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 CH ₄ 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°2′E, 31°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 × 10⁶ mol year ^-1. Mean CH₄ emission rate from the sediments of the Changjiang Estuary in spring was 1.97 μmol 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 μmol 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.