CH oxidation in a boreal lake during the development of hypolimnetic hypoxia.

Freshwater ecosystems represent a significant natural source of methane (CH). CH produced through anaerobic decomposition of organic matter (OM) in lake sediment and water column can be either oxidized to carbon dioxide (CO) by methanotrophic microbes or emitted to the atmosphere. While the role of CH oxidation as a CH sink is widely accepted, neither the magnitude nor the drivers behind CH oxidation are well constrained. In this study, we aimed to gain more specific insight into CH oxidation in the water column of a seasonally stratified, typical boreal lake, particularly under hypoxic conditions. We used CH incubations to determine the active CH oxidation sites and the potential CH oxidation rates in the water column, and we measured environmental variables that could explain CH oxidation in the water column. During hypolimnetic hypoxia, 91% of available CH was oxidized in the active CH oxidation zone, where the potential CH oxidation rates gradually increased from the oxycline to the hypolimnion. Our results showed that in warm springs, which become more frequent, early thermal stratification with cold well-oxygenated hypolimnion delays the period of hypolimnetic hypoxia and limits CH production. Thus, the delayed development of hypolimnetic hypoxia may partially counteract the expected increase in the lacustrine CH emissions caused by the increasing organic carbon load from forested catchments.