Regulation of Mesoscale Eddies on Oceanic Methane Production, Oxidation, and Emissions

Abstract

The ocean is a source of atmospheric methane (CH₄), yet the impact of mesoscale processes on CH₄ cycling remains largely unconstrained. In this study, we combined high-resolution underway observations and site-specific geochemical analyses conducted in September 2020, with methane oxidation (MOx) rates measurements and molecular analysis in September 2022, to investigate the regulation of mesoscale eddies on CH₄ production, methanotrophic activity, and emission fluxes in the South China Sea (SCS). Underway observation revealed that cyclonic eddies (CEs) increased surface CH₄ concentrations, while anticyclonic eddies (AEs) generally exhibited lower CH₄ levels. CEs observed in September 2020 after summer, enhanced CH₄ production associated with phytoplankton by transporting coastal nitrate-rich waters into the eddy core. Particulate dimethylsulfoniopropionate (DMSP_p) produced by phytoplankton was identified as a significant source of CH₄ within the mixed layer based on the significant correlations between DMSP and CH₄ (r = 0.79; p < 0.01). In contrast, elevated MOx rates and pmoA gene abundance were observed in the AEs, driven by convergence and stratification of surface seawater caused by downwelling of water masses. Compared to reference sites, the CH₄ air–sea fluxes in CEs increased by 204%, whereas the CH₄ emission flux in AEs was reduced by 25.1%. Collectively, mesoscale eddies significantly influence CH₄ cycle by altering phytoplankton composition, nutrient dynamics and microbial communities, ultimately leading to the divergent CH₄ emissions. Our results illustrated the control of mesoscale eddies on CH₄ production and oxidation and highlighted the importance of physical processes on biogeochemical cycling and greenhouse gas emissions.