Beschreibung:
<p>Lakes are important components of the global methane (CH₄) cycle. In seasonally ice-covered lakes, CH₄ 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 CH₄ emissions at icemelt. To understand the fate of CH₄ trapped below ice, we measured depth-distributions of CH₄ 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 CH₄ oxidation. During most sampling occasions, we found steep CH₄ 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 CH₄ oxidation can occur at low temperatures typical for the water-ice interface. CH₄ oxidation was observed as a significant decrease in CH₄ concentration, a significant increase in stable isotope <sup>13</sup>C signature, and an increase in MOB during the incubation. Thus, CH₄ accumulating in the top 20 cm of the water column, fed by diffusion from CH₄ in trapped bubbles, may fuel significant CH₄ oxidation. Since northern latitude lakes can be ice-covered for many months of the year and significant amounts of CH₄ accumulate below the ice, the extent of CH₄ oxidation under these low temperature-conditions is important for understanding the potential CH₄ emissions to the atmosphere during ice-melt.</p>