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Methane Dynamics in Lakes

Hartmann, Jan Frederik

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Abstract

Within the framework of this study a fast and sensitive method for the continuous determination of methane (CH4) and its stable carbon isotopic values (δ13C-CH4) in surface waters was developed. The gas is extracted by applying a vacuum to a gas-liquid exchange membrane and measured by a portable cavity ring-down spectroscopy analyser (M-CRDS). The M-CRDS was calibrated and characterised for CH4 concentration and δ13C-CH4 with synthetic water standards. Deployments in the laboratory as well as during fieldwork showed a very good agreement of CH4 measured simultaneously by the M-CRDS and conventional analytical methods. Therefore, the M-CRDS provides the continuous analyses of dissolved CH4 concentrations and δ13C-CH4 values of surface water at a very high temporal resolution. The newly developed method (M-CRDS) was successfully deployed at three lakes with different trophic states: Lake Willersinnweiher (Germany), Lake Stechlin (Germany) and Lake Erken (Sweden). The studies revealed for the first time high spatial and temporal variability of CH4 behaviour in the water column and the sediments. In addition, these studies showed that the presence of anaerobic (AOM) and aerobic (MOx) oxidation of methane in the sediment and within the water column, respectively, is a very effective barrier to CH4 emission into the atmosphere in thermal stratified lakes. Nonetheless, CH4 oversaturation with respect to the atmosphere was observed in the surface water layer of all three lakes during all seasons of the year. Most surface water CH4 concentrations are derived by horizontal transport processes from littoral zones. Groundwater-fed lakes such as Lake Willersinnweiher might be enriched in CH4 by groundwater contributing to its methane pool. Surface water CH4 could also be produced in-situ by the photoautotroph community, directly linked to algae dynamics and algae abundances. The temporal and spatial variability of CH4 is thereby unambiguously controlled by CH4 accumulation within the oxic water layer and its CH4 loss to the atmosphere. These studies also demonstrated that wind-induced changes in the upper water column of lakes lead to highly variable CH4 emissions from lakes. Growing eutrophication and climate warming will both have major effects on the CH4 pool of lakes. Accordingly, CH4 emissions from freshwater environments will further increase in the future with far-reaching consequences for climate change.

Item Type: Dissertation
Supervisor: Isenbeck-Schröter, Prof. Dr. Margot
Place of Publication: Heidelberg
Date of thesis defense: 18 October 2018
Date Deposited: 11 Apr 2019 06:47
Date: 2019
Faculties / Institutes: Fakultät für Chemie und Geowissenschaften > Institut für Geowissenschaften
Subjects: 550 Earth sciences
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