Distributions and sea-to-air fluxes of nitrous oxide in the coastal and shelf waters of the northwestern South China Sea

Abstract

Nitrous oxide (N2O) in the coastal and shelf waters of the northwestern South China Sea was determined during five campaigns carried out from 2006 to 2009. N2O distributions showed temporal fluctuations in both concentration and saturation. In the coastal waters along the eastern Hainan Island, highest mean surface N2O concentrations were observed in winter (10.44 ± 1.45 nM, 140 ± 20% saturation) and the lowest values (6.28 ± 0.47 nM, 110 ± 8.1% saturation) were observed in summer after a strong typhoon. In the shelf region of the northwestern South China Sea, N2O concentrations varied from 5.51 to 7.74 nM, with the saturation of 99%–123% in August 2008. N2O distributions in the coastal and shelf waters of South China Sea were influenced by complex interactions of several processes, including land source input, air-sea exchanges, in situ biological production and physical mixing. N2O in the intermediate waters of the shelf region of the South China Sea was formed mainly through nitrification. N2O yield from nitrification was estimated to be 0.065% based on correlations of excess N2O versus apparent oxygen utilization and NO3- concentrations. Average sea-to-air N2O fluxes from the coastal waters were estimated to be 3.2 ± 3.5 and 5.6 ± 6.1 μmol m−2 d−1 using the LM86 and W92 equation, and sea-to-air N2O fluxes from the shelf region were 1.1 ± 1.0 and 1.7 ± 1.6 μmol m−2 d−1 using the LM86 and W92 relationships, respectively. The coastal and shelf waters of the northwestern South China Sea behaved as a net source of N2O to the atmosphere, and N2O emission can be enhanced by either typhoon related upward mixing or strong air-sea exchange.