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.

Whole‐ecosystem oxygenation experiments reveal substantially greater hypolimnetic methane concentrations in reservoirs during anoxia

“…For waterbodies with anoxic hypolimnia, there is significant variability in the fraction of hypolimnetic GHGs that are directly emitted to the atmosphere vs. oxidized in the water column (CH 4 : Bastviken et al 2008;Vachon et al 2019;Saarela et al 2020) or taken up by phytoplankton (CO 2 : Balmer and Engel et al 2019). Encinas Fernández et al (2014 estimated that up to 50% of the total hypolimnetic CH 4 stored in anoxic hypolimnia is directly released to the atmosphere during turnover.…”

Vertical stratification patterns of methanotrophs and their genetic controllers in water columns of oxygen-stratified boreal lakes

“…2A and B ), which agrees with the measured rates of potential CH 4 oxidation (Saarela et al . 2020 ). In Lovojärvi, δ 13 C and concentration data suggested that CH 4 oxidation took place around the oxic-anoxic interface as well as deeper in the hypoxic and anoxic water column between 11.9 m and 5.9 m; however, confirmation of CH 4 oxidation at the deeper zone would require additional analyses, as the change in δ 13 C of CH 4 in the deeper zone was actually very slight (Fig.…”

“…Data from Lake Kuivajärvi for four sampling occasions from May to September 2016 were already presented in a previous paper (Saarela et al . 2020 ). However, in this paper, data are presented for September to allow for easier comparison with the microbial data.…”

Overview: Recent advances on the understanding of the Northern Eurasian environments and of the urban air quality in China - Pan Eurasian Experiment (PEEX) program perspective

“…Methane is mainly produced in the bottom sediments and/or hypolimnion, where most of anaerobic decomposition of organic matter take place, and then is either oxidized to CO2 in the water column or emitted to the atmosphere. At Kuivajärvi, a typical meso humid lake located in Southern Finland, it was found that 91% of available CH4 was oxidized in the active CH4 oxidation zone during hypolimnetic hypoxia (Saarela et al, 2020). In warm springs, the early onset of thermal stratification with cold and well-oxygenated hypolimnion delays the period of hypolimnetic hypoxia and thus limiting the production of methane.…”