Analysing seasonal variations of methane oxidation processes coupled with denitrification in a stratified lake using stable isotopes and numerical modeling

Since the discovery of nitrite (n-damo) and nitrate dependent anaerobic oxidation of methane in 2006, evidence has arisen about the occurrence of this process in freshwater ecosystems. Anaerobic oxidation of methane (AOM) linked with denitrification has been proposed to be a previously overlooked sink of methane in freshwater environments. However, the occurrence and relevance of AOM coupled with denitrification in methane removal and, therefore, mitigating methane emissions in the environment is so far largely unquantified. We investigated methane oxidation processes in the water column at the seasonally stratified lake Fohnsee located in Southern Germany to determine the seasonal dynamics and to quantitatively describe the contribution of anaerobic and aerobic oxidation of methane removing CH₄ from the water column throughout nearly one year. Vertical concentration profiles and corresponding stable isotope ratios of methane (δ¹³C) and nitrate (δ¹⁵N and δ¹⁸O), together with dissolved oxygen concentrations were measured, and a numerical model was developed to evaluate whether anaerobic oxidation of methane coupled with denitrification is a key biogeochemical process at lake Fohnsee. Our data set together with the results of the numerical model revealed a redox zone within the water column where both denitrification and anaerobic oxidation of methane are linked, contributing to approximately 70% of methane removal between June and September. Therefore, AOM linked with denitrification represents the dominant methane sink in the water column of lake Fohnsee during summer stratification. The data patterns also showed that the anaerobic oxidation of methane zone was located near the lake bottom in May, and moved upwards following the seasonal vertical displacement of the oxycline and the availability of nitrate. This redox dynamic within the water column of lake Fohnsee had also an effect on the relevance of aerobic and anaerobic oxidation of methane removing methane from the water column during the year and the formation of a bacterial sulfate reduction zone close to the lake sediments.