Spatial Distribution of Methane in Surface Water and Sediment of Jiulongjiang Estuary and the Effect Environment factors of It

Distribution of methane in surface water and sediment of Jiulongjiang Estuary was investigated during July,2009 through head-space method.The concentration of methane varies from 10.7 to 456.7 nmol·L-1 in the surface water at 56 sampled stations,and supersaturates relative to equilibrium with atmospheric methane.The concentration of methane decreases rapidly from estuarine upside margin to the open coastal ocean,resulting from mixing between high CH4-containing fresh water and low CH4-containing seawater.The sediment cores are situated in the upper estuarine coast and seaward boundary along the estuarine salinity gradient,representing the freshwater,half-brackish and marine water environment.Distribution of methane in porewater is consistent with that of surface water,which decreases rapidly from B1,B2,B3 to B4 stations,from 2 212 μmol·L-1 to 5 μmol·L-1.The concentration of sulfate in porewater increases gradually from B1,B2,B3 to B4 stations,with average value of 0.13,0.64,5.3 and 16.3 mmol·L-1 respectively.The trends of methane in surface water and porewater have illustrated a large amount of methane is generated via the process of organic matter degradation mediated by methanogens,moved across sediment-water interface,and entered to overlying water.In seaward boundary sediment with an abundance of sulphate in sediment,and sulphate in porewater inhibits the methanogenesis,the methane input from the sediment rapidly decreases.Depth profiles of methane in porewater B2 and B3 stations show an increase in concentration from 43 and 10 μmol·L-1 near the sediment-water interface to about 1 051 and 57 μmol·L-1 at core end.According to the vertical profile of methane,total organic carbon(TOC) and sulfate trend,a large amount of methane is depleted via anoxic oxidation in methane-sulfate transition.The methane released from the low concentration of sulfate sediment intertidal wetland situated in upper estuarine could be the most important source in Jiulongjiang estuary.