Influence of physical processes and freshwater discharge on the seasonality of phytoplankton regime in the Bay of Bengal

We have examined the seasonality of phytoplankton in the western and northern Bay of Bengal using shipboard data collected during three seasons as well as ocean colour imagery from OCTS and SeaWiFS. Seasonal changes in the hydrography of the bay observed during these seasons gave rise to striking differences in biomass and primary productivity. Heavy fresh water influx from rivers and the resulting vertical stratification impeded vertical transfer of nutrients. Although such a nutrient regime resulted in an oligotrophic environment, chlorophyll a and primary production were substantially enhanced by physical processes that could erode the strong halocline. In March–April (pre-southwest monsoon), the poleward flowing East India Coastal Current brought to the surface, nutrient laden cooler waters that enriched the coastal region, but highest biomass (Chl a, 53 mg m−2) and productivity (4.5 g C m−2 d−1) were in the region of an eddylike structure along the coast and in the region between 13° and 16°N lat. Its appearance in satellite images of two consecutive years suggests the structure to be an annual feature. Wind driven coastal upwelling and increased river runoff during the following season, the southwest monsoon (July–August), increased phytoplankton biomass dramatically (92 mg m−2) but productivity averaged only 0.3 g C m−2 d−1 suggesting light limitation due to intense cloud cover. With a reduction in cloud cover and enhanced irradiance during the following season, the northeast monsoon (January–February), primary production increased especially in the northern part of the bay where phytoplankton appeared to benefit from both improved light conditions and nutrient inputs from estuarine mechanisms and river runoff.