Methane accumulation and its potential precursor compounds in the oxic surface water layer of two contrasting...

Methane (CH₄) supersaturation in oxygenated waters is a widespread phenomenon despite the traditional perception of strict anoxic methanogenesis. This notion has recently been challenged by successive findings of processes and mechanisms that produce CH₄ in oxic environments. While some of the processes contributing to the vertical accumulation of CH₄ in the oxygenated upper water layers of freshwater lakes have been identified, temporal variations as well as drivers are still poorly understood. In this study, we investigated the accumulation of CH₄ in oxic water layers of two contrasting lakes in Germany: Lake Willersinnweiher (shallow, monomictic, eutrophic) and Lake Stechlin (deep, dimictic, eutrophic) from 2019 to 2020. The dynamics of isotopic values of CH₄ and the role of potential precursor compounds of oxic CH₄ production were explored. During the study period, persistent strong CH₄ supersaturation (relative to air) was observed in the surface waters, mostly concentrated around the thermocline. The magnitude of vertical CH₄ accumulation strongly varied over season and was generally more pronounced in shallow Lake Willersinnweiher. In both lakes, increases in CH₄ concentrations from the surface to the thermocline mostly coincided with an enrichment in ¹³C-CH₄ and ²H-CH₄, indicating a complex interaction of multiple processes such as CH₄ oxidation, CH₄ transport from littoral sediments and oxic CH₄ production, sustaining and controlling this CH₄ supersaturation. Furthermore, incubation experiments with ¹³C- and ²H-labelled methylated P-, N- and C- compounds clearly showed that methylphosphonate, methylamine and methionine acted as potent precursors of accumulating CH₄ and at least partly sustained CH₄ supersaturation. This highlights the need to better understand the mechanisms underlying CH₄ accumulation by focusing on production and transport pathways of CH₄ and its precursor compounds, e.g., produced via phytoplankton. Such knowledge forms the foundation to better predict aquatic CH₄ dynamics and its subsequent rates of emission to the atmosphere.