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

• Published in: Frontiers in Earth Science
• Main Authors: Schenk, Jonathan, Sawakuchi, Henrique O., Sieczko, Anna K., Pajala, Gustav, Rudberg, David, Hagberg, Emelie, Fors, Kjell, Laudon, Hjalmar, Karlsson, Jan, Bastviken, David
• Other Authors: European Research Council, Vetenskapsrådet, Svenska Forskningsrådet Formas, Knut och Alice Wallenbergs Stiftelse
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Frontiers Media SA 2021
• Subjects: Subarctic
• Online Access: http://dx.doi.org/10.3389/feart.2021.722215; https://www.frontiersin.org/articles/10.3389/feart.2021.722215/full

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.