Picophytoplankton Distribution and Their Contribution to Particulate Organic Carbon in the Northern Indian Ocean

Abstract Picophytoplankton are drivers of biogeochemical cycling and serve as key indicators of seafood productivity, ocean health, and climate change. The Arabian Sea and the Bay of Bengal—two basins of the northern Indian Ocean that support the livelihoods of over a billion people—share similar climatic conditions but differ in their hydrographic and ecological characteristics. However, the environmental factors governing picophytoplankton abundance and their implications on carbon biomass in these regions remain poorly understood. This study investigated the abundance, carbon biomass, and environmental relationships of three groups of picophytoplankton: Prochlorococcus , Synechococcus , and picoeukaryotes. We observed Synechococcus dominating the surface waters (47% in the Arabian Sea and 63% in the Bay of Bengal), while Prochlorococcus showed the highest abundance and variability in the water column between basins. Picoeukaryotes contributed more to the total picophytoplankton carbon biomass in both basins. In particular, picophytoplankton carbon biomass contribution to particulate organic carbon were not substantially distinct between the two basins. The estimated picophytoplankton carbon stock was 1.05 Tg C in the Arabian Sea and 0.5 Tg C in the Bay of Bengal. Furthermore, we determined key environmental factors—temperature, salinity, light, dissolved inorganic phosphorus (DIP), and dissolved inorganic silicate (DSi)—that shape picophytoplankton dynamics in the region. Temperature and salinity primarily influenced picophytoplankton distribution in the Arabian Sea, while nutrients were critical in the Bay of Bengal. Overall, we underscore the role of physicochemical dynamics in carbon cycling by presenting the contribution of picophytoplankton carbon biomass to the region's particulate organic carbon pool.