Influence of water column stratification and mixing patterns on the fate of methane produced in deep sediments of a small eutrophic lake

<jats:title>Abstract</jats:title><jats:p>Methane (CH<jats:sub>4</jats:sub>), a potent greenhouse gas, is produced in and emitted from lakes at globally significant rates. The drivers controlling the proportion of produced CH<jats:sub>4</jats:sub> that will reach the atmosphere, however, are still not well understood. We sampled a small eutrophic lake (Soppensee, Switzerland) in 2016–2017 for CH<jats:sub>4</jats:sub> concentrations profiles and emissions, combined with water column hydrodynamics to investigate the fate of CH<jats:sub>4</jats:sub> produced in hypolimnetic sediments. Using a mass balance approach for the periods between April and October of both years, net CH<jats:sub>4</jats:sub> production rates in hypolimnetic sediments ranged between 11.4 and 17.7 mmol m<jats:sup>−2</jats:sup> d<jats:sup>−1</jats:sup>, of which 66–88% was stored in the hypolimnion, 13–27% was diffused to the epilimnion, and 6–7% left the sediments via ebullition. Combining these results with a process‐based model we show that water column turbulent diffusivity (<jats:italic>K</jats:italic><jats:sub> <jats:italic>z</jats:italic></jats:sub>) had a major influence on the fate of produced CH<jats:sub>4</jats:sub> in the sediments, where higher <jats:italic>K</jats:italic><jats:sub> <jats:italic>z</jats:italic></jats:sub> values potentially lead to greater proportion being oxidized and lower <jats:italic>K</jats:italic><jats:sub> <jats:italic>z</jats:italic></jats:sub> lead to a greater proportion being stored. During fall when the water column mixes, we found that a greater proportion of stored CH<jats:sub>4</jats:sub> is emitted if the lake mixes rapidly, whereas a greater proportion will be oxidized if the water column mixes more gradually. This work highlights the central role of lake hydrodynamics in regulating CH<jats:sub>4</jats:sub> dynamics and further suggests the potential for CH<jats:sub>4</jats:sub> production and emissions to be sensitive to climate‐driven alterations in lake mixing regimes and stratification.</jats:p>