Dimethylated Sulfur, Methane and Aerobic Methane Production in the Yellow Sea and Bohai Sea

This paper presents a comprehensive study of biogenic dimethylated sulfur compounds and methane (CH4) from 27 March to 16 April 2018 (spring) and from 24 July to 10 August 2018 (summer) in the Yellow Sea and Bohai Sea. The overall distributions of dimethylsulfide (DMS), dimethylsulfoniopropionate (DMSP, precursor of DMS), dimethylsulfoxide (DMSO, oxidation product of DMS) and CH4 in surface waters were characterized by elevated concentrations in summer and in coastal waters, coupled to phytoplankton biomass and terrestrially sourced inputs. Surface waters were oversaturated with CH4. The flux distributions of DMS and CH4 were generally consistent with their concentration distributions. In situ incubation experiments revealed that microbial consumption was the main removal mechanism for DMS, which could remove up to 72.1% of the total DMS compared to sea-to-air exchange in the surface layer. High DMS release and enhanced DMS yields (34%-62%) suggested more obvious influence from high DMSP concentrations (1 or 5 mu m L-1) on DMS production in oligotrophic waters with lower bacterial sulfur demand in contrast to near-shore waters. Positive correlations were found between CH4 and DMSP (dissolved) in summer and DMSO (particulate and dissolved) in spring. A DMSP addition experiment suggested that DMSP could act as a precursor for aerobic methanogenesis, and CH4 preferentially occurs under nitrogen-stressed conditions in the surface layers of the Yellow Sea and Bohai Sea. Plain Language Summary Dimethylated sulfur (DMS) is considered to be a cooling-effect biogenic gas derived from the decomposition of dimethylsulfoniopropionate (DMSP). DMSP is associated with the aerobic production of methane (CH4), a powerful greenhouse gas with global warming potential. Oceanic emission remains a highly uncertain term in the budgets of atmospheric CH4 and DMS because of sparse sampling and the variable marine environment. In order to clarify their key biogeochemical processes and provide more field data for quantifying global and regional budgets, distributions and emission fluxes of DMS and CH4, and possible CH4 production from DMSP in upper waters, were investigated in the Yellow Sea and Bohai Sea. Biogenic dimethylated sulfur compounds including DMS, DMSP, dimethylsulfoxide (the oxidation product of DMS), and CH4 exhibited high concentrations in summer and in coastal waters mainly due to the influence of phytoplankton biomass and terrestrial sources. Surface waters were oversaturated with CH4, and the flux distributions of DMS and CH4 generally followed their concentration distributions. High levels of DMSP had a more obvious influence on DMS emission in offshore waters because of various microbial demands. CH4 was found to be produced from DMSP in oxic waters, especially in offshore waters with nitrate limitation.