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 northe rn Bay of Bengal using shipboard data collected during three seasons as wel l as ocean colour imagery from OCTS and SeaWiFS. Seasonal changes in the hy drography of the bay observed during these seasons gave rise to striking di fferences in biomass and primary productivity. Heavy fresh water influx fro m rivers and the resulting vertical stratification impeded vertical transfe r of nutrients. Although such a nutrient regime resulted in a oligotrophic environment, chlorophyll a and primary production were substantially enhanc ed by physical processes that could erode the strong halocline. In March-Ap ril (pre-southwest monsoon), the poleward flowing East India Coastal Curren t brought to the surface, nutrient laden cooler waters that enriched the co astal 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 co ast and in the region between 13 degrees and 16 degrees N lat. Its appearan ce in satellite images of two consecutive years suggests the structure to b e an annual feature. Wind driven coastal upwelling and increased river runo ff during the following season, the southwest monsoon (July-August), increa sed phytoplankton biomass dramatically (92 mg m(-2)) but productivity avera ged only 0.3 g C m(-2) d(-1) suggesting light limitation due to intense clo ud cover. With a reduction in cloud cover and enhanced irradiance during th e following season, the northeast monsoon (January-February), primary produ ction increased especially in the northern part of the bay where phytoplank ton appeared to benefit from both improved light conditions and nutrient in puts from estuarine mechanisms and river runoff. (C) 2000 Elsevier Science Ltd. All rights reserved.