Lay summary
Methane emissions from lakes and reservoirs to the atmosphere contribute significantly to the global methane cycle . Lakes and reservoirs have so far not been part of global budgets but they could contribute up to 10% to the atmospheric flux. Such estimates are poorly constrained because methane is produced at highly variable rates in anoxic freshwater sediments, it can be oxidized in the sediment and in the water column, and transferred to the atmosphere either by diffusion or by rising bubbles.Over the last years, several pathways of methane oxidation in the absence of oxygen have been discovered. Anaerobic methane oxidation via sulfate reduction is now firmly established for marine systems. Recent laboratory evidence further proves that microbial methane oxidation is also proceeding with nitrate, nitrite, iron- and manganese oxides as electron acceptors. These new pathways could stabilize methane oxidation rates under fluctuating oxygen conditions in lakes. In this project we will address the following research questionsoWhich pathways of methane oxidation are operating at different redox environments in lakes and which fraction of methane is transformed by different electron acceptors?oHow are the concentrations and the fluxes of methane, oxidizing agents and micronutrients like copper controlling oxidation pathways and their rates?oWhat are the consequences of such coupled redox cascades for the overall methane oxidation, for the isotopic signature of methane, and for microbial production in the absence of light?We will address these questions with field studies in four selected lakes in Switzerland, Spain, Central Africa and Western Canada which exhibit redox interfaces in the water column with contrasting boundary conditions. The lakes differ in the availability of methane, their redox conditions, the vertical mixing regime. Our biogeochemical analysis will combine our expertise in newly established high-resolution profiling and sampling techniques, our experience in stable isotope and biomarker analysis, and our know-how in reaction-transport modeling. By focusing on consistent biogeochemical analysis of methane oxidation across several systems the project aims at providing general insights and reference data for improved global estimates.