Revising the methane cycling in lakes: sources and sinks in two German lakes with specific consideration of methane accumulation in oxic waters

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Recently, methane (CH4) accumulation in oxic waters has been found in freshwater and marine waters worldwide. In our Aquameth project (GR1540/21-1) we have reviewed and evaluated potential mechanisms for this phenomenon in meso-oligotrophic Lake Stechlin. By establishing an online CH4 measurement system, we could show a close spatio-temporal relationship between the dynamics of alga (e.g. cyanobacteria and cryptophytes) and CH4 in the oxic water layers of the lake. Whilst the recently established methyl phosphonate metabolism might be present in the lake we found first evidence that algae directly produce CH4 during photosynthesis. However, the possible mechanism and its contribution to CH4 produced in oxic waters to total CH4 fluxes remains largely unknown. By combining the expertise of two established research groups - that ideally complement each other including instrumental applications, we aim to evaluate the precise chemistry and biology of CH4 formation and oxidation processes to better understand the role of lakes in regional and global CH4 cycling. Therefore, the complete CH4 budget of two lakes in Germany will be quantified in detail, i.e. CH4 bulk sources and sinks will be evaluated by a detailed mass balance approach combined with in situ incubation experiments. Our contrasting field sites/lakes (oligo-mesotrophic Lake Stechlin and eutrophic Willersinnweiher) represent 2 main temperate lake types (deep /nutrient-poor and shallow /nutrient-rich) well studied and biogeochemically characterized by the two research institutes. In both lakes, the presence of specific processes of CH4 production, accumulation and its release to the atmosphere depend on a complicated interplay of physical, chemical and biological factors favoring certain organisms and related processes. Thus our main objectives are to entangle the complicated interplay between environmental variables and processes of CH4 formation, accumulation and release, and to provide a seasonal CH4 budget of the lake to determine the relevance of oxic CH4 formation for regional and global CH4 cycling. We hypothesize that (1) Methane production is directly linked to photosynthesis and CH4 can be directly formed by various photoautotrophic organisms at specific environmental conditions, e.g. nutrient limitation. (2) Methane formation is decoupled from oxidation due to spatial and/or temporal decoupling of methanotrophic activity in oxic waters. (3) Methane at the thermocline is the product of a complicated interplay between biological, chemical and physical processes. (4) Increased CH4 concentrations in the upper oxic water layer facilitate the gas exchange between water and atmosphere. Although CH4 accumulation in the upper water layers has been largely neglected, it may represent a major missing link in the global CH4 budget. To address these hypotheses, field measurements and lab experiments will be combined by close interactions between both teams.