Dynamics of dissolved organic carbon in the northern Indian Ocean

Oceanic Dissolved Organic Carbon (DOC) represents one of the largest carbon reservoirs on Earth, and its distribution and biogeochemical cycles play important roles in carbon cycling and other biogeochemical processes in the ocean. The DOC concentrations were determined in the vertical profiles of seawater samples collected from the Arabian Sea and the Bay of Bengal. Unusual DOC concentration gradients in the water column of the northern Indian Ocean basin were noticed, which is generally not observed in other oceanic basins. High DOC observed in both the basins of the northern Indian Ocean (NIO) ranged from 75-100 µM. The Bay of Bengal has subsurface high DOC compared to surface water. The Bay of Bengal receives huge amount of fresh water flux and sediment, whereas the Arabian Sea experiences very high biological productivity due to seasonally reversing monsoon systems. This can be attributed to the release of DOC from the enormous flux of (0.5 × 10¹² mol C yr ^-1) particulate organic carbon (POC) from the Ganga-Brahmaputra river system. In the Arabian Sea, surface DOC concentration decreases gradually while going from north to south, which is similar with the primary productivity of the region. While deep water in this both the basins have high DOC content (>45 µM) as the deep waters in these basins are from polar regions, which are supposed to be very old and different in DOC content. The oxygen consumption related to DOC remineralisation evaluated by the relation between Apparent Oxygen Utilisation (AOU) and DOC has been calculated to be ~18 % in the Bay of Bengal.