Variations in Dissolved Methane in the Yellow Sea During the Spring Algal Blooms of 2009

Methane（CH4） is an important greenhouse gas and oceans are net sources of atmospheric CH4. The effects of environmental factors on the CH4 variation during different phases of the spring algal blooms were examined during two cruises conducted in the Yellow Sea（YS） from February to April of 2009. During the pre-bloom period from February to March, low CH4 saturation（< 134%） was observed in the surface water, except at two nearshore stations where the CH4 levels were above 140% in March due to mixing with the coastal water. During the bloom period, CH4 increased obviously at two bloom-tracking stations, especially at the surface with mean saturations of 140% and 170%. The increase in CH4 concentration/saturation is thought to be the result of in situ CH4 production. The particulate organic carbon（POC） and chlorophyll a contents were believed to be important factors that influenced the CH4 production. In addition, the presence of different dominant phytoplankton species and the grazing pressure may have stimulated the CH4 production by supplying potential methanogenic substrates(such as dimethylsulphoniopropionate（DMSP）). Both the incubation data and the in situ estimations further evidenced the significant influence of the spring blooms on the CH4 production. The calculated sea-to-air CH4 fluxes during the bloom period were not significantly higher than those during the pre-bloom period despite the bloom-increased CH4 saturation. This is due to the variation in physical forcing（such as wind speed）, which is the main driver for determining the CH4 flux. Finally, we estimated the annual CH4 flux in the YS as 9.0 μmol m-2 d-1; the findings suggest that the YS is a natural source of atmospheric CH4.