Shift in the phytoplankton composition associated with reversal in east India coastal currents in the Bay of Bengal

The alteration of phytoplankton composition due to variations in physical mixing, salinity, nutrient concentrations and their stoichiometric ratios would impact the overall ecosystem, affect the food web and influence the carbon cycle. The seasonal reversal in the East Indian Coastal Currents (EICC) significantly modifies stratification, physical mixing and nutrient levels in the coastal waters. To examine the impact of reversal in EICC on nutrient stoichiometry and phytoplankton composition, coastal waters were measured at a monthly time scale in the inshore and offshore regions of the Bay of Bengal for one year. The EICC flows northward between February and September, bringing saline waters along the coast, associated with winds from the south-west direction, promotes coastal upwelling. Elevated concentrations of nutrients (nitrate, phosphate and silicate) and phytoplankton biomass were observed associated with coastal upwelling dominated by fucoxanthin (Bacillariophyceae), and chlorophyll-b (Chl-b; green algae). CHEMTAX analysis suggests that diatoms contributed significantly (60–70 %) during the upwelling period, with up to 90 % of the contribution coming from microplankton in both coastal and offshore regions. The EICC reverses its flow towards the south between October and January, bringing low saline, nutrient-poor waters along the coast, resulting in a decrease in Chlorophyll-a (Chl-a). During this period, the coastal waters were dominated by zeaxanthin (cyanobacteria). Despite nutrients not being limiting, picoplankton dominance during this period is caused by the transport of cyanobacteria from the river to the coastal region due to their abundance in the river water. The CHEMTAX analysis suggested that diatoms, prochlorophytes, and cyanobacteria contributed significantly in the inshore region, whereas chlorophytes and cyanobacteria contributed to the offshore region. Picoplankton (>50 %) contributed significantly, followed by micro (30 %) and nanoplankton (∼20 %). Fucoxanthin and Chl-b displayed a significant linear relationship with salinity, whereas zeaxanthin displayed an inverse relationship with salinity. The fraction of microplankton linearly correlated with salinity, whereas nano- and picoplankton inversely correlated. This study highlights the effect of temporal variabilities in salinity associated with reversing the EICC modified the phytoplankton composition, and it may have a significant impact on the coastal ecosystem. Understanding intra- and interannual variability in the EICC and associated salinity changes may allow for the prediction of phytoplankton composition in the coastal Bay of Bengal.