| Nitrous oxide (N2O) is an important greenhouse gas in the atmosphere just after Corbon dioxide (CO2) and Methane (CH4). It plays significant roles in the global warming and atmospheric chemistry. Coastal waters, especially rivers and bays which are influenced severely by human activities, are net natural sources of atmospheric N2O. Since production from capture fisheries has leveled off, aquaculture will play a significant role to meet the increasing demand of aquatic foods. Considering the microbial nitrification and denitrification within the aquaculture systems may affect the production and emission of N2O to the atmosphere, Hence studies on the biogeochemistry of dissolved N2O in the representative aquaculture systems will be helpful to understand the impact of human activities on atmospheric N2O and their effect on environment.Distribution, emission, seasonal variation and production mechanism of dissolved N2O were studied in this thesis based on nine cruises in the representative aquaculture area (Sanggou Bay). The main results are as follows:Nine cruises on Sanggou Bay and its adjacent sea areas were carried out in June, September of 2012, April, July, October of 2013, January, May of 2014, respectively, to obtain the spatial and temporal variations of N2O in different aquaculture areas. In winter, N2O concentration of in Sanggou Bay was influenced mainly by water temeparture with an average of (16.58±177nmol/L). Average Concentration of N2O in Spring (11.59±1.29) was slightly higher than those in Autumn (11.07±1.19) nmol/L. In Summer, the distribution of N2O concentration in Sanggou Bay was on the opposite trend with water temperature with an average of 10.9±0.71 nmol/L.Dissolved N2O in the surface waters of Sanggou Bay in Winter (11.89±0.72) and Spring (11.31 ± 11.25) were higher than those in the bottom layers (10.36±0.88 and 11.03±0.57nmol/L, respectively), due to temperature and water column disturbance caused by harvest of kelpt.. Dissolved N2O in the bottom waters of Sanggou Bay were higher than those in surface in Summer and Autumn because of release from the sediments. Temperature has dual influences to dissolved N2O in the Sanggou Bay. The saturation of N2O had negative correlation with temperature when it is lower than 10℃. However, the saturation of N2O had positive correlation with temperature when it is higher than 10"C.N2O released from the sediments is one of the important sources for dissolved N2O in the Sanggou Bay. Annual average sediment-water N2O fluxes was 0.18 μmol·m-2·d-1, and the season variation was influenced by aquacuture, temperature and DIN.The distribution of dissolved N2O in Sanggou Bay in spring and autumn was greatly influenced by the activities of aquaculture. However, the average N2O in surface waters out of the bay(12.53±0.77nmol/L)was higher than those in the Sanggou Bay (11.43±2.13nmol/L),which indicates that the comprehensive Multi trophic level cultivation model in the Sanggou Bay can effectively reduce the emission of N2O.Dissolved N2O in the groundwaters around the Sanggou Bay did not indicate seasonal variation with an annual average of 90.2 nmol/L. Discharge of ground waters have great influence on the distribution of dissolved N2O in the bottom waters. Dissolved N2O in the riverine waters indicated obvious seasonal variation with an average of 27.4 nmol/L. Riverine input had certain influence to the distribution of N2O in the Bay during the wet seasons.Nitrification is the main process to produce N2O in the Sanggou Bay Waters and the nitrification rates in waters ranged between (8.64±13.12) μmol·L-1·d-1. N2O in the surafce waters of the Sanggou Bay were supersaturated generally at all seasons with an annual average value of (129±118)%, indicates the Sanggou Bay is a net source of N2O in the Atmosphere. Sources and sinks of N2O in the Sanggou Bay was preliminary estimated using box model, which indicates net annual production rate of N2O in Sanggou Bay to be (-60.6～1.97) nmol·L-1·d-1. N2O input via groundwaters (97.4mol·d-1) is the major source of dissolved N2O in the Sanggou Bay, while the contribution from sediment release (24.0 mol·d-1), river input (15.3 mol·d-1), exchange between inner bay and open sea (13.2 mol·d-1) are relatively small. Sea-to-air release (85.3～152.0 mol·d-1) is the major sink of the N2O in Sanggou Bay. |
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