Emission and oxidation of methane in a meromictic, eutrophic and temperate lake (Dendre, Belgium)

We sampled the water column of the Dendre stone pit lake (Belgium) in spring, summer, autumn and winter. Depth profiles of several physico-chemical variables, nutrients, dissolved gases (CO₂, CH₄, N₂O), sulfate, sulfide, iron and manganese concentrations and δ¹³C-CH4 were determined. We performed incubation experiments to quantify CH₄ oxidation rates, with a focus on anaerobic CH₄ oxidation (AOM), without and with an inhibitor of sulfate reduction (molybdate). The evolution of nitrate and sulfate concentrations during the incubations was monitored. The water column was anoxic below 20 m throughout the year, and was thermally stratified in summer and autumn. High partial pressure of CO₂ and CH₄ and high concentrations of ammonium and phosphate were observed in anoxic waters. Important nitrous oxide and nitrate concentration maxima were also observed (up to 440 nmol L^-1 and 80 μmol L^-1, respectively). Vertical profiles of δ¹³C-CH₄ unambiguously showed the occurrence of AOM. Important AOM rates (up to 14 μmol L^-1 d^-1) were observed and often co-occurred with nitrate consumption peaks, suggesting the occurrence of AOM coupled with nitrate reduction. AOM coupled with sulfate reduction also occurred, since AOM rates tended to be lower when molybdate was added. CH₄ oxidation was mostly aerobic (∼80% of total oxidation) in spring and winter, and almost exclusively anaerobic in summer and autumn. Despite important CH₄ oxidation rates, the estimated CH₄ fluxes from the water surface to the atmosphere were high (mean of 732 μmol m^-2 d^-1 in spring, summer and autumn, and up to 12,482 μmol m^-2 d^-1 in winter).