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

Abstract

Freshwater ecosystems represent a significant natural source of methane (CH 4 ). CH 4 produced through anaerobic decomposition of organic matter (OM) in lake sediment and water column can be either oxidized to carbon dioxide (CO 2 ) by methanotrophic microbes or emitted to the atmosphere. While the role of CH 4 oxidation as a CH 4 sink is widely accepted, neither the magnitude nor the drivers behind CH 4 oxidation are well constrained. In this study, we aimed to gain more specific insight into CH 4 oxidation in the water column of a seasonally stratified, typical boreal lake, particularly under hypoxic conditions. We used 13 CH 4 incubations to determine the active CH 4 oxidation sites and the potential CH 4 oxidation rates in the water column, and we measured environmental variables that could explain CH 4 oxidation in the water column. During hypolimnetic hypoxia, 91% of available CH 4 was oxidized in the active CH 4 oxidation zone, where the potential CH 4 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 4 production. Thus, the delayed development of hypolimnetic hypoxia may partially counteract the expected increase in the lacustrine CH 4 emissions caused by the increasing organic carbon load from forested catchments.

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