Methane dynamics in a stratified temperate reservoir: Accumulation, redistribution and hotspots

Freshwater reservoirs are increasingly recognized as important contributors to global methane (CH₄) emissions. However, most studies focus on hypolimnetic CH₄ accumulation during stratification, while CH₄ dynamics in intermediate and surface layers remain poorly resolved. Here, we present a multiannual, spatially explicit dataset from a meso‑eutrophic temperate drinking-water reservoir (Římov, Czechia), combining vertical and longitudinal dissolved CH₄ measurements (2016-2023) with environmental gradients, hydrodynamic indicators (modeled water age), and measurements of CH₄ oxidation potential (2020-2021). Consistent with previous work, CH₄ accumulated strongly under hypolimnetic anoxia, including two persistent storage hotspots: the deep lacustrine part near the dam and an upstream transition-zone hotspot. We also observed recurrent deviations from the classical "bottom-up" structure. Episodic epilimnetic CH₄ enrichment occurred most frequently in the upstream transition zone, creating a consistent diffusive flux hotspot, while metalimnetic CH₄ maxima developed downstream during stratification, suggesting redistribution via density-driven intrusions and internal circulation. Generalized Linear Mixed-effects Models identified water age, oxygen availability, conductivity, and depth strata as key predictors of spatial variability in CH₄ concentrations. A comparison of CH₄ turnover time with modeled water age provides a first-order conceptual framework to contrast microbial oxidation potential with hydrodynamic transport timescales. Our results highlight that epilimnetic and intermediate-depth CH₄ pools can contribute disproportionately to diffusive fluxes and potentially to downstream export, emphasizing the need for monitoring and modelling frameworks that explicitly resolve both longitudinal transport and mid-depth processes in managed reservoirs.