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

• Published in: Journal of geophysical research. Biogeosciences Vol. 128; no. 11
• Main Authors: Pajala, Gustav, Sawakuchi, Henrique O., Rudberg, David, Schenk, Jonathan, Sieczko, Anna, Gålfalk, Magnus, Seekell, David, Sundgren, Ingrid, Thanh Duc, Nguyen, Karlsson, Jan, Bastviken, David
• Format: Journal Article
• Language: English
• Published: 01.11.2023
• Subjects: dissolved oxygen; emissions; methane; methane oxidation; storage
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2022JG007185

Lakes contribute 9%–19% of global methane (CH4) emissions to the atmosphere. Dissolved molecular oxygen (DO) in lakes can inhibit the production of CH4 and promote CH4 oxidation. DO is therefore often considered an important regulator of CH4 emissions from lakes. Presence or absence of DO in the water above the sediments can affect CH4 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 CH4 accumulation in deep waters and potentially large emissions upon water column turnover. However, the relative importance of these two DO effects on CH4 fluxes is still unclear. We assessed CH4 fluxes from two connected lake basins in northern boreal Sweden where one was experimentally oxygenated. Results showed no clear difference in summer CH4 emissions attributable to water column DO concentrations. Large amounts of CH4 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 CH4 reaching oxic water layers. Accordingly, ≤24% of the stored CH4 was likely emitted in the experimental lake. Overall, our results suggest that hypolimnetic DO and water column CH4 storage might have a smaller impact on CH4 emissions in boreal forest lakes than previous estimates, yet potential fluxes associated with water column turnover events remain a significant uncertainty in lake CH4 emission estimates. Plain Language Summary Lakes represent 9%–19% of the total sources of the greenhouse gas methane (CH4), and the concentration of dissolved oxygen is considered important for regulating lake CH4 cycling. In this study we measured CH4 emissions from two connected lake basins, one where we increased concentrations of dissolved oxygen in the water column (Experimental basin), and one that was left in a natural state, resulting in deep water oxygen depletion (Reference basin). In the Experimental basin, the dissolved oxygen addition resulted in negligible CH4 accumulation in the water column. In the Reference basin, CH4 accumulated in anoxic water layers. However, no clear difference in CH4 emissions among the two basins could be detected during the summer. The basins only partially mixed in fall and spring and depending on the degree and intensity of water column mixing, we estimated that 0%–24% of CH4 stored in the water column was released upon fall and spring turnover, while the rest was converted to carbon dioxide by methane‐oxidizing bacteria. Accordingly, water column anoxia may not be important for CH4 emissions from all boreal lakes but can represent a main share of the yearly CH4 emissions in some lakes. Key Points Hypolimnetic oxygenation strongly reduced CH4 storage Lake CH4 fluxes during summer stratification period were not influenced by water column dissolved oxygen concentrations 0%–24% of the stored CH4 escaped oxidation and was emitted at turnover periods, constituting 0%–44% of the yearly CH4 emissions