Physical and Biogeochemical Processes Driving Methane Sources, Sinks and Emissions in Aquatic Systems: The Past, Present and Future under Global Change

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.

Inland waters are important global sources, and occasional sinks, of CO₂, CH₄, and N₂O to the atmosphere, but relatively little is known about the contribution of GHGs of constructed waterbodies, particularly small sites in agricultural regions that receive large amounts of nutrients (carbon, nitrogen, phosphorus). Here, we quantify the magnitude and controls of diffusive CO₂, CH₄, and N₂O fluxes from 20 agricultural reservoirs on seasonal and diel timescales. All gases exhibited consistent seasonal trends, with CO₂ concentrations highest in spring and fall and lowest in mid-summer, CH₄ highest in mid-summer, and N₂O elevated in spring following ice-off. No discernible diel trends were observed for GHG content. Analyses of GHG covariance with potential regulatory factors were conducted using generalized additive models (GAMs) that revealed CO₂ concentrations were affected primarily by factors related to benthic respiration, including dissolved oxygen (DO), dissolved inorganic nitrogen (DIN), dissolved organic carbon (DOC), stratification strength, and water source (as δ¹⁸O_water). In contrast, variation in CH₄ content was correlated positively with factors that favoured methanogenesis, and so varied inversely with DO, soluble reactive phosphorus (SRP), and conductivity (a proxy for sulfate content), and positively with DIN, DOC, and temperature. Finally, N₂O concentrations were driven mainly by variation in reservoir mixing (as buoyancy frequency), and were correlated positively with DO, SRP, and DIN levels and negatively with pH and stratification strength. Estimates of mean CO₂-eq flux during the open-water period ranged from 5,520 mmol m⁻² year¹ (using GAM-predictions) to 10,445 mmol m⁻² year⁻¹ (using interpolations of seasonal data) reflecting how extreme values were extrapolated, with true annual flux rates likely falling between these two estimates.