3.1.3. Chlorophyll-a
Pre-Monsoon (March-May) and Northeast Monsoon (November - February) are low productive periods in the SEAS along the southwest coast of India (Supplementary Figure 3). This situation is caused by surface layer stratification through solar heating during the Pre-Monsoon and the intrusion of low saline oceanic Bay of Bengal waters during the Northeast Monsoon22. Both these cause meagre nutrient availability in the upper euphotic column creating low phytoplankton biomass (chlorophyll-a) concentration22. The situation significantly changes by late-May/June as a result of coastal upwelling, and enhanced chlorophyll-a concentration starts appearing along the entire coastline by June and persists up to October. GoM has always moderate to the high level of chlorophyll-a present in the nearshore waters along the Indian coast. Enhancement of chlorophyll-a in the GoM starts by late-May, just after the Pre-Monsoon, and it peaks during July-August and then declines by September-October. During the Northeast Monsoon, though the chlorophyll-a concentration decreases in the offshore waters in the GoM, its moderate concentration persists in the shelf waters along the Indian coastline as a continuity of the water masses intruded from the Bay of Bengal through the Palk Bay.