Methane Production, Oxygenation, and Flux Patterns: Snapshot Analyses in a Shallow Hypereutrophic Danish Lake

Phosphorus- and carbon-enriched lakes sustain high algal production and sedimentation of labile organic carbon, leading to substantial methane (CH4) production and emissions that exacerbate global warming. Accurately assessing their role in global climate change and developing restoration strategies to limit carbon release into the atmosphere requires robust knowledge of CH4 production drivers, removal processes (oxidation), and spatial variability in production and emission patterns – factors often constrained by costly instrumentation. <br/>While lakes with complex morphometry exhibit strong spatial heterogeneity in CH4 dynamics, it remains unclear whether similarly large variability occurs in morphometrically uniform lake systems. <br/>We conducted high-resolution summer “snapshot” measuring campaigns in a small (10-hectare), shallow, hypereutrophic Danish lake (2022–2025) to assess spatial variability in sediment CH4 production and surface fluxes. At 37 stations, we deployed low-cost automated vented floating chambers (‘DIY’ design) to record surface CH4 fluxes, sampled water column CH4 concentrations using the headspace equilibrium technique, and sampled sediments for incubation in the laboratory to quantify CH4 production and assess sediment geochemical drivers. <br/>Despite the lake’s overall homogeneous depth and sediment characteristics, we observed pronounced horizontal and vertical gradients in CH4 dynamics. Production potential increased with sediment nutrient content, but correlations between local sediment production and surface emissions were weak - likely reflecting frequent sporadic ebullitive CH4 release and substantial CH4 oxidation in the oxygenated surface water layer.<br/>Restoration strategies that permanently remove the nutrient-enriched surface sediments to improve water quality and limit algal production may prevent long-term anoxia and effectively mitigate CH4 emissions to the atmosphere.