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

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 (2.360.28C) 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 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. Methane (CH4) is a potent greenhouse gas (GHG) with a warming potential that is 28 times greater than that of carbon dioxide (CO2) and is also an important component of the global carbon cycle (IPCC 2013). Although CH4 production has increased due to anthropogenic activity, microbial activity is still the main producer of CH4 (Conrad 2009), and the significance of the process will likely increase if temperatures continue to rise (Duc et al. 2010). In lakes, archaeal CH4 production under anoxic conditions mainly takes place in the sediments (Bastviken 2009), with subsequent CH4 emissions from the freshwater to the atmosphere influencing the global GHG balance (Bastviken et al. 2011). Fortunately, from a CH4 emission point of view, the activity of methaneoxidizing bacteria (MOB), i.e., methanotrophs, holds the potential to limit emissions of such biogenic CH4 to the atmosphere. MOB use CH4 as a carbon and energy source, and thus, contribute to the cycling of carbon into the pelagic food web (Bastviken et al. 2003). Within a lake system, the production and consumption of CH4 varies seasonally (e.g., Martinez-Cruz et al. 2015). For example, many lakes in the northern hemisphere are icecovered during an extensive part of the year, where the ice acts as a barrier to atmospheric exchange (Striegl et al. 2001). During this winter-period, CH4 can accumulate below the ice (Greene et al. 2014), a process fueled mainly by ebullition of CH4-enriched bubbles released from lake sediments (Walter et al. 2006). CH4 bubbles released from the sediment are rapidly transported through the water column bypassing CH4 oxidation in bottom waters. At ice-melt, the CH4 accumulated below ice can cause a significant episodic release of CH4 into the atmosphere (Phelps et al. 1998; Karlsson et al. 2013). Earlier studies investigating CH4 turnover in the water *Correspondence: [email protected] Monica Ric~ao Canelhas and Blaize A. Denfeld shared first authorship. The copyright line for this article was changed on 22 June 2016 after original online publication. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made. Special Issue: Methane Emissions from Oceans, Wetlands, and Freshwater Habitats: New Perspectives and Feedbacks on Climate Edited by: Kimberly Wickland and Leila Hamdan S78 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 61, 2016, S78–S90 VC 2016 The Authors Limnology and Oceanography published by Wiley Periodicals, Inc. on behalf of Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10288