Dynamics of nitrous oxide and methane in the southeastern Arabian Sea

Abstract

This study examined the factors controlling the intra- and inter-seasonal variations of dissolved methane (CH4) and nitrous oxide (N2O) in the southeastern Arabian Sea (SEAS). Time-series measurements of CH4, N2O and allied biogeochemical parameters were carried out during the monthly campaigns in the coastal waters and a seasonal campaign in the shelf waters of the SEAS. The southwest monsoon period (SWM) brought drastic changes in the regional hydrography through the incursion of hypoxic waters due to coastal upwelling, which increased N2O concentrations substantially but reduced CH4 levels. The ranges of N2O and CH4 during the upwelling period were 8–89 nM and 9–165 nM, respectively, and the non-upwelling period was 2–27 nM and 5–271 nM, respectively. The significant positive correlations of N2O with apparent oxygen utilisation (AOU), the sum of dissolved nitrate and nitrite (NO2− + NO3−) and excess N2O (ΔN2O), as well as a negative correlation with dissolved oxygen indicates that nitrification is the major process in this region during the non-upwelling period. In contrast, during SWM, N2O did not correlate with NO2− + NO3−; however, it exhibited a significant negative correlation with dissolved nitrite (under hypoxia), suggesting the possibility of nitrifier-denitrification as an active process during hypoxia. The high (low) levels of CH4 recorded during the oxic spring inter-monsoon (hypoxic during the SWM) period showed a direct dependency on the changes in the benthic community. The high abundance of the adult macrofauna and active bioturbation resulted in high sedimentary CH4 release, which led to enhanced water column CH4 concentrations (17–271 nM) during the spring inter-monsoon period. In addition, the breakdown of methylated organic compounds under nutrient-limited conditions may also support the elevated CH4 levels in surface waters. A low macrofaunal abundance and reduced bioturbation led to a considerable reduction of subsurface CH4 concentrations during hypoxia. Overall, the SEAS is found to be a net source of CH4 and N2O to the atmosphere, with its sea-to-air fluxes ranging from 1.7 to 85.8 μM m−2 d−1 (19.88 ± 22.20 μM m−2 d−1) for N2O and 4–756 μM m−2 d−1(133 ± 158 μM m−2 d−1) for CH4.