LAUSR

Abstract The westward propagating Rossby waves and associated currents transport low salinity nutrient-rich water mass from the Bay of Bengal into the southeastern Arabian Sea (SEAS) during winter monsoon, and ensuing changes in physicochemical properties of water column regulate phytoplankton distribution in the latter region. Herein, we elucidate the influence of Rossby wave propagation on picophytoplankton community composition in the SEAS. We compare the picophytoplankton community structure between the Rossby front (low saline, high nitrate concentration and strong current) and the surrounding non-frontal regions (high saline, low nitrate and weak current) along a cruise track in the SEAS during early winter monsoon (November 2016). The higher abundance of phycoerythrin-rich Synechococcus (‘dim’ and ‘bright’ types) and lower abundance of Prochlorococcus in the frontal region than the non-frontal regions suggest that salinity and nitrate concentration in the water column significantly influence the picophytoplankton community dynamics. We argue that the advection of low saline water mass from the Bay of Bengal associated with Rossby wave propagation introduces a ‘dim’ phycoerythrin-rich Synechococcus group in the frontal region, and the ensuing physicochemical changes in water column favor their growth, whereas low cell abundance of Prochlorococcus and Phycocyanin-rich Synechococcus suggests restricted growth of both the groups. These inferences indicate that Rossby wave associated circulation pattern and physicochemical changes in the water column regulate picophytoplankton community composition and its contribution to food web dynamics in the SEAS.