Methane oxidation at the water‐ice interface of an ice‐covered lake

Lakes are important components of the global methane (CH 4 ) cycle. In seasonally ice‐covered lakes, CH 4 transported by ebullition (bubbling) from anoxic sediments gets trapped at the water‐ice interface. If not oxidized by methane‐oxidizing bacteria (MOB), this can potentially lead to high episodic CH 4 emissions at ice‐melt. To understand the fate of CH 4 trapped below ice, we measured depth‐distributions of CH 4 concentrations in the water column near bubbles trapped below ice in Lake Erken. We also performed a 21 d incubation experiment at low temperature (2.3 ± 0.2°C) to investigate the potential for CH 4 oxidation. During most sampling occasions, we found steep CH 4 concentration gradients just below the ice with a 13‐fold decrease from the surface to a depth of 20 cm. In vitro incubations revealed that CH 4 oxidation can occur at low temperatures typical for the water‐ice interface. CH 4 oxidation was observed as a significant decrease in CH 4 concentration, a significant increase in stable isotope 13 C signature, and an increase in MOB during the incubation. Thus, CH 4 accumulating in the top 20 cm of the water column, fed by diffusion from CH 4 in trapped bubbles, may fuel significant CH 4 oxidation. Since northern latitude lakes can be ice‐covered for many months of the year and significant amounts of CH 4 accumulate below the ice, the extent of CH 4 oxidation under these low temperature‐conditions is important for understanding the potential CH 4 emissions to the atmosphere during ice‐melt.