Net Community Production in the Northern Indian Ocean

The monthly/seasonal variability in net community production (NCP) is controlled by the monsoon either through increasing nutrients to the euphotic zone or an increase in river runoff in the northern Indian Ocean. Net autotrophy was found during the southwest monsoon (SWM) in the Arabian Sea, and heterotrophy was found during the spring and fall intermonsoon periods. Large spatial variability in NCP occurs in the Arabian Sea, with dominant autotrophy in the west and heterotrophy in the eastern Arabian Sea. Differential monsoon forcing in these two regions might cause such variability. In contrast to the Arabian Sea, net heterotrophy is noticed during the SWM in the Bay of Bengal when peak river discharge occurs. The bacterial biomass and production rates were found to be higher during the SWM in the northern Bay of Bengal and strongly suggest that enhanced bacterial respiration could be possible during the SWM, resulting in net heterotrophy. On the basis of the sediment trap data, it was suggested that enhanced river discharge increases particles sinking rates to the deep sea, therefore, removing atmospheric CO 2 . On the contrary, increased river discharge leads to heterotrophy because of enhanced community respiration, resulting in higher surface pCO 2 above the atmospheric levels and thus flux to the atmosphere during the SWM in the Bay of Bengal. The annual mean NCP to the northern Indian Ocean estimated by different models amounts to 300―318 Tg C. If the entire NCP sinks to the aphotic zone, on an annual scale, it is sufficient to support bacterial carbon demand in the deep ocean.