Nitrous Oxide in the Central Bay of Bengal During Summer Monsoon

Abstract:

Nitrogen biogeochemistry of the Bay of Bengal is susceptible to anthropogenic inputs derived through riverine inputs and atmospheric deposition. The spatial and temporal extent of these inputs on N biogeochemistry, particularly on nitrous oxide (N₂O), is still under investigation. In this regard, vertical distribution of dissolved N₂O along with nutrients concentration were measured from surface to 2000 m at 8 locations between 7°N and 16°N in the central Bay of Bengal onboard ORV SagarNidhi during summer monsoon (July-August) 2018. The concentrations of dissolved N₂O in surface waters ranged from 4.96 to 6.63 nM with saturation level ranging from 81 to 108%. The vertical profiles of dissolved N₂O showed small variation with depth and remained undersaturated throughout except for slight supersaturation at the surface and mixed layer depths. The dissolved N₂O concentrations in oxic (surface) and oxygen depleted waters were significantly lower than previously reported for the coastal Bay of Bengal and the Arabian Sea (up to 1200 % saturation). Negative correlations of excess N₂O (ΔN₂O) with nitrate and ΔN₂O with oxygen in oxygen depleted waters suggested the likelihood of N₂O undersaturated due to denitrification. Out of eight sampled, four locations acted as a minor source (average flux rate = 2.12 ± 1.50 μmol m^-2d^-1), whereas others were a minor sink (average flux rate = -3.63 ± 3.38 μmol m^-2d^-1) of N₂O in the central Bay of Bengal during monsoon