Application of concentration and 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake
Teresa Einzmann, Moritz Schroll, Jan F. Kleint, Markus Greule, Frank Keppler
Abstract
Methane (CH 4 ) emissions from aquatic systems have recently been comprised to account for up to 50% of global CH 4 emissions, with lakes representing one of the largest CH 4 sources within this pool. However, there is large uncertainty associated with CH 4 emissions from freshwater environments to the atmosphere, because of a lack of understanding in the spatial and temporal dynamics of CH 4 sources and sinks, as well as underlying mechanisms and processes. In this study, we investigated the concentrations and stable carbon (δ 13 C-CH 4 ) and hydrogen (δ 2 H-CH 4 ) isotope composition of CH 4 in a small eutrophic lake (Lake Willersinnweiher) with seasonal stratification and its spatial and temporal variation. We found that while supersaturation of CH 4 in the entire water column was present throughout the whole year, the isotopic composition of CH 4 in sediment and water column varied depending on lake stratification, physiochemical conditions, and lake depth. During the stratification period, isotopic characteristics of pelagic surface water CH 4 differed from littoral and sedimentary CH 4 , suggesting likely mixing of CH 4 from different sources including vertical and lateral input as well as groundwater input and potentially oxic methane production in the mixed surface water layer. Aerobic CH 4 oxidation indicated by a strong increase in both δ 13 C-CH 4 and δ 2 H-CH 4 values at the bottom of the oxycline was found to significantly reduce upward migrating CH 4 released at the sediment-water interface. In the sediment, stable isotope characteristics of CH 4 showed an increasing dominance of the acetoclastic CH 4 formation pathway from the pelagic towards the littoral area. Furthermore, the occurrence of sulfate-dependent anaerobic methane oxidation in the sediment was suggested by an increase in δ 13 C-CH 4 and δ 2 H-CH 4 values. During the mixing period, the isotopic CH 4 composition of the water column was distinctively less negative than during the stratification period potentially resulting from a greater impact of groundwater CH 4 input compared to the stratification period. Our findings implicate that the application of concentrations and dual isotope measurements of CH 4 is a promising approach for constraining CH 4 sinks and sources in Lake Willersinnweiher and potentially other small lakes to clearly disentangle the complex CH 4 dynamics in lakes both spatially and seasonally.