Large methane emissions from a subarctic lake during spring thaw: Mechanisms and landscape significance

Abstract The ice‐cover season and subsequent spring thaw are thought to be of particular importance for the biogeochemical cycle of northern lakes and wetlands. Yet the magnitude of their methane emissions during an entire cold season is uncertain due to scarce measurements. While wetlands are known to be the highest natural emitters of methane, emissions from northern lakes are an uncertain component of terrestrial carbon budgets. To evaluate the importance of methane emissions from a subarctic lake during winter and spring, surface methane fluxes were recorded with the eddy covariance method in a subarctic fen‐type wetland and in an adjacent shallow lake, from freezeup to complete ice out. The fen was a steady emitter of methane throughout winter. While no detectable flux was observed from the ice‐covered lake surface during winter, it was the largest methane source of the landscape in spring, with a cumulative release 1.7‐fold higher than at the fen, accounting for 53% of annual lake emissions. The high temporal resolution of the measurements allowed making a direct link between breakdown of the temperature stratification after ice breakup and the highest release of methane from the lake surface. A sediment upwelling at the end of the thaw season likely contributed to these emissions. We suggest that, unlike wetlands, shallow seasonally ice‐covered lakes can have their highest methane emission potential in the cold season, likely dominating the spring methane release of subarctic landscapes with high lake coverage.