Methane oxidation controls methane emissions during winter overturning in a stratified subtropical reservoir

Methane (CH4) oxidation represents a critical methane sink in freshwater systems, yet its dynamics during seasonal mixing remain poorly understood. By combining high-resolution vertical profiles of CH4 concentrations and methane oxidation rates (MORs), we demonstrate that high methanotrophic activity controls CH4 emission during winter overturning in a subtropical reservoir. Oxycline served as the hotspot of methane oxidation, intercepting approximately 90% of upward-diffusing CH4. Above oxycline, ~1-24% of CH4 was further oxidized in the oxic surface waters, resulting in only 0.3% of stored CH4 emitted to the atmosphere. Anaerobic oxidation of methane (AOM) contributes up to 61.6% of total MOR in the oxycline and extremely high rates (58.5 μmol L-1 d-1) might be achieved through non-classical electron transfer pathways. These results highlight the pathways and roles of methane oxidation in controlling methane emissions during overturning, providing insights into the dynamics and controls of CH4 cycling in freshwater reservoirs.