Distribution and emission of methane from the Changjiang

Field surveys along the Changjiang were carried out in January 2008 and September 2008, persistently. Monthly survey at Xuliujing of the Changjiang Estuary was carried out from September 2007 to August 2008. Concentrations of methane in the Changjiang were measured by purge and trap-gas chromatography and the atmospheric methane fluxes were calculated according to the equation by Wanninkhof. The mean concentration in surface waters of the Changjiang was (330.8±186.9) nmol·L^-1 in January 2008 and (80.9±58.3) nmol·L^-1 in September 2008, persistently. Concentrations in bottom waters were consistent with those in surface waters. Supersaturated in methane with an average air-water flux of (385.1±278.0) μmol·(m²·d)^-1. Besides, average methane concentration at Xuliujing was (167.5±91.4) nmol·L^-1 with an average air-water flux (690.9±291.6) μmol·(m²·d)^-1, the high values appeared in February and July. Along the middle reaches of the Yangtze River, methane concentrations were increased, and the lower reaches were reduced. In the estuary, methane concentrations decreased rapidly from Xuliujing to the sea, showed negative correlation with salinity. The input of CH₄-rich water form tributaries and lakes impacted the methane concentration trend of the Changjiang. The Changjiang was estimated to contribute 208 Gg CH₄ to the atmosphere annually. However, the freshwater CH₄ discharge to the East China Sea was 112×10⁶ mol·a^-1 in the estuary.