Response of phytoplankton biomass to nutrient stoichiometry in coastal waters of the western Bay of Bengal

Abstract

The hydro-biological parameters such as thermohaline structure, nutrients, and chlorophyll-a (chl-a) during pre-southwest monsoon (PRSWM) and post-southwest monsoon (POSWM) periods of 2017, was investigated and compared along a coastal transect in the western Bay of Bengal (BoB). The impact of cold-core and warm-core eddies on chl-a in the very near coastal waters of the western BoB was analyzed. During PRSWM period, the water column was recharged with phytoplankton growth-promoting nutrients entrained by cold-core eddies that resulted in an increment in chl-a concentration. A three-times increase in chl-a concentration was observed due to the cold-core eddy, whereas the presence of warm-core eddy reduced the chl-a concentration by 87%. However, during POSWM period, despite having higher nitrate concentration and adequate light in the water column, the surface chl-a was substantially lesser than PRSWM. The molar ratio of ambient inorganic macronutrients during POSWM period played a significant role resulting in lower chl-a concentration. Nitrogen to phosphate (N:P) and nitrogen to silicate (N:Si) ratios were less than the Redfield values. The chl-a concentration off Godavari, a biological hotspot, was the lowest during POSWM period. This study highlights phosphate limitation off Godavari during the high river discharge period.