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Revising the methane cycling in lakes: sources and sinks in two German lakes with specific consideration of methane accumulation in oxic waters

Subject Area Hydrogeology, Hydrology, Limnology, Urban Water Management, Water Chemistry, Integrated Water Resources Management
Microbial Ecology and Applied Microbiology
Mineralogy, Petrology and Geochemistry
Term from 2013 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 241479293
 
Final Report Year 2023

Final Report Abstract

In light of the recently discovered methane (CH4 ) accumulation in oxic waters, it is crucial to understand the precise CH4 production mechanisms under varying environmental conditions. This understanding is essential for better estimating its contribution to the global methane budget and reducing uncertainties regarding CH4 sources and sinks. Previously, it was assumed that the magnitude of CH4 accumulation in the water column primarily depended on complex interactions among phytoplankton, methanogenic Archaea, and methanotrophic bacteria. However, the widely reported 'methane paradox' in oceans and lakes, characterized by CH4 oversaturation in the oxygenated mid-water column, challenges the prevailing paradigm of exclusive anoxic methanogenesis via classical methanogenesis. The overall aim of this research was to comprehensively characterize the complete CH4 cycle in two contrasting but common lake types by quantifying the CH4 bulk sources and sinks using a detailed mass balance approach, combined with multiple stable isotope measurements and in situ incubation experiments. Generally, we closely followed the project according to the proposed research plan and most aspects of the work packages were addressed successfully. Our research project provides a comprehensive multi-year dataset of field work investigating sources and sinks of CH4 in the sediment and water column of lakes Stechlin and Willersinnweiher combining concentrations as well as stable carbon (δ13C-CH4 ) and hydrogen (δ2H-CH4 ) values and redox/nutrient status during seasonal variations between 2019 and 2021 as well as laboratory experiments where we identified precursor compounds of oxic methane production (OMP) in both lakes. Furthermore, our project opened up new avenues regarding CH4 formation from cyanobacteria and CH4 formation in the oxic epilimnion of marine and freshwater environments. This work was complemented with measurements of stable carbon isotope signatures and fractionations of CH4 emitted from phytoplankton. In addition, we performed 3-D hydrodynamical modelling to unravel the lateral flux of CH4 emitted from littoral sources (submitted) to obtain an improved certainty of our CH4 mass balance and OMP contribution to CH4 emissions from Lake Stechlin. Most of our results are published in peer-reviewed journals (21 publications in Nature, Nature Communications, Science Advances, L&O, etc., 5 are common publications between the groups, 2 further manuscripts are submitted). Nevertheless, there was work that could not be performed. This was the case with our proposed photosynthesis inhibitor experiments (part of work package 3 D “Role of photosynthesis - inhibitor experiments”). This was mainly due to the corona pandemic leading to logistic problems and time constraints regarding the collection of field samples and the availability of axenic cultures of cyanobacteria and green algae. We hope we can address these issues in the near future.

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