Accumulation of dissolved organic carbon and nitrogen in the photic zone in the nitrogen-depleted waters of the Bay of Bengal

Abstract

Dissolved organic matter (DOM) accumulates in the upper ocean for several days to years and supports both autotrophic and heterotrophic activities. To examine the source of DOM and its role in plankton metabolic processes, 18 stations were sampled in the Bay of Bengal (BoB) during June 2019. The concentration of DOC in the upper ocean exhibited a significant relationship with Chlorophyll-a (Chl-a), particulate organic carbon (POC) and DOC exudation rates, suggesting possible sources through in situ biological processes. The mean DOC: DON ratio (8.3 ± 2) and isotopic composition of carbon in DOC (delta 13CDOC of -21.2 ±1‰) indicates a marine origin in the BoB during the study period. On the other hand, dissolved organic nitrogen (DON) exhibited a significant weak inverse relationship with salinity in the northern BoB indicating some contribution from the riverine source. Phytoplankton uptake rates of amino acids (glycine or aspartic acid) were 10 times lower than dissolved inorganic nitrogen (DIN). The accumulation time of semi-labile DOC, produced by phytoplankton, in the upper ocean varied between 2.5 and 7.5 months in the BoB with the longer rate in the anticyclonic (ACE; 5.6 ± 1 months) than cyclonic eddy (CE; 3.6 ± 0.5 months) regions. This study suggests that DOM is produced through in situ processes and supports phytoplankton biomass in the BoB than hitherto hypothesized as river-borne sources and refractory in nature.