Distribution and air–sea exchange of nitrous oxide in the coastal Bay of Bengal during peak discharge period (southwest monsoon)

Abstract

In order to examine the impact of river discharge from the Indian subcontinent on the concentration and air–sea exchange of Nitrous Oxide (N₂O) a study was conducted during peak discharge period in the coastal Bay of Bengal, The study revealed that freshwater discharge exerts a dominant control on the N₂O cycling in the surface waters of the coastal Bay of Bengal. The surface concentration of N₂O in the Southwestern (SW) coastal Bay of Bengal was high (7.4 ± 1.6 nM) and supersaturated (126 ± 27%) whereas contrasting trend was found in the Northwestern (NW) region (4.9 ± 0.3 nM and 81 ± 6%). Such spatial differences in N₂O concentration and saturation were resulted from variable characteristics of the discharged waters and vertical stratification. The NW region of the coastal Bay of Bengal was under the influence of the discharge from the Ganges River having N₂O below the saturation in the estuary (82 ± 5%) while the SW region was under the influence of peninsular river discharges that were super-saturated (187 ± 29%). The low N₂O concentration at NW region resulted from low concentrations in the source water (Ganges) as these waters were formed by melting of the Himalayan glacier where low ammonium concentrations were observed due to less human settlement resulting in lower nitrification rates. Higher concentration of N₂O in the SW region was attributed to the discharge from monsoonal rivers containing high N₂O concentrations, high nitrification rates and mild coastal upwelling. The sea-to-air fluxes of N₂O suggest that NW region is a sink for atmospheric N₂O due to discharge of under saturated water from Ganges and strong stratification while SW region is a source caused by coastal upwelling and discharge of highly saturated water from monsoonal rivers.