Methane in the Changjiang (Yangtze River) Estuary and its adjacent marine area: riverine input, sediment release and atmospheric fluxes

Dissolved methane (CH4) was measured in the waters of the Changjiang (Yangtze River) Estuary and its adjacent marine area during five surveys from 2002 to 2006. Dissolved CH4 concentrations ranged from 2.71 to 89.2 nM and had seasonal variation with the highest values occurring in summer and lowest in autumn. The horizontal distribution of dissolved CH4 decreased along the freshwater plume from the river mouth to the open sea. Dissolved CH4 in surface waters of the Changjiang was observed monthly at the most downstream main channel station Xuliujing (121 degrees 2'E, 31 degrees 46'N), which ranged from 16.2 to 126.2 nM with an average of 71.6 +/- 36.3 nM. The average annual input of CH4 from the Changjiang to the Estuary and its adjacent area was estimated to be 2.24 mol s(-1) equal to 70.6 x 10(6) mol year(-1). Mean CH4 emission rate from the sediments of the Changjiang Estuary in spring was 1.97 mu mol m(-2) day(-1), but it may be higher in summer due to hypoxia in the bottom waters and higher temperatures. The annual sea to air CH4 fluxes from the Changjiang Estuary and its adjacent marine area were estimated to be 61.4 +/- 22.6 and 16.0 +/- 6.1 mu mol m(-2) day(-1), respectively, using three different gas exchange models. Hence the Changjiang Estuary and its adjacent marine area are net sources of atmospheric CH4.