Highly Variable Methane Production and Fluxes in a Hypereutrophic Lake Despite Uniform Bathymetry and Sediment Chemistry

Abstract Hypereutrophic lakes support high sedimentation of labile organic carbon, promoting sediment anoxia and GHG potential production. Lakes with complex morphometry exhibit extensive spatial heterogeneity in sediment methane (CH 4 ) and carbon dioxide CO 2 dynamics. We wondered: do lakes with uniform bathymetry show low spatial variation in sediments' potential CH 4 and CO 2 production and surface emissions? To address this question, we studied a uniform 11‐ha shallow hypereutrophic Danish lake. Lake emission was measured over five summer days using fully automated, vented floating chambers; sediments' chemistry and potential production of CH 4 and CO 2 were measured ex situ. We found small spatial variations in total nitrogen (TN), total phosphorus (TP), and total carbon contents in the bulk 0–10 cm of surface sediments, but high spatial variability in potential CH 4 production (0.15–20.01 mmol m −2 d −1 ). Anoxic potential sediment CO 2 production was negligible (<0.04 mmol m −2 d −1 ). Potential sediment CH 4 production was significantly correlated with sediments' TN and TP, but with low determination coefficients. Lake surface CH 4 emissions varied 5–10‐fold among stations, peaking above the deepest sites. No spatial relationship was found between potential CH 4 production rates (obtained from laboratory incubation experiments with repeated measurements over a 13‐day incubation period) and surface ebullition, likely due to temporally variable and highly localized bubble release from the sediment, causing a mismatch. Surface emission was 56% of the sediment's potential CH 4 production. Despite the lake's uniform morphology, spatial variations in potential CH 4 production and emissions were high, with hotspots detected at sites likely experiencing high organic sedimentation and frequent anoxia. High spatial resolution is important to obtain reliable GHG estimates and evaluate the underlying drivers.