Abstract

Thermokarst (thaw) lakes are projected to be an increasing source of atmospheric methane (CH4), a potent greenhouse gas, as climate warms and permafrost thaws. However, the magnitude of CH4 cycling, particularly the role of aerobic CH4 oxidation (methanotrophy), among thermokarst lakes, is not well constrained. This study assesses year-round methanotrophic CH4 oxidation in relation to total CH4 emissions at Big Trail Lake, a young Interior Alaska thermokarst lake which formed in icy, Pleistocene-aged Yedoma permafrost and which is characterized by large CH4 emissions. By integrating field measurements, incubations, and modeling, we demonstrate that methanotrophy reduced dissolved gas emissions (i.e., diffusive fluxes) by 57-100% during the open water period. However, its impact on total annual lake emissions was much lower (12-19%) due to a paucity of oxygen in the ice-covered water column during winter and high rates of ebullition that bypass methanotrophy in summer. Methanotrophy was most active during short shoulder seasons when snowmelt and autumnal turnover supplied oxygen to CH4-rich water columns, yet these periods were insufficient to significantly offset the annual CH4 budget dominated by ebullition emissions. Model predictions for scenarios of shortened ice-cover periods due to future warming suggest marginal increases in CH4 emissions. This study highlights the limited role of methanotrophy against the backdrop of vast permafrost-derived CH4 lake emissions in a warming Arctic.