Nitrous oxide in the central Bay of Bengal during the summer monsoon

Abstract

Although oceans contribute significantly to the global nitrous oxide (N2O) emissions, there are less than desired data related to N2O concentrations and fluxes in the global oceans and, in particular, the Indian Ocean. Within the Indian Ocean, the quantification and understanding of N2O dynamics in the open Bay of Bengal (BOB) is lacking, specifically during the southwest monsoon. The present study focused on quantifying the dissolved N2O concentrations in the water column along with water–atmosphere fluxes of N2O from the central Bay of Bengal during peak monsoon. For this purpose, a study was conducted during July–August 2018, where dissolved N2O concentrations were measured from the surface to 2000 m depth at eight stations. The concentrations of dissolved N2O in the surface waters ranged from 4.93 to 6.33 nM with saturation levels varying from 81 to 108%. Dissolved N2O showed a small variation with depth and appeared to be undersaturated at all depths except at the surface and at the depth of deep chlorophyll maximum. Flux calculations revealed that out of eight, four stations acted as a minor source, whereas three were minor sink for N2O. The low concentrations of surface N2O and small vertical gradients of N2O in the water column suggested either low production of N2O or significant consumption in the water column. However, the lack of comprehensive complementary data limits us to conclude the exact interplay of the mechanism leading to such observation. Comparison of the present data with available studies in the coastal BOB suggested significantly lower concentrations and fluxes in the open BOB. However, the range reported during the present study is closer to the only study carried out in the central BOB around twenty years ago; suggesting that N2O dynamics in the BOB have not changed drastically in recent times.