Methane Oversaturation in Boreal Lakes: Towards Process-based Understanding

English Abstract:

Methane (CH4) oversaturation in lakes, particularly in the surface waters, is a common finding of limnological research on gas dynamics. A few possible explanations have been put forth to explain this ubiquitous condition of the landscape, but the interaction of these possible processes and their ability to truly explain the oversaturation remain unresolved. In this context, 11 lakes in the Laurentian region of southern Quebec, Canada were surveyed in detail for CH4 dynamics during summer 2014. Surface sampling throughout the lakes and a cross-section of depth profiles across the deepest point of the lake revealed a persistent CH4 oversaturation in each lake at the surface, as well as at several other depths with depleting concentrations in between. No significant ebullition was found in the profundal zone of any lake, while substantially high CH4 emissions were observed from floating chamber measurements in the littoral zones. Higher dissolved CH4 concentrations in the littoral along with the floating chamber measurements, as well as submerged bubble trap surveys in the littoral zones of three of the lakes, suggest that littoral methane production is mostly responsible for the CH4 oversaturation in Laurentian lakes. Mass balance models will help in determining the CH4 contribution from the littoral zone and other possible sources as well as provide the basis for testing a lake-level process-based model at the landscape and regional level of Quebec.