Methane emissions from small lakes: Dynamics and distribution patterns (MethDyn)
Final Report Abstract
We have investigated diffusive fluxes at lake surfaces and ebullition fluxes of CH4 utilizing intensive field measurements and modelling approaches. Based on data from Lake Illmensee we demonstrated that diffusive CH4 fluxes estimated with the Cole & Caraco model agree well with flux chamber measurements during stratified but not during unstratified conditions. During unstratified conditions, K600, derived from flux chamber measurements combined with dissolved gas concentrations was substantially larger than during stratified conditions suggesting that near-surface convection may have enhanced K600. Although K600 determined from CO2 and CH4 fluxes were consistent under unstratified conditions, they deviated during stratified conditions, suggesting that concentration gradients near the surface may affect estimates of gas fluxes. In Lake Illmensee diffusive CH4 emissions during autumn overturn differ substantially between years and contribute 40% to 60% of the diffusive emissions between Mai and December. The CH4 emissions during overturn are affected by overturn dynamics and CH4 oxidation in the water column, but predominately depend on the mass of CH4 stored in anoxic deep water at the end of the stratification period. The data suggests that the mass of stored methane is strongly linked to the primary production during the season until the onset of the overturn and that the inter-annual variability of methane emissions during overturn is mainly controlled by the inter-annual difference in seasonal primary production. Thus, methane emissions from holomictic lakes that develop anoxic deep water during the stratified period can be expected to increase with higher nutrient loadings. Our numerical modeling results indicate that the CH4 mass balances in many lakes and reservoirs do not require oxic methanogenesis as a source to compensate CH4 emissions to the atmosphere during stratified conditions. In contrast, field data and modelling results suggest that reasonable CH4 fluxes from sediments in shallow waters are sufficient to explain diffusive CH4 emissions from lakes and reservoirs, and also explain the seasonal changes in CH4 concentrations and CH4 distributions in the surface waters of lakes. Hence, the modelling results support the hypothesis that diffusive fluxes from shallow sediments and not oxic methanogenesis are the main source of the CH4 emitted from lakes and reservoirs during stratified conditions. According to the applications of a 1-D model simulating methane emissions from Arctic lakes in response to climate warming, methane concentrations and emissions will increase in a warmer climate. In the simulations, the main causes for the increase in the emissions are warmer sediment temperatures enhancing CH4 production and shorter duration of ice cover leading to longer time periods for diffusive emissions and ebullition flux to the atmosphere. The results from this modelling study indicate that the feedback between CH4 dynamics and climate change is important for the assessment of future, atmospheric GHG concentrations and thus of future warming. Measurements of ebullition fluxes of CH4 from numerous active seeps in Upper Lake Constance revealed that many of the seeps investigated had ebullition fluxes orders of magnitude larger than the typical diffusive fluxes in lakes. During weeks to months ebullition fluxes from individual seeps can be rather constant. The ebullition fluxes are modulated by changes in ambient pressure leading to e.g. daily changes in ebullition fluxes with daily changes in atmospheric pressure and long-term changes due to water-level fluctuations altering hydrostatic pressure. However, some seeps show sudden shifts in the ebullition flux and others only intermittent phases of ebullition. Thus, the establishment of relations between the ebullition flux and properties of seep location and environmental conditions is difficult. Furthermore, ebullition fluxes are spatially very heterogeneous. Hence, overall assessment of the ebullition flux from more than 1,100 active seeps is rather uncertain. Using the average of the mean ebullition flux determined for 42 different seeps provides a crude estimate of the annual release of CH4 from 1100 active seeps in Lake Constance on the order 130 t CH4 yr^-1. This ebullition flux from seeps provides about 50 to 100 times more methane than released annually by diffusive emissions from Lake Illmensee and ~ 40% of the annual diffusive emissions from Upper Lake Constance. Thus, ebullition from seeps, which is only part of the overall ebullition flux in Lake Constance, constitutes a considerable source of CH4, which has not yet been included in the global estimates of CH4 emissions from lakes.
Publications
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(2019). Sediment fluxes rather than oxic methanogenesis explain diffusive CH4 emissions from lakes and reservoirs. Nature – Scientific Reports 9: 42
Peeters, F., J. Encinas-Fernandez, and H. Hofmann
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(2021) Interannual variability of methane storage and emission during autumn overturn in a small lake. Journal of Geophysical Research:Biogeosciences
Ragg, R., F. Peeters, J. Ingwersen, P. Teiber-Siessegger, and H. Hofmann
