| Dissolved nitrous oxide (N2O) is an important greenhouse gas, playing a significant role in the global climate system. The world ocean is believed to be a net natural source of atmospheric N2O, among which the coastal ocean is responsible for a large part of the oceanic N2O emissions. In this dissertation, we examined the N2O and other related parameters in a world major subtropical maginal sea, the South China Sea (SCS) based on a cruise conducted in summer, between July and September2007. We also examined the spatial distribution and seasonal variations of N2O in a large perturbed estuary, the Pearl River Estuary (PRE), based on two cruises conducted in spring (April2007) and summer (August2008).N2O was analyzed using a purge and trap system at room temperature coupled with gas chromatography. Fourteen depth profiles of N2O were obtained during the cruise in the SCS. All of the depth profiles showed that N2O was supersaturated throughout the water column. The lowest concentrations, with an average of6.5±0.5nmol kg-1, were found in the surface water, while the highest value, up to25.7±0.7nmol kg-l was observed at approximate800m, where the dissolve oxygen (DO) minimum zone was located. It is interesting that in the surface water of a center of cyclonic eddy, we observed enhanced N2O concentrations, which was7.9nmol kg-1, as compared to6.2nmol kg-1at the edge of the eddy. We estimate a net sea-air flux in summer of SCS at a range of1.5-8.1μmol m-2d-1. In the intermediate water mass (100-600m) between the bottom of photic zone and the deep-water N2O maximum, the excess N2O (△N2O) was overall correlated with the apparent oxygen utilization (AOU), and was also correlated with NO3-, suggesting that most of the N2O may come from nitrification. Below600m, however this correlation disappeared, using a one-dimensional steady-state diffusion advection model, we revealed that the N2O was mainly controlled by water mixing in the part of SCS deeper than800m. In the PRE, the entire estuary was supersaturated with N2O during our survey seasons. Concentrations of N2O ranged from276nmol kg-1(42times supersaturated) in the02-depleted upper estuary, down to about7nmol kg-1(slight supersaturated) at the mouth of the estuary. The distribution of N2O showed coherent relationships with the distribution patterns of ammonium, oxygen, nitrate and nitrification activity. Based on the zonal distribution of N2O, we estimated a net water-air N2O fluxes in the PRE at a level of (139±81)×103mol d-1in spring and (78±48)×103mol d-1in summer. It appeared the N2O flux in upper estuary was much controlled by the nitrification and denitrification in the water column and sediments; while excess N2O load from upstream and nitrification in the water column controlled the N2O flux from the mid estuary. Mixing with outer seawater dominated the N2O level in the lower estuary.Distributions of N2O in the two systems under study were much different. The concentrations, saturations and sea-air fluxes of N2O in the SCS were much lower. The surface N2O was dominated by sea-air exchange and nitrification in the bottom of the photic zone, and the N2O in the deep water in SCS was mainly controlled by water mixing. While the PRE was a large perturbed estuary, the concentrations, saturations and water-air fluxes of N2O waried greatly. Duing to the strong nitrification and denitrification in the water column and sediments, the N2O fluxes in upper estuary were120orders of magnitude higher than SCS. |
PDF Full Text Request