Significant difference in the vertical distribution of nitrous oxide in the central Bay of Bengal from that in the western area

The distribution of nitrous oxide (N2O) concentrations in the central Bay of Bengal was examined to evaluate this oceanic area, with oxygen-depleted waters in the intermediate layer, as a natural N2O source. The vertical distributions of chemical properties of nitrate (NO-3), nitrite (NO-2), N2O, phosphate (PO3-4), oxygen (O2) and chlorophyll a (chl a) were determined at 12 stations between 7°N and 15°N in the central area of the Bay of Bengal during the two RV Shinyo Maru cruises conducted in February 1995 and February 1996. Although supersaturated N2O concentrations were reported in the western Bay of Bengal (average surface saturation: 125%) and the Arabian Sea (average surface saturation: 186%), we found that N2O concentrations were relatively low in the central Bay of Bengal. The concentrations of N2O in surface water ranged from 6.06 to 7.30 nm, and surface N2O saturation ranged from 102 to 119%, with an average of 113% during February 1996 and 119% during February 1995. Vertical distributions of N2O showed that there was not a large concentration change with depth in the water columns. The saturation levels of N2O in both surface oxic and oxygen-depleted waters were significantly lower (up to 240% during February 1995 and 250% during February 1996) than those found in the western Bay of Bengal and the Arabian Sea (up to 1260%). The strong stratification and associated nutrient depletion in the euphotic zone may cause lower productivity in the central Bay of Bengal. The low concentration of surface N2O and small vertical gradients of N2O in the water column suggested that the production of N2O in the oxygen-depleted layer was low and the emissions of N2O to the atmosphere were small from the central Bay of Bengal in this season.