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The methane dynamics in the waters of Lakes Michigan and Superior, components of the North American Great Lake system, were investigated using measurements of methane concentration and natural radiocarbon (C-14-CH4) dissolved in these lake waters. All C-14-CH4 measurements were above modern levels regardless of location and depth with a range of 117-145% modern carbon (pMC). Methane concentrations in the deep basin of both lakes were low, ranging from 3.3 to 4.3 nM, with minimal vertical variation. However, the concentrations of CH4 increased toward coastal areas in both lakes, possibly due to higher groundwater inputs and aerobic methanogenesis associated with primary productivity. Except for one site, C-14-CH4 dissolved in the waters of Lake Michigan was greater than in Lake Superior by similar to 12 pMC, a difference that was likely due to inputs of excess C-14-CH4 from nuclear power plants along the coast of Lake Michigan.

Plain Language Summary Methane is a greenhouse gas whose concentration is increasing rapidly in the modern atmosphere, and freshwater lakes globally provide a significant source to the atmosphere. Here we investigate the dynamics of methane dissolved in waters of the North American Great Lakes, one of the largest liquid freshwater environments on Earth. We found that methane dynamics in both Lakes Michigan and Superior are impacted by atmospheric input, in situ aerobic methanogenesis, groundwater, and nuclear power plants, as well as rapid vertical mixing in the water column.