Biogeochemistry Of Methane And Nitrous Oxide In The Water And Sediments Of JiaoZhou Bay

CH₄ and N₂O are important greenhouse gases, which play significant roles in the global warming and atmospheric chemistry. Global oceans are net natural sources of atmospheric CH₄ and N₂O. Although continental shelves, estuaries and bays only occupy a small part of the world ocean, they appear to be responsible for a large part of the oceanic CH₄ and N₂O emission. Therefore studies on the biogeochemistry of dissolved CH₄ and N₂O in coastal waters, especially on the production and consumption mechanism, will be helpful to enrich the list about sources and sinks of global atmospheric CH₄ and N₂O, and to expand the studies about biogeochemistry of dissolved CH₄ and N₂O in continental shelves, estuaries and bays.In this dissertation, the production and consumption mechanism, sediment-water flux, distributions, sources and sinks in Jiaozhou Bay which is impacted by human activities severely, are studied detailedly.1. Nitrification rates, denitrification rates and the sediment-water fluxs were determined during surveys at Dagu estuary and Loushan estuary in December 2007, Septemberand November 2008 and February 2009. The results showed that the nitrification rates and denitrification rates in tidal flat sediment had obviously seasonal and spatialvariations, and the latter is higher than the former. At Dagu estuary, the denitrification rates ranged from 9.55 to 104.43μmol·N·m^-2·h^-1 with maximum values occurring in spring and lowest values occurring in winter due to the influence of temperature and biomass. The maximum nitrification rate was determined in spring, but it hardly happened in summer. Based on the denitrification rates, the area of the Jiaozhou Bay and mean inputs of nitrate from Dagu river into the Jiaozhou Bay, the proportion of the nitrate inputs removed via denitrification is estimated to be 18% in spring, 5% in summer, 10% in autumn and 2% in winter. At Loushan estuary, the denitrification rate in summer was higher than that in winter due to the influence of organic matterand the concentration of nitrate in the overlying water. The nitrification hardly happened in summer also. The sediment-water N₂O fluxes showed obvious seasonal variations. At Dagu estuary, the sediment released N₂O into the overlying water allthe year except in winter. The sediment at Loushan estuary released N₂O not only in summer but also in winter. On an annual scale, the tidal flat sediments surroundingJiaozhou Bay is a main source of N₂O in the water column. The sediment-water CH₄fluxes showed obvious seasonal variations. At Dagu estuary, the sediment absorbede CH₄ from the overlying water in all seasons. But the sediment at Loushan estuary release CH₄ in summer and winter.2. CH₄ and N₂O concentrations in surface waters of the Jiaozhou Bay, riverine water and sewage water were determined during surveys between December 2007 and February 2009., Concentrations and saturations of CH₄ showed obvious seasonal variation with maximum values occurring in summer and lowest values occurring in winter. The seasonal variations of dissolved N₂O concentrations in the the Jiaozhou Bay were not obvious, but the saturations showed obvious seasonal variations with higher values occurring in autumn. The dissolved CH₄ in the water column showed an obvious decrease from eastern and northern to western and southern part of the bay, while N₂O showed decreases from both the eastern and northwestern coast to the central part. The distribution of CH₄ and N₂O in the Jiaozhou Bay are influenced strongly by the land-soure input. In situ incubation experiments of sediment cores in April 2008 showed that the CH₄ release rates from two stations were 0.07 and 0.29μmol·CH₄·m^-2·h^-1, and the N₂O release rates were 0.04 and 0.11μmol·m^-2·h^-1, which indicates that the sediment of the Jiaozhou Bay could can act as a source of dissolved CH₄ and N₂O in the water column. The annual flux from sediments is estimated to be 1.11×10⁶mol·a^-1 for CH₄ and 0.46×10⁶mol·a^-1 for N₂O. Biological activities in the surface waters of the Jiaozhou Bay varied seasonally and spatially, The production of N₂O by biological activities was estimated to be 0.31×10⁶ mol·a^-1. Concentrations of CH₄ and N₂O in the riverine water around the Jiaozhou Bay ranged from 500 to 5000 nmol/L and 9.8 to 1400 nmol/L, and the annual riverine input was estimated to be 2.69×10⁶ mol·a^-1 and 0.25×10⁶ mol·a^-1, respectively. The annual input via sewage water is estimated to be 0.02×10⁶ mol·a^-1 and 0.29×10⁶ mol·a^-1, respectively. So the primary source of the dissolved CH4 in the water of the Jiaozhou Bay is the riverine water input, while N₂O is the sewage water input.Air-sea fluxes of dissolved CH4 and N₂O showed obvious seasonal and spatial variations. The studied area is a net source of atmospheric CH4 all the year around, while from winter till autumn, it transforms from sink of atmospheric N₂O to source. The annual CH4 and N₂O emission from the Jiaozhou Bay are estimated to be 10×10⁶ -17.4×10⁶ mol·a^-1and 0.76×10⁶ -1.40×10⁶ mol·a^-1.