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

Abstract

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...

Application of concentration and 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake

“…Hence, due to high SO 4 2− concentrations in Lake Willersinnweiher, the potential for sulfate-dependent AOM is high compared to most other limnic environments. The main process of organic matter turnover in Lake Willersinnweiher is degradation via sulfate reduction (Kleint et al, 2021). This results in the production of sulfide (S 2− ) in the lake sediment, causing diffusive release into the bottom water and consequently leading to euxinic conditions in the hypolimnion during the stratification period.…”

“…Pyrite oxidation in the Quaternary river sediments of the Rhine cause high SO 4 2− concentrations in the catchment upstream of the lake (Schröder, 2004;Isenbeck-Schröter et al, 2016). A detailed review of geochemical processes in the lake water and sediment of Lake Willersinnweiher can be found in a recent study by Kleint et al (2021).…”

Salinity causes widespread restriction of methane emissions from inland waters

“…An abundance of ions favors more energetically efficient reactions, with sulfate (SO4 2-) and iron (Fe 3+ ) reducers outcompeting methanogens for labile C substrate and hydrogen ions 11,15,16 . The reduction of SO4 2-, nitrate (NO3 -), and Fe 3+ ions can be coupled to anaerobic CH4 oxidation [17][18][19][20][21][22][23] , further supressing ambient CH4 concentrations at high salinity levels. Salinity can also interact with other key CH4 controls, with organic C availability modulating the SO4 2inhibition of methanogenesis 11,18,24,25 , and nutrient availability changing with salt content due to sorption to sediments 18,26 .…”

Significance of anaerobic oxidation of methane (AOM) in mitigating methane emission from major natural and anthropogenic sources: a review of AOM rates in recent publications

“…106,107 AOM driven by other electron acceptors has also been reported. For example, metal oxides and sulfates can also be electron acceptors, 16,69 but the rates are usually two or three orders of magnitude lower than those of the DAMO pathways. 108,109 In this section, AOM and relevant methane emissions from lakes/reservoirs/ponds and rivers/streams are discussed separately because the two water bodies are different in several aspects.…”

“…AOM has also been observed in anoxic freshwater and terrestrial environments, such as lakes, rivers, wetlands, paddy systems, and even deep underground in fractured granitic rocks. 4,[16][17][18][19][20] DAMO (denitrifying anaerobic methane oxidation) has been demonstrated as a major AOM pathway in terrestrial environments. [21][22][23][24] DAMO entails two different processes, i.e.…”