Exploring Sources and Biogeochemical Dynamics of Dissolved Methane in the Central Bohai Sea in Summer
Yong Zhang, Bing Chen, Weidong Zhai
Abstract
To clarify the sources and biogeochemical dynamics of dissolved methane (CH 4 ) in shallow-water coastal oceans, we investigated the concentrations, sea-to-air fluxes, and net cycling rate of CH 4 in the central Bohai Sea in summer 2018. During the survey, both summertime stratification and dissolved oxygen (DO) deficit were observed. In the surface layer, CH 4 concentration ([CH 4 ]) ranged from 3.08 to 10.27 nmol kg -1 . The average sea-to-air flux was estimated at 6.46 3.32 mol m -2 d -1 , indicating that the Bohai Sea serves as a source of atmospheric CH 4 . In the bottom layer, [CH 4 ] ranged from 4.29 to 31.04 nmol kg -1 . The downward increase in [CH 4 ] within the water column indicated substantial sources of CH 4 in the seafloor. Based on the complex relationship between the saturation ratio of CH 4 and DO, three types of CH 4 release from the seafloor were identified, including diagenesis of buried organic matter, natural leakage from geological settings, and anthropogenic release from offshore oil/gas development. The saturation ratios of CH 4 in bottom waters were positively correlated with the degree of stratification of the water column. Using a two-layer box model, sedimentary CH 4 release rates at two oxygen-deficient stations were estimated to be 56.0-60.8 mol m -2 d -1 , which was much higher than the sea-to-air fluxes. The modeling results also indicated that the majority of seafloor released CH 4 (>90%) was consumed in the water column of this shallow-water coastal sea. This is the first comprehensive study on CH 4 cycling in the Bohai Sea. This work shows the indicative role of DO in identifying multiple CH 4 sources and highlights the dominance of water stratification and microbial consumption in local CH 4 dynamics.