Final Report Summary - RECONMET (Reconstruction of methane flux from lakes: development and application of a new approach)
Methane is an important greenhouse gas and lakes are a relevant source of methane to the atmosphere. Currently, limited information is available about how methane concentrations and methane cycling in lakes have responded to past changes in climate or environmental conditions. It is therefore challenging to predict how the methane emissions of lakes will respond to future environmental change. Methane oxidizing bacteria (MOB) incorporate carbon from methane and, as a consequence, are characterized by a distinctive carbon isotopic composition. In lakes where MOB are a relevant food source this also affects the carbon isotopic composition of some aquatic invertebrates and organic fossilizing structures produced by these organisms. Within RECONMET stable isotope analyses on chitinous invertebrate remains were developed as a tool for examining past variations in methane abundance and methane cycling in lakes. Laboratory experiments confirmed that the carbon isotopic composition of food ingested by invertebrates determines the isotopic composition of not only their soft tissue, but also the fossilizing structures (e.g. exoskeleton fragments, resting eggs) that can be found in the sediments. In a survey of small lakes in western, central and northern Europe a wide range of invertebrate remains were screened to identify groups that are characterized by a carbon isotopic composition typical for organisms that feed on MOB, and that are related in their carbon isotopic composition with in-lake methane concentrations. The results indicate that isotopic analyses of the fossils of a number of invertebrate groups, especially filter-feeding taxa, have the potential to be used to reconstruct past variations in methane and carbon cycling in lakes. Down-core sediment records developed within the project show that the relevance of methane-derived carbon in the foodweb of small lakes can respond dynamically to changes in nutrient concentration, land use, oxygen concentrations and climate. Lags between environmental shifts and major changes in the relevance of methane-derived carbon in the foodwebs in the order of decades to centuries were observed. This suggests that non-linear responses should be expected of the methane cycle of lakes to external forcing. The project results set the stage for a systematic application of carbon isotopic analyses on chitinous remains from different invertebrate groups for reconstructing past changes in methane cycling at a larger number of sites and over a range of time scales. In this way, the project results allow the development of detailed information on how the abundance of methane in lakes, and the carbon sources of lacustrine foodwebs, are expected to change as a consequence of climatic change and human impact on lake ecosystems.