ALBERT

Description: We measured dissolved methane (CH4) concentrations, saturations, and fluxes from sea into air and from sediment into water during cruises in March, May, August, October, and December of 2011 in the East China Sea (ECS) and the Yellow Sea (YS). CH4 concentrations had obvious spatial and seasonal variability due to the complex effects of different water masses and other variables. Maximal CH4 concentration, sea–air and sediment–water fluxes all occurred during the summer. CH4 concentration decreased gradually from the coastal area to the open sea, and high levels of CH4 generally appeared near the Changjiang Estuary and outside the Hangzhou Bay. During early spring and winter, CH4 had a uniform distribution from the surface to the bottom, but CH4 concentration increased gradually with depth during other seasons. The subsurface CH4 maximum occurred at a depth of about 200 m during May, October, and December. The CH4 level at the bottom was generally higher than at the surface, and this was enhanced during summer due to hypoxia in the bottom waters. Changjiang-diluted water, the Kuroshio Current, and the Taiwan Warm Current Water affected the geographic distribution of CH4 in the ECS, and these water bodies contributed about 3.45, 2.97, 14.60 mol s−1 of CH4 during summer and 2.11, 8.58, 5.20 mol s−1 CH4 during winter, respectively. Sediment was also a significant source of dissolved CH4 in the ECS, and we estimated the average sediment–water CH4 flux of the ECS and YS as about 1.02 μmol m−2 d−1. We also used a box model to calculate the CH4 budget in the ECS. The results suggested that in situ CH4 production in the water column was the major source of CH4, and accounted for 0.21 μmol m−3 day−1 during summer and 0.11 μmol m−3 day−1 during winter. Air–sea exchange was the major sink of CH4 in the ECS. We estimated total CH4 emission from the ECS and YS as about 4.45 x 109 mol during 2011. Our results indicated that the ECS and YS were active areas for CH4 production and emission.