Water mass mixing controls methane cycling and emission in highly hydrodynamic regions of the open ocean.

Ocean circulations and water mass exchange can exert significant influences on seawater biogeochemistry, microbial communities, and carbon cycling in marine systems. However, the detailed mechanisms of the impacts of physical processes in the open ocean on the cycle of greenhouse gases, particularly methane, remain poorly understood. In this study, we integrated high-resolution underway observations, experimental incubations, radioisotope labelling, and molecular analysis to constrain the controls of methanogenic pathways, methanotrophic activity, and emission fluxes in the highly hydrodynamic Kuroshio and Oyashio Extension (KOE) region of the Northwest Pacific. The mixing of high-temperature, nutrient-rich Kuroshio waters with methane-rich Oyashio currents significantly affected not only methane abundance, but also methane production pathways and oxidation rates. Water mass mixing caused changes in the dominance of phytoplankton communities to Bacillariophyta, with less production of the methane precursor dimethylsulphoniopropionate, thus reducing dimethylsulphoniopropionate-dependent methanogenesis. The alteration of nutrient levels due to mixing of Kuroshio and Oyashio at KOE is also likely to affect microbial utilization of dissolved organic phosphorus, thus influencing methane production from the C-P cleavage of methylphosphonate. Furthermore, the abundances of methanotrophs, such as Methylocystis and Methylosinus, were much higher at the KOE sites than those observed at the Oyashio Extension, which contributed to elevated methane oxidation rates in the mixing region. Microbial oxidation as a biological sink of methane accounted for ~43.7% ± 28.8% of the total methane loss, which reduced methane emissions to the atmosphere. These data highlight the physical controls on biogeochemical methane cycling, indicating that intensive mixing of water masses may regulate methane emissions from the open oceans.