Methane in Lakes: Variability in Stable Carbon Isotopic Composition and the Potential Importance of Groundwater Input
Jonathan Schenk, Henrique O. Sawakuchi, Anna Sieczko, Gustav Pajala, David Rudberg, Emelie Hagberg, Kjell Fors, Hjalmar Laudon, Jan Karlsson, David Bastviken
Abstract
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.