Decline in C:N:P ratios of particulate organic matter during the fall intermonsoon in the northern Indian Ocean

Recent studies have delineated the stoichiometric (C:N:P) dependency of phytoplankton on environmental and biological factors in the ocean and leveraged these findings in global climate models. However, model projections for low latitude regions remain largely uncertain due to insufficient observations. Here, we have investigated the regional scale stoichiometric variations of particulate organic matter (POM) in the northern Indian Ocean, encompassing the Bay of Bengal and the Arabian Sea, during the fall intermonsoon. The mean C:N:P ratios of POM in the Arabian Sea (126:24:1) and in the Bay of Bengal (148:25:1) were lower than those during the other seasons in the northern Indian Ocean. While N:P and C:P ratios remain invariant, C:N ratio showed variation in the top layer across the basins (surface to 25–60 m in the Arabian Sea and surface to 42–60 m in the Bay of Bengal). We suggest that the Prochlorococcus and Synechococcus control the C:N ratio in the Bay of Bengal, while nutrients and temperature regulate the ratio in the Arabian Sea. Contrary to the expected increase in C:N ratio (8–9) in oligotrophic conditions, the ratio was close to the Redfield ratio (6.6). The decrease in C:N ratio could be attributed to the heterotrophic bacterial (C:N = 3.8–4.5) dominance in the plankton community during the fall intermonsoon. Our analysis might be useful in evaluating the regional importance of temperature, nutrients, and plankton community composition on biogeochemical elemental cycling.