The impact of seasonal sulfate–methane transition zones on methane cycling in a sulfate‐enriched freshwater environment

Lake Willersinnweiher located in south‐western Germany is a small eutrophic gravel pit lake fed by sulfate‐enriched groundwater. The aim of this study was to investigate the total methane (CH 4 ) mass balance of Lake Willersinnweiher with a particular focus on the interaction of carbon and sulfur cycling within the lake sediments and the redoxcline of the water column. Our results show that Lake Willersinnweiher permanently releases CH 4 to the atmosphere throughout the whole year 2018 at rates ranging from 5 to 120 mol d −1 . Sediment data show the presence of intense anaerobic oxidation of CH 4 in the upper sediment layers during early summer. Here, CH 4 is most likely consumed via sulfate in sulfate–methane transition zones (SMTZs) that have been observed for a few specific freshwater environments only. Seasonal dynamics in biogeochemical processes trigger the non‐steady state conditions within the sediments and the CH 4 consumption in the SMTZs. In parallel, CH 4 released from the sediments is completely consumed by aerobic oxidation processes in the redoxcline indicated by minimum CH 4 concentrations with high δ 13 C–CH 4 values. This zone acts as an effective barrier, minimizing CH 4 release into the surface water and the atmosphere and thus CH 4 oversaturation along with near‐atmospheric isotopic composition indicate the presence of an additional CH 4 source in the epilimnion of Lake Willersinnweiher.