The long-term distribution of in situ optically active substances (OAS), accuracy assessment of satellite retrieved chlorophyll-a (chl-a) and its long-term trend has been carried out at a coastal site of the north-western Bay of Bengal. The temporal distribution of chl-a, total suspended matter (TSM) and absorption due to coloured dissolved organic matter at 440 nm (aCDOM440) discerned a common peak during southwest monsoon season (August-October). Chl-a also showed a prominent peak during pre-southwest monsoon period (March-April). The spatial variability of TSM and aCDOM440 was maximum during southwest monsoon, whereas in the case of chl-a, it was during pre-southwest monsoon. The accuracy assessment of chl-a retrieved from Moderate Resolution Imaging Spectroradiometer-Aqua (MODISA), Ocean Colour Monitor-2 (OCM-2) and Visible Infrared Imager Radiometer Suite (VIIRS) showed overestimation in nearshore waters. The error in satellite measurement of chl-a was within the range of 33 to 51%. The chl-a retrieved from MODISA was most accurate as indicated by statistical analysis. The long-term trend in satellite chl-a clearly indicated bi-modal distribution with a primary peak during pre-southwest monsoon attributed to recurrent phytoplankton bloom that was mostly confined to nearshore waters. Whereas, the secondary peak in chl-a, during the end of southwest monsoon, spreads far offshore.
Keywords: Bay of Bengal; Bloom; Chlorophyll-a; MODISA; Monsoon.