Methane And Nitrous Oxide In The Yellow River Estuary, The Yellow Sea And The Bohai Sea

CH4 and N2O are important greenhouse gases, which play significant roles in the global warming and atmospheric chemistry. Rivers, estuaries, coastal waters are net natural sources of atmospheric CH4 and N2O. Hence studies on the biogeochemistry of dissolved CH4 and N2O in these typical water systems will be helpful to understand the emission of CH4 and N2O from different water systems and their effect on environment.In this dissertation, the seasonal variation of distributions, fluxes and, influencing factors of CH4 and N2O in the Yellow River and its estuary, the Bohai Sea and the Yellow Sea are-studied detailedly. The main results are as follows:1. The seasonal variation of CH4 and N2O was studied during surveys at Station Kenli at the Yellow River from November 2008 to December 2009. The results showed that CH4 concentrations ranged from 8.73 to 30.65 nmol/L with an average of 20.97±7.57 nmol/L and had seasonal vatiations with the highest values occurring in autumn and the lowest in spring. CH4 saturations ranged from 251.9% to 1243.6% with an average of 649.7%. CH4 concentrations in the Yellow River correlated positively with the monthly flow rates. Dissolved N2O concentrations ranged from 8.78 to 24.26 nmol/L with an average of 17.80±4.90 nmol/L. The higher N2O concentrations occurred in spring and winter and the lower values appeared in summer and autumn.The average input of CH4 and N2O from the Yellow River to the Bohai Sea is estimated to be 2.22×105mol/yr and 1.72×105 mol/yr, respectively. The continuous observation during the Yellow River water and sediment regulating period from June 19 to July 18 of 2009 showed that dissolved CH4 and N2O in the Yellow River reached maximum earlier than the flow rate maximum. They increased to the highest values following with the arrival of the water and then decreased gradually till stable. CH4 and N2O input to the Bohai Sea via Yellow River is estimated to be 2.15×104 mol and 1.86×104 mol in only one month respectively, accounting for 9.7% and 10.8% of total CH4 and N2O annual flux respectively. Three cruises were conducted on the Yellow River estuary from June to July of 2009, the results showed that the average concentrations, saturations and fluxes of CH4 and N2O are on the lower level compared to other estuaries. The concentration ranges of CH4 are larger than N2O. The horizontal distributions of CH4 and N2O illustrate a decrease from onshore to offshore with the highest values occurs in the Yellow River mouth. The vertical distributions show that CH4 and N2O concentrations in bottom waters are higher than those in surface. CH4 and N2O in the Yellow River estuary are supersaturated at most stations.2. Distributions and fluxes of CH4 and N2O were determined during surveys on the Bohai Sea in Spetember and October of 2008. The results showed that the distributions of CH4 and N2O in the Bohai Sea are affected obviously by inputs of fresh water and release of sediments. Both CH4 and N2O concentrations and their variation range in bottom waters are larger than those in the surface.The highest CH4 concentrations were observed in the coastal bottom waters of Qinhuangdao. The distributions of N2O concentration in the surface waters of Bohai Sea are different from the bottom ones. Higher N2O concentrations are found in surface waters of the west coast of Dalian, and the north of Laizhou Bay. Higher N2O concentrations in bottom waters were found in the southwest of the central Bohai Sea and in the area between Dalian and Qinhuangdao. CH4 and N2O at all stations are supersaturated with respect to the atmosphere. The annual CH4 and N2O emission from the Bohai Sea are estimated to be 3.6×108g-9.3×108g and 1.99×108g-5.40×108g, respectively.3. Distributions and fluxes of CH4 and N2O were determined during two surveys on the Yellow Sea in February and March-April 2009. The results showed that dissolved CH4 and N2O in both surface and bottom waters were uniformly in two seasons. CH4 concentrations illustrated a decrease from onshore to offshore in both seasons. In winter, CH4 in surface waters are influenced strongly by the Yangtze River diluted water, and showed higher values in the southwestern region. Higher CH4 in bottom waters were found in the northwestern region and lower values occuured in the southestern region. In spring, CH4 in surface waters are high in the coastal area of Rongcheng, while CH4 in bottom waters are high in the north and east of coastal waters. N2O distributions in both surface and bottom waters of two seasons showed a gradually decrease from the western and northern coastal waters to the open sea. In winter, N2O concentrations in bottom waters had negative correlations with temperature and salinity. CH4 and N2O in the Yellow Sea are supersaturated slightly, and sea-to-air fluxes are low. The annual contributions of Chinese coastal waters to the global oceanic CH4 and N2O emissions are estimated to be 3.9%and 3.5%, respectively.