Time Series Observation on Phytoplankton Dynamics in the Coastal Waters of the World Heritage Site of Indian Sundarban Mangrove Forest, NE Coast of Bay of Bengal

Sea levels on the Indian sub-continent are increasing at the rate of about 2.5 mm/year; the rate of increment is greater in the eastern coast, with an estimated sea level rise of about 3.14 mm/year. This suggests that mean annual sea levels in the Indian sub-continent will be some 15 cm higher in 2060 than what it was during 2000. The Indian Sundarbans in northeast coast of the country, at the apex of the Bay of Bengal is an extremely dynamic deltaic lobe sustaining a wide spectrum of mangrove flora and fauna. It is estimated that sea level in this deltaic lobe has increased by about 15 cm since the 1950s and this has been correlated with changes in the pattern and rates of erosion and accretion in the islands of the Indian Sundarbans. Such geo-physical phenomena may not only pose serious impact on the adjacent aquatic system by way of increasing turbidity, nutrient budget, salinity, pH etc., but the phenomenon has every possibility to shift the biodiversity spectrum of the adjacent land masses (supporting mangrove and mangrove associate species) and aquatic system due to salinization of land, alteration of soil pH, increased erosional activities leading to reduction of water transparency, increased salinity of the water bodies and invasion of the areas with more number of stenohaline species (preferably phytoplankton). The present paper is an attempt to scan the time series observation on phytoplankton dynamics in the coastal waters of Indian Sundarban mangrove forest, NE coast of Bay of Bengal. Twenty four stations have been selected in the present programme at different salinity gradients to evaluate the temporal variations of selective hydrological parameters like surface water temperature, salinity, pH, dissolved oxygen, transparency and nutrient load. Simultaneous data on phytoplankton diversity was also assessed to critically analyze the impact of temporal oscillation of hydrological parameters on the tiny, free floating, drifting, primary producer community. Duncan test applied on the data set of 25 years (1990 to 2015) revealed significant temporal variations of surface water temperature, salinity, pH, transparency, nitrate concentration, phosphate concentration and phytoplankton composition. Since 1990, nine stenohaline phytoplankton species have been recorded in the upstream regions of the deltaic lobe indicating a gradual shifting of aquatic phase towards high salinity. Few environmental variables like dissolved oxygen and silicate did not show variations at significant level. Although the time span is too short to predict potential impact of aquatic climate change on phytoplankton community of the mangrove dominated Indian Sundarbans, but significant temporal variation in the phytoplankton community since 1990 speaks in favour of using these tiny, free floating, drifting, primary producer community as potential bioindicators of aquatic climate change in short term scale.