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

摘要

Nitrous oxide (N_2O) in the coastal and shelf waters of the northwestern South China Sea was determined during five campaigns carried out from 2006 to 2009. N_2O distributions showed temporal fluctuations in both concentration and saturation. In the coastal waters along the eastern Hainan Island, highest mean surface N_2O 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, N_2O concentrations varied from 5.51 to 7.74 nM, with the saturation of 99%-123% in August 2008. N_2O 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. N_2O in the intermediate waters of the shelf region of the South China Sea was formed mainly through nitrification. N_2O yield from nitrification was estimated to be 0.065% based on correlations of excess N_2O versus apparent oxygen utilization and NO_3~- concentrations. Average sea-to-air N_2O fluxes from the coastal waters were estimated to be 3.2 ± 3.5 and 5.6 ± 6.1 umol m~(-2) d~(-1) using the LM86 and W92 equation, and sea-to-air N_2O 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 N_2O to the atmosphere, and N_2O emission can be enhanced by either typhoon related upward mixing or strong air-sea exchange.