The nutrient level of seawater is a significant factor for increase in the fish catch.…”

“…Only a handful of authors like Shah et al (2008) are known to have conducted researches on phytoplankton and related nutrient distribution in the Bangladesh part, Manna et al (2010) at Indian Sundarbans, Vijayakumaran (2005) at Visakhapatnam, Madhupratap et al (2003) in central and western Bay of Bengal. Nutrient dynamic studies of the Bay of Bengal region has been done by different authors, viz.…”