The role of littoral zones as source of methane in lakes: Dynamics, distribution patterns, and emissions

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Small lakes (<1 km2) have been identified as a significant source of methane in the global methane budget. These lakes dominate in area the global extent of natural, inland water bodies and have higher methane fluxes per unit area than large lakes. However, the estimation of lake-wide methane emissions underlies large uncertainties, because measurements that account for the temporal and spatial variability of methane in and between lakes are rare. The aim of the project is to investigate methane dynamics and distribution patterns in and methane emissions from small lakes on a regional and inter-regional scale. Thereby, the focus will be on the identification of systematic patterns and proxies for the dissolved methane concentration and methane emissions that allow the generalization and spatial up-scaling of the results obtained from a selected number of lakes. The main hypothesis are that (1) the dissolved methane concentration and methane emissions differ substantially between lakes due to their different lake properties; (2) lake and catchment properties as well as abiotic parameters may serve as proxies for the dissolved methane concentration and methane emissions on a regional or even larger spatial scale; (3) the formation and extent of an anoxic hypolimnion in combination with mixing dynamics during overturn periods (i.e., in autumn and after ice break-up) determine the annual, diffusive methane emissions from small lakes; (4) apart from lake characteristics and abiotic conditions, differences in the composition of the microbial community (methane producing and oxidizing bacteria) between lakes may explain differences in the lake-wide methane emissions from lakes. These hypotheses will be tested by intensive field experiments measuring the internal dynamics, spatial heterogeneity, lake-wide distribution, and seasonal variability of dissolved methane and methane emissions together with abiotic conditions in and between small Swabian and Bavarian lakes. State of the art in-situ measuring techniques (e.g., an eddy-covariance system, single- and multi-beam echosounders, methane probes, automated diffusive flux chambers and funnels, oxygen- and carbon dioxide-optodes, CTD, and thermistors), will be combined with intensive water sampling and water sample analysis (dissolved methane, methane isotopic composition, composition of the microbial community, and other water constituents) to meet the requirements of sufficient temporal resolution and reliable absolute data on all abiotic parameters and in specific on the dissolved methane concentration.