Exploring Sources and Biogeochemical Dynamics of Dissolved Methane in the Central Bohai Sea in Summer

• Main Authors: Yong Zhang, Bing Chen, Wei-dong Zhai
• Format: Article
• Language: English
• Published: Frontiers Media S.A. 2020-02-01
• Series: Frontiers in Marine Science
• Subjects: methane; sea-to-air flux; microbial consumption; dissolved oxygen; stratification; Bohai Sea
• Online Access: https://www.frontiersin.org/article/10.3389/fmars.2020.00079/full

To clarify the sources and biogeochemical dynamics of dissolved methane (CH4) in shallow-water coastal oceans, we investigated the concentrations, sea-to-air fluxes, and net cycling rate of CH4 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, CH4 concentration ([CH4]) ranged from 3.08 to 10.27 nmol kg–1. The average sea-to-air flux was estimated at 6.46 ± 3.32 μmol m–2 d–1, indicating that the Bohai Sea serves as a source of atmospheric CH4. In the bottom layer, [CH4] ranged from 4.29 to 31.04 nmol kg–1. The downward increase in [CH4] within the water column indicated substantial sources of CH4 in the seafloor. Based on the complex relationship between the saturation ratio of CH4 and DO, three types of CH4 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 CH4 in bottom waters were positively correlated with the degree of stratification of the water column. Using a two-layer box model, sedimentary CH4 release rates at two oxygen-deficient stations were estimated to be 56.0–60.8 μmol 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 CH4 (>90%) was consumed in the water column of this shallow-water coastal sea. This is the first comprehensive study on CH4 cycling in the Bohai Sea. This work shows the indicative role of DO in identifying multiple CH4 sources and highlights the dominance of water stratification and microbial consumption in local CH4 dynamics.