九龙江河口表层水体及沉积物中甲烷的分布和环境控制因素研究

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

利用静态顶空法在2009年7月测定了九龙江河口表层水体和沉积物孔隙水中甲烷浓度以及相关的环境参数,并对甲烷浓度分布特征和控制因素进行了相关的分析.结果显示56个河口表层水的甲烷浓度在10.7～456.7 nmol.L-1之间,饱和度远超过大气平衡甲烷浓度,由河口上端向中下端逐渐减小.4个站位(B1、B2、B3和B4站位)孔隙水中平均甲烷浓度(分别为2 212、447、28和5μmol.L-1)从河口上端向下端快速减小,与水体甲烷浓度水平变化趋势基本一致.B1～B4站位孔隙水中硫酸盐的浓度依次增大,其平均值分别为0.13、0.64、5.3和16.3 mmol.L-1.九龙江河口表层水和孔隙水中甲烷浓度变化趋势,表明河口上端沉积物中产甲烷菌降解有机质产生甲烷,并以扩散的形式通过沉积物-水界面进入上部水体,导致河口上端甲烷浓度增加;而在河口下端海相区随着孔隙水中硫酸盐浓度增加,沉积物中产甲烷过程逐渐受到硫酸盐还原过程的抑制,河口下端孔隙水和表层水甲烷浓度相应降低.B2和B3站位孔隙水中甲烷浓度随着深度增加分别由43和10μmol.L-1增加至1 051和57μmol.L-1,结合总有机碳(TOC)和硫酸盐在沉积柱剖面上的变化趋势,表明大量甲烷在沉积物硫酸盐-甲烷过渡带中被厌氧氧化,这进一步抑制了沉积物中甲烷的释放强度.九龙江河口沉积物中甲烷的产生过程除有机质以外还受到孔隙水中硫酸盐浓度的控制,而水体甲烷主要来源于河口上端盐度相对较低且富有机质的红树林潮间带湿地的释放.

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.