Methane distribution and sea-to-air flux in the East China Sea during the summer of 2013: Impact of hypoxia

We measured dissolved methane (CH4) at different depths and calculated sea-to-air CH4 fluxes at 65 stations in the East China Sea (ECS) from August 4 to 31 of 2013. CH4 concentrations in surface waters ranged from 2.07 to 27.39 nM and concentrations in bottom waters ranged from 1.76 to 31.35 nM. The concentration of CH4 also decreased with distance from the Changjiang (Yangtze River) Estuary. Analysis of the depth profiles of CH4 indicated great variations among the different stations due to the influence of different water masses and variations in other factors. A region of significant bottom-water hypoxia, with an oxygen level less than 1.5 mg L−1, occurred at the northern region of the ECS. This hypoxic region also had enhanced CH4 production in the water column and sedimentary release of CH4. CH4 saturations in the surface waters ranged from 113% to 1364%, with an average of 317% (±236). We estimated the sea-to-air CH4 flux as 6.5±7.4 μmol m−2 d−1 using the LM86 equation, and as 11.5±11.9 μmol m−2 d−1 using the W92 equation. The overall estimated CH4 emission rate from the ECS was 6.4×10−2 Tg yr−1. The saturation and sea-to-air flux of CH4 varied significantly among the stations. Taken together, our data suggest that the ECS is a net source of atmospheric CH4.