Aerobic Methane (CH4) Oxidation in Surface Water of the Arctic Lakes in Stordalen Mire, Sweden

Arctic lakes are important sources of methane (CH4), a potent greenhouse gas. Methane emissions from aquatic ecosystems are determined by a balance between the production of CH4 in sediments by anaerobic methanogenic archaea and its consumption either in sediments by anaerobic methanotrophic archaea or in the water column by aerobic methanotrophic bacteria. Understanding rates of CH4 oxidation (or consumption) in the water column of Arctic lakes is critical to assessing current and future potential CH4 emissions in freshwater aquatic systems. In this study, CH4 oxidation rates were measured in the surface waters of two well-studied Arctic lakes in northern Sweden, Inre Harrsjön and Villasjön, during the summer of 2022. We used the headspace technique to measure the concentration of dissolved CH4 in surface waters and changes in dissolved CH4 over a four-day incubation to calculate oxidation rates from the decrease in dissolved CH4 over time. We measured oxidation in waters from both the edges and centers of the two lakes. The lakes differed in their depth and surrounding landscape. Inre Harrsjön is deeper than Villasjön (6 and 1 m, respectively), as such we measured oxidation rates in the center of Inre Harrsjön at two different depths. We also measured oxidation rates in surface waters from a stable and thawing lake edge in Inre Harrsjön. We observed CH4 oxidation in surface waters from all sampled locations in both lakes. Methane oxidation rates were highly variable across sampling locations within both lakes, and on average were 68.01 ± 27.11 nmol CH4 L-1 d-1 in Inre Harrsjön and 40.02 ± 25.83 nmol CH4 L-1 d-1 in Villasjön. Future studies of water column CH4 oxidation in Inre Harrsjön and Villasjön should measure CH4 oxidation rates across the ice-free season and at different depths. In addition, to better understand CH4 cycling in these lakes, future work should investigate the relationships between water quality (pH, dissolved O2, etc.), water column microbial communities, and CH4 oxidation rates.