CH₄ oxidation in a boreal lake during the development of hypolimnetic hypoxia

Freshwater ecosystems represent a significant natural source of methane (CH₄). CH₄ produced through anaerobic decomposition of organic matter (OM) in lake sediment and water column can be either oxidized to carbon dioxide (CO₂) by methanotrophic microbes or emitted to the atmosphere. While the role of CH₄ oxidation as a CH₄ sink is widely accepted, neither the magnitude nor the drivers behind CH₄ oxidation are well constrained. In this study, we aimed to gain more specific insight into CH₄ oxidation in the water column of a seasonally stratified, typical boreal lake, particularly under hypoxic conditions. We used ¹³CH₄ incubations to determine the active CH₄ oxidation sites and the potential CH₄ oxidation rates in the water column, and we measured environmental variables that could explain CH₄ oxidation in the water column. During hypolimnetic hypoxia, 91% of available CH₄ was oxidized in the active CH₄ oxidation zone, where the potential CH₄ 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 CH₄ production. Thus, the delayed development of hypolimnetic hypoxia may partially counteract the expected increase in the lacustrine CH₄ emissions caused by the increasing organic carbon load from forested catchments.