Methane And Nitrous Oxide In The Changjiang And Typical Waters In Eastern Part Of Hainan

CH4 and N2O are important atmospheric trace gases and play significant roles in atmospheric chemistry and global warming. Rivers, estuaries, oceans and other water systems are important natural sources of atmospheric CH4 and N2O. Hence studies on the biogeochemistry of CH4 and N2O in these typical water systems will be helpful to understand the increasese of CH4 and N2O in atmosphere over the past two centuries, as well as to predict the future variations of atmospheric CH4 and N2O.In this paper, distributions, water-air fluxes and influencing factors of CH4 and N2O in the Changjiang, which locates in the subtropical regions, as well as typical waters in eastern part of Hainan, such as rivers, estuaries, lagoons and shelf sea are studied detailedly. The main research results are as follows:1. Distributions and fluxes of CH4 and N2O are determined in the waters of the middle and lower reaches of Changjiang and its estuary during the surveys in January and September 2008. The results show that the mean concentrations of CH4 and N2O in the Changjiang and its estuary are (181.8±149.0) nmol?L-1 and (20.4±19.4) nmol?L-1, respectively. Concentrations in bottom waters are consistent with those in surface waters. Inputs of CH4-rich and N2O-rich waters in tributaries and lakes impact the distributions of CH4 and N2O in the mainstream of Changjiang. Either CH4 or N2O Concentrations in the Changjiang Estuary show a decrease from fresh to salt waters, which is a common characteristic distribution in estuaries. Besides, seasonal variations are also observed, eg., dissolved CH4 and N2O in the lower reaches of the Changjiang in January 2008 are higher than those in September. Monthly survey at Xuliujing of the Changjiang Estuary from June 2007 to October 2008 also show that CH4 and N2O concentrations are affected by water temperature, high CH4 values appeare in Febrary and July, while N2O concentrations in summer and winter are much higher than those in spring and autumn. CH4 and N2O in surface waters of the Changjiang are persistently supersaturated, which indicate that the Changjiang is net sourcese of atmospheric CH4 and N2O. The Changjiang is estimated to contribute 173Gg CH4 and 28.5Gg N2O–N to the atmosphere annually. Dissolved CH4 and N2O–N inputs to the East China Sea from the Changjiang are estimated to be 1.8Gg?a-1 and 0.43Gg?a-1, respectively, which play important roles in the biogeochemical cycle of C and N in the estuary.2. Distributions and fluxes of CH4 and N2O are determined during the survey of typical waters in eastern part of Hainan from July 23th to September 6th in 2008. The results show that CH4 and N2O concentrations in rivers and lagoons are much higer than those in the shelf sea. Either CH4 or N2O Concentrations in Bamen Bay vary over a wide range but are much higher than the atmospheric equilibrium, CH4 concentrations vary from 6.5nmol?L-1 to 560.1nmol?L-1,N2O from 6.5nmol?L-1 to 111.4nmol?L-1, which are nonlinearly correlated with salinity. This general pattern is due to river inputs which are a major contributor to CH4 and N2O found in estuarine waters. While the concentrations of CH4 are in the range of 8.1 to 180.6 nmol?L-1, and concentrations of N2O are in the range of 6.0 to 16.4 nmol?L-1 in Boao lagoon, which are also affected by inputs of river. CH4 and N2O in surface waters of the rivers, estuaries and lagoons in eastern part of Hainan are persistently supersaturated, which indicate that they are net sources of atmospheric CH4 and N2O. The Lagoons in eastern part of Hainan is estimated to contribute 108.9Mg?a-1 CH4 and 7.3Mg?a-1 N2O–N to the atmosphere annually.In the shelf sea of eastern part of Hainan, the waters near shore may be influenced by the CH4-rich and N2O-rich rivers and lagoons effluent, the horizontal distributions of CH4, as well as N2O, in the surface waters illustrate a decrease from onshore to offshore. A big difference is shown in this area after the typhoon because of the wind. CH4 concentrations in surface waters in northern South China Sea vary from 2.7 to 22.3 nmol?L-1, and N2O from 5.5 to 7.7 nmol?L-1. The horizontal distributions are similarly dominated by river inputs, especially the Pearl River. The vertical distributions show that CH4 and N2O concentrations in surface waters are higer than those in bottom, and subsurface methane maxima is observed in northern South China Sea with depth of more than 1000m. The surface waters in all stations of northern South China Sea are supersaturated with CH4 and N2O, which may be contribute 17.1Gg?a-1 CH4 and 1.9Gg?a-1 N2O–N to the atmosphere annually, and CH4 and N2O in the shelf sea of eastern part of Hainan account for 11.7% and 24.2%, respectively. Human and natural impacts arround the water systems in eastern part of Hainan are also observed. For example, Groundwater and shrimp pond are both the sources of CH4 and N2O for the nearshore area, even the estuaries. The mangrove ecosystems around the seacoast are also net CH4 sources for estuaries, sea and atmosphere, which contribute 137.7Mg?a-1 CH4 to atmosphere in eastern part of Hainan.