Application of concentration and 2-dimensional stable isotope measurements of methane to constrain sources and sinks in a seasonally stratified freshwater lake

Abstract

Methane (CH4) emissions from aquatic systems have recently been comprised to account for up to 50% of global CH4 emissions, with lakes representing one of the largest CH4 sources within this pool. However, there is large uncertainty associated with CH4 emissions from freshwater environments to the atmosphere, because of a lack of understanding in the spatial and temporal dynamics of CH4 sources and sinks, as well as underlying mechanisms and processes. In this study, we investigated the concentrations and stable carbon (δ13C-CH4) and hydrogen (δ2H-CH4) isotope composition of CH4 in a small eutrophic lake (Lake Willersinnweiher) with seasonal stratification and its spatial and temporal variation. We found that while supersaturation of CH4 in the entire water column was present throughout the whole year, the isotopic composition of CH4 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 CH4 differed from littoral and sedimentary CH4, suggesting likely mixing of CH4 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 CH4 oxidation indicated by a strong increase in both δ13C-CH4 and δ2H-CH4 values at the bottom of the oxycline was found to significantly reduce upward migrating CH4 released at the sediment-water interface. In the sediment, stable isotope characteristics of CH4 showed an increasing dominance of the acetoclastic CH4 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 δ13C-CH4 and δ2H-CH4 values. During the mixing period, the isotopic CH4 composition of the water column was distinctively less negative than during the stratification period potentially resulting from a greater impact of groundwater CH4 input compared to the stratification period. Our findings implicate that the application of concentrations and dual isotope measurements of CH4 is a promising approach for constraining CH4 sinks and sources in Lake Willersinnweiher and potentially other small lakes to clearly disentangle the complex CH4 dynamics in lakes both spatially and seasonally.

Stable Carbon Isotope Signature of Methane Released From Phytoplankton

“…Similar to the observation of oxic CH 4 production in the surface waters of oceans, there has been a controversial discussion about the occurrence of CH 4 formation in the context of CH 4 supersaturation in the epilimnion of lakes (Bižić, Klintzsch, et al, 2020;Encinas Fernández et al, 2016;Grossart et al, 2011;Günthel et al, 2020;Hartmann et al, 2020;Morana et al, 2020;Peeters et al, 2019;Tang et al, 2014Tang et al, , 2016. The δ 13 C values of dissolved CH 4 in various studies have been used as a diagnostic tool, in order to assign the observed CH 4 supersaturation to one or multiple sources (Blees et al, 2015;Einzmann et al, 2022;Hartmann et al, 2020;Morana et al, 2020;Tang et al, 2014;Thottathil et al, 2022;Tsunogai et al, 2020). However, to our knowledge only two recent studies by Hartmann et al (2020) and Thottathil et al (2022), estimated the δ 13 C-CH 4_source values for the CH 4 produced in the oxic water column of Lake Stechlin in Germany and five lakes in Canada.…”

“…In addition, the δ 13 C-CH 4_source values of phytoplankton complement our understanding of isotopic carbon source signatures of CH 4 in the environment. In the context of the aquatic CH 4 paradox, the results could help to differentiate between CH 4 produced by methanogenic archaea in anoxic microsites, the intestinal tract of zooplankton or sedimentary sources, and those produced from phytoplankton as for example, recently applied by Einzmann et al (2022) to constrain sources and sinks of CH 4 in a small lake in Southern Germany.…”

“…Interestingly, a very recent study (Perez‐Coronel & Beman, 2022) that applied freshwater incubation experiments under different treatments suggested multiple sources act simultaneously to explain aerobic CH 4 production in aquatic environments. Several recent studies have applied stable isotope techniques to better constrain the origin and fate of CH 4 in lakes (Einzmann et al., 2022; Hartmann et al., 2020; Taenzer et al., 2020; Thottathil et al., 2022; Thottathil & Prairie, 2021; Tsunogai et al., 2020). The stable carbon isotope ratio ( 13 C/ 12 C) of CH 4 (expressed as δ 13 C‐CH 4 values) depends on the production, degradation, and transport processes within the aquatic system.…”

Enigmatic persistence of aerobic methanotrophs in oxygen-limiting freshwater habitats

“…2 ; e.g. [ 23 , 30 , 57 , 60 , 65 ]), although one previous study reported 13 CH 4 enrichment in anoxic waters of a stratified boreal lake [ 17 ]. Despite no evident change in stable isotopes of CH 4 , the presence of “Ca.…”

Stable Carbon Isotope Signature of Methane Released from Phytoplankton

“…Interestingly, a very recent study (Perez-Coronel & Beman, 2022) that applied freshwater incubation experiments under different treatments suggested multiple sources act simultaneously to explain aerobic CH 4 production in aquatic environments. Several recent studies have applied stable isotope techniques to better constrain the origin and fate of CH 4 in lakes (Einzmann et al, 2022;Hartmann et al, 2020;Taenzer et al, 2020;Thottathil & Prairie, 2021;Thottathil et al, 2022;Tsunogai et al, 2020). The stable carbon isotope ratio ( 13 C/ 12 C) of CH 4 (expressed as δ 13 C-CH 4 values) depends on the production, degradation, and transport processes within the aquatic system.…”