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Methane (CH 4 ) is an important component of the carbon (C) cycling in lakes. CH 4  production enables carbon in sediments to be either reintroduced to the food web  via  CH 4  oxidation or emitted as a greenhouse gas making lakes one of the largest natural sources of atmospheric CH 4 . Large stable carbon isotopic fractionation during CH 4  oxidation makes changes in  13 C: 12 C ratio (δ 13 C) a powerful and widely used tool to determine the extent to which lake CH 4  is oxidized, rather than emitted. This relies on correct δ 13 C values of original CH 4  sources, the variability of which has rarely been investigated systematically in lakes. In this study, we measured δ 13 C in CH 4  bubbles in littoral sediments and in CH 4  dissolved in the anoxic hypolimnion of six boreal lakes with different characteristics. The results indicate that δ 13 C of CH 4  sources is consistently higher (less  13 C depletion) in littoral sediments than in deep waters across boreal and subarctic lakes. Variability in organic matter substrates across depths is a potential explanation. In one of the studied lakes available data from nearby soils showed correspondence between δ 13 C-CH 4  in groundwater and deep lake water, and input from the catchment of CH 4  via  groundwater exceeded atmospheric CH 4  emissions tenfold over a period of 1 month. It indicates that lateral hydrological transport of CH 4  can explain the observed δ 13 C-CH 4  patterns and be important for lake CH 4  cycling. Our results have important consequences for modelling and process assessments relative to lake CH 4  using δ 13 C, including for CH 4  oxidation, which is a key regulator of lake CH 4  emissions.

Methane in Lakes: Variability in Stable Carbon Isotopic Composition and the Potential Importance of Groundwater Input