Dissolved methane in the East China Sea: Distribution, seasonal variation and emission

Abstract We present a dataset of dissolved methane (CH 4 ) in the East China Sea (ECS) during five cruises in March, May, August, October and December 2011. CH 4 distribution in this region showed pronounced spatial and seasonal variability due to the complex mixing of different water masses and other variables. Surface CH 4 concentrations gradually decreased from the coast to the open sea, with maximum values occurring near Changjiang estuary or outside the Hangzhou Bay. The annual mean CH 4 concentration of the surface layer was 9.1 ± 1.6 nmol L −1 in the coastal area, which was nearly twice as large as that in the open sea (4.3 ± 1.3 nmol L −1 ). CH 4 was distributed evenly from the surface to the bottom in the shelf region during March and December, while it increased gradually with depth during May and October. CH 4 depth profiles exhibited various distribution features along the coast, in the middle and on the edge of continental shelf. CH 4 levels at the bottom were generally higher than at the surface during all seasons, indicating obvious CH 4 sources from sediments. Incubation experiments of sediment cores onboard suggested that sediment release was an important source of CH 4 in the water column of the ECS. We estimated that the annual average CH 4 release rate from sediments was about 1.11 μmol·m −2 ·d −1 on the continental shelf of the ECS. The maximum CH 4 concentration and sediment-water CH 4 flux both occurred in summer, which might be related with the occurrence of hypoxia in the bottom water. Surface seawater of the ECS was oversaturated with CH 4 relative to the atmosphere over most of the five cruises, indicating that the ECS was a net source of atmospheric CH 4 . The annual mean area-weighted sea-air flux density of CH 4 in the ECS was estimated to be about 10.7 μmol·m −2 ·d −1 in 2011. Accordingly, an area-weighted, seasonally adjusted annual rate of CH 4 efflux was determined to be 2.98 × 10 9  mol yr −1 (~0.05 Tg CH 4 yr −1 ) from the ECS to the atmosphere.