Lake Willersinnweiher located in south-western Germany is a small eutrophic gravel pit lake fed by sulfateenriched 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.The emission of the greenhouse gas methane (CH 4 ) from freshwater lakes has been suggested to play a substantial role in the global methane budget (e.g., Bastviken et al. 2004). Here, significant amounts of CH 4 are emitted, even though CH 4 produced in aquatic systems is largely consumed by anaerobic and aerobic methanotrophs (up to 30-99%; Bastviken et al. 2008). The amount of emitted CH 4 is thereby depending on bioproduction, degradation and mineralization of organic substances in the sediments and the water column of the freshwater lake.Carbon mineralization in anoxic lake sediments is affected by manganese (Mn) and iron (Fe) reducing bacteria metabolizing competitive substrates, outcompeting CH 4 forming microorganisms (methanogens) within the upper sediment layers (e.g., Whiticar 1999). Sulfate (SO 4 2− ) reduction often has minor implications on organic matter degradation due to low SO 4 2− availability in most freshwater environments (Holmer and Storkholm 2001), so that SO 4 2− is rapidly depleted with sediment depth and SO 4 2− reducing bacteria become inactive or are absent. As a consequence, methanogenesis is the most important process in overall carbon mineralization in anoxic lacustrine sediments (e.g., Rudd and Hamilton 1978). Competitive metabolisms are not only affecting methanogenesis, but also consumption of CH 4 by bacteria (methanotrophs) in the sediments. The anaerobic oxidation of methane in the sediments of lakes is usually coupled to the reduction of the energetically more favorable electron acceptors, such as nitrate and nitrite (e.g., Raghoebarsing et al. 2006) or Fe(III) and/or Mn(I...

Methane (CH4) emissions from aquatic systems have recently been comprised to account for up to 50% of global CH4 emissions, with lakes representing one of the largest CH4 sources within this pool. However, there is large uncertainty associated with CH4 emissions from freshwater environments to the atmosphere, because of a lack of understanding in the spatial and temporal dynamics of CH4 sources and sinks, as well as underlying mechanisms and processes. In this study, we investigated the concentrations and stable carbon (δ13C-CH4) and hydrogen (δ2H-CH4) isotope composition of CH4 in a small eutrophic lake (Lake Willersinnweiher) with seasonal stratification and its spatial and temporal variation. We found that while supersaturation of CH4 in the entire water column was present throughout the whole year, the isotopic composition of CH4 in sediment and water column varied depending on lake stratification, physiochemical conditions, and lake depth. During the stratification period, isotopic characteristics of pelagic surface water CH4 differed from littoral and sedimentary CH4, suggesting likely mixing of CH4 from different sources including vertical and lateral input as well as groundwater input and potentially oxic methane production in the mixed surface water layer. Aerobic CH4 oxidation indicated by a strong increase in both δ13C-CH4 and δ2H-CH4 values at the bottom of the oxycline was found to significantly reduce upward migrating CH4 released at the sediment-water interface. In the sediment, stable isotope characteristics of CH4 showed an increasing dominance of the acetoclastic CH4 formation pathway from the pelagic towards the littoral area. Furthermore, the occurrence of sulfate-dependent anaerobic methane oxidation in the sediment was suggested by an increase in δ13C-CH4 and δ2H-CH4 values. During the mixing period, the isotopic CH4 composition of the water column was distinctively less negative than during the stratification period potentially resulting from a greater impact of groundwater CH4 input compared to the stratification period. Our findings implicate that the application of concentrations and dual isotope measurements of CH4 is a promising approach for constraining CH4 sinks and sources in Lake Willersinnweiher and potentially other small lakes to clearly disentangle the complex CH4 dynamics in lakes both spatially and seasonally.