Distinctive phytoplankton size responses to the nutrient enrichment of coastal upwelling and winter convection in the eastern Arabian Sea

Abstract

Even though the seasonal pattern of phytoplankton biomass distribution in the seas around India is reasonably well understood, there is relatively little information on phytoplankton size classes. The current study shows how phytoplankton of different size classes respond to nutrient enrichment caused by vertical convective mixing during the Northeast Monsoon [(NEM) November-February] and coastal upwelling during the Southwest Monsoon [(SWM) June-September] in the Eastern Arabian Sea (EAS). In the Northeastern Arabian Sea (NEAS), phytoplankton biomass [Chlorophyll-a (Chl-a)] and primary production (PP) were moderate and comparable during both the NEM (Chl-a av. 21 mg m-2 and PP av. 601 mg C m-2 d-1) and the SWM (Chl-a av. 27 mg m-2 and PP av. 516 mg C m-2 d-1). The Southeastern Arabian Sea (SEAS) was oligotrophic (Chl-a av. 12 mg m-2 and PP av. 197 mg C m-2 d-1) during the NEM due to strong surface stratification, but due to coastal upwelling, it turned into a very productive system along the shelf waters during the SWM (Chl-a av. 45 mg m-2 and PP av. 1201 mg C m-2 d-1). Interestingly, the subsurface Chl-a maximum (SCM) was almost absent in the NEAS during both seasons. Similarly, SCM was absent in the entire coastal upwelling zone of the EAS during the SWM. In these areas and situations, Chl-a was found to accumulate in the nutrient-rich surface layers of the shallow euphotic zones.