The Effects of Water Column Dissolved Oxygen Concentrations on Lake Methane Emissions—Results From a Whole‐Lake Oxygenation Experiment

Abstract Lakes contribute 9%–19% of global methane (CH 4 ) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH 4 and promote CH 4 oxidation. DO is therefore often considered an important regulator of CH 4 emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH 4 production and emissions by (a) influencing if methane production can be fueled by the most reactive organic matter in the top sediment layer or rely on deeper and less degradable organic matter, and (b) enabling CH 4 accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH 4 fluxes is still unclear. We assessed CH 4 fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH 4 emissions attributable to water column DO concentrations. Large amounts of CH 4 accumulated in the anoxic hypolimnion of the reference basin but little of this may have been emitted because of incomplete mixing, and effective methane oxidation of stored CH 4 reaching oxic water layers. Accordingly, ≤24% of the stored CH 4 was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH 4 storage might have a smaller impact on CH 4 emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH 4 emission estimates.