Distribution and sea-to-air fluxes of nitrous oxide and methane from a seasonally hypoxic coastal zone in the southeastern Arabian Sea.

ABSTRACT

The seasonal variability, pathways, and sea-to-air fluxes of nitrous oxide (N2O) and methane (CH4) in the coastal environment, where coastal upwelling and mudbanks co-exist are presented based on the monthly time-series measurements from November 2021 to December 2022. Upwelling-driven hypoxic water's shoreward propagation and persistence were the major factors controlling the N2O concentrations, while the freshwater influx and sedimentary fluxes modulate CH4 concentrations. The N2O concentrations were high during the southwest monsoon (up to 35 nM; 19 ± 8 nM)), followed by spring inter-monsoon (up to 19 nM; 10 ± 5 nM), and lowest during the northeast monsoon (up to 13 nM; 8 ± 2 nM), whereas the CH4 levels were high during the spring inter-monsoon (8.4 to 65 nM), followed by southwest monsoon (6.8 to 53.1 nM) and relatively lower concentrations during the northeast monsoon (3.3 to 32.6 nM). The positive correlations of excess N2O with Apparent Oxygen Utilisation (AOU) and the sum of nitrate and nitrite (NOx) indicate that nitrification is the primary source of N2O in the mudbank regime. The negative correlation of CH4 concentrations with salinity indicates considerable input of CH4 through freshwater influx. CH4 exhibited a highly significant positive correlation with Chlorophyll-a throughout the study period. Furthermore, it displayed a statistically significant positive correlation with phosphate (PO43-) during the northeast monsoon while a strong negative correlation with PO43- during the spring inter-monsoon, pointing towards the role of aerobic CH4 production pathways in the mudbank regime. N2O and CH4 exhibited a contrasting seasonal pattern of sea-to-air fluxes, characterised by the highest N2O fluxes during the southwest monsoon (hypoxia) (13 ± 10 µM m-2 d-1), followed by spring inter-monsoon (12 ± 16 µM m-2 d-1), and the lowest during the northeast monsoon (0.6 ± 3 µM m-2 d-1). Conversely, the highest sea-to-air fluxes of CH4 were noticed during the spring inter-monsoon (74 ± 56 µM m-2 d-1), followed by the southwest monsoon (45 ± 35 µM m-2 d-1), and the lowest values during the northeast monsoon (19 ± 16 µM m-2d-1). Long-term time-series measurements will be invaluable in understanding the longer-term impacts of climate-driven variability on marine biogeochemical cycles in dynamic nearshore systems.