Dynamics and drivers of aerobic methane oxidation in water of a shallow eutrophic lake

Anthropogenic eutrophication and global warming have rendered shallow lakes a critical natural source of atmospheric CH₄. However, critical knowledge gaps remain regarding the vertical and seasonal dynamics, response to eutrophication, and regulatory mechanisms of aerobic methane oxidation in shallow lake waters, limiting our ability to predict CH₄ cycling and develop effective emission mitigation strategies. Seasonally on-board studies were carried out to examine the methane oxidation rate (MOR) and the specific methane oxidation rate (SMOR) in the water, as well as the abundance and community structure of methanotrophs in Lake Chaohu, located in the lower reaches of the Yangtze River in China. Annual pmoA gene copy numbers across water layers ranged from 246 ± 148 to 742 ± 473 copy mL^-1 at the eight study sites. The two dominant methanotroph genera were Methylocystis and Methylosinus, belonging to Type II, and together accounting for over 90% of the methanotrophs on annual average. The MOR and SMOR ranged from 1.32 ± 0.30 to 31.9 ± 6.0 nmol L^-1 h^-1 and from 0.40 ± 0.09 d^-1 to 2.75 ± 0.51 d^-1, respectively. The highest MOR and SMOR values were observed in summer, and the bottom water exhibited higher MOR and SMOR than the surface water. Water temperature, CH₄, pmoA gene, suspended particulate matter, and chlorophyll a were identified as the main driving factors. Seasonally, aerobic methane oxidation reduced 62.3 ± 14.9% - 76.6 ± 9.1% of the diffusive CH₄ emissions in the lake. This study underscores the crucial role of aerobic methane oxidation in reducing diffusive CH₄ emissions in shallow eutrophic lakes with the limited water depth.