Physical and Biogeochemical Investigations of the Methane Cycle in a Small, Eutrophic Lake (Soppensee, Switzerland)

Abstract

This thesis studied in-lake processes driving lake methane (CH4) cycling in a small eutrophic lake (Soppensee). Modelling of dissolved noble gas in sediments suggested that CH4 ebullition started ~175 yrs ago, which was confirmed by changes of bulk sediment carbonate, organic carbon and their stable isotopes. Current sediment CH4 production, ebullition and diffusion were measured, and a novel approach was developed to quantify ebullition based on the % CH4 in sediment bubbles. Mechanistic modelling further revealed that in Soppensee, ebullition has been enabled by high carbonate sedimentation. In the water column, a CH4 mass balance showed that lower turbulent diffusivity in the thermocline reduces the oxidized CH4 proportion. A novel analysis approach yielded highly resolved CH4 oxidation rates. The presented understandings of CH4 ebullition and oxidation contribute to improved predictions of lake CH4 emissions under changing climate and eutrophication and should be considered in lake and reservoir management.