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Tonya DelSontro
Department of Earth and Environmental Sciences
Introduction
Atmospheric concentrations of methane have more than doubled during the industrial era. The global warming potential of methane is about 80 times higher than carbon dioxide, thus making reducing emissions a priority for mitigating climate change. Lakes represent about 25 percent of natural methane sources, but large uncertainties remain about the contribution of internal sources and sinks.
Contrasting the paradigm that methane is only produced in anoxic conditions, recent discoveries show that oxic methane production (OMP) occurs in surface waters worldwide. OMP drivers and their contribution to global lake methane emissions remain unclear. While studies show the occurrence of OMP in lakes across geographic and trophic gradients, OMP has not been investigated in pre-alpine lakes which are disproportionately experiencing climate change. The study calculates comprehensive methane budgets for four adjacent Swiss pre-alpine lakes under identical climate forcing but with different trophic states.
Methodology
Net methane production rates (Pnet) were estimated for four pre-alpine lakes in the Swiss Alps: Lac de Bretaye, Lac Noir, Lac des Chavonnes and Lac Lioson, which are eutrophic, meso/eutrophic, mesotrophic and oligotrophic respectively. Pnet was defined as the balance between OMP (which adds methane) and methane oxidation (which removes methane) in the surface mixed layer, the component that contributes to diffusive emissions. Pnet in the surface mixed layer was estimated using two independent mass balance approaches: a 1-D lateral transport model (Figure 1a) and a 0-D full-scale mass balance (Figure 1b). The full-scale mass balance assumed the surface mixed layer as a well-mixed reactor where each component was based on measured values. The lateral transport model also used in situ measurements but estimated the diffusive flux to the atmosphere using the mass transfer coefficient with Pnet rates obtained by finding the simulated transect methane concentrations that best-fit the measured concentrations.
Figure 1: Conceptual schematic of the methane budget components in the surface mixed layer and methodological approaches. Methane mass balance components: diffusive methane emissions to the atmosphere (Fa), vertical transport (Fz), bubble dissolution (Rdis), littoral sediment flux (Fs).
Outcomes
In three study lakes, Pnet values were positive, indicating that OMP was greater than methane oxidation, and that Pnet acted as a methane source during daytime conditions over the stratified season. Pnet was near zero in Lac Chavonnes, a meso-oligotrophic lake which also had the largest water level changes throughout the summer (Figure 2).
Figure 2: Methane production (Pnet) rate estimations in the surface mixed layer of (a) eutrophic and (b) oligotrophic lakes using two approaches (full-scale mass balance, filled boxes; lateral transport model, open boxes). Boxes show the first and third quartiles with the median (line), whiskers extend to most extreme data point within 1.5 times the interquartile range from the box. The white dot represents the average of the Pnet distribution. Note different scales on y-axes of the two panels.
Pnet rates were temporally variable in each lake and varied between study sites. While Pnet was relatively constant during the stratified season in the oligotrophic lakes, highly positive Pnet rates in the eutrophic lakes at the beginning of the summer indicated that OMP was an active source of methane to the atmosphere. The eutrophic lakes had Pnet rates one order magnitude higher than the more oligotrophic lakes, suggesting that Pnet may also be related to trophic state. The dominant sources of methane to lake surface waters were Pnet and littoral sediment flux, but results suggested no relationship between the contribution of these sources and trophic state even though each were higher in more productive systems.
Considering the importance of Pnet contributions to atmospheric methane emissions, defining approaches to estimate and upscale Pnet are critical. While it is plausible that the OMP proportion to diffusive emissions may partially depend on lake bathymetry, study results indicate that OMP is a complex phenomenon that is also related to lake trophic properties. The study observed that Pnet for an individual lake can be explained mostly by changes in light climate (LC) which defines the average light intensity that phytoplankton can be exposed to in the surface mixed layer during the day. The study found that increases in LC strongly increase Pnet rates in eutrophic lakes, whereas Pnet is nearly independent of LC in oligotrophic lakes (Figure 3a). A more robust empirical approach using additional trophic state parameters -chlorophyll a concentration and Secchi depth - was proposed to upscale Pnet in a variety of lake ecosystems (Figure 3b), once again highlighting the interaction between trophic state and methane in aquatic systems.
Figure 3: Linking net methane production (Pnet) in the surface mixed layer with trophic variables.
a Relationship between Pnet and light climate (LC, m m−1) and trophic state. The minimum Pnet rate (Pnet,min) and the minimum LC (LCmin) were subtracted in each lake to be able to compare the slope of each curve. Pnet becomes more independent of LC in more oligotrophic lakes. b Interaction between Pnet and the average surface concentration of chlorophyll-a, LC and Secchi depth (Zs, m) suggest a direct role of photosynthesis on OMP.
Conclusions
The study quantified Pnet rates in the oxic surface mixed layer of four pre-alpine lakes using two models that have previously produced contradictory results when resolving OMP in lowland lakes. Results indicated that three of four lakes had a positive Pnet responsible for up to 85 percent of atmospheric methane emissions occurring at the beginning of summer. Good agreement between mass balance approaches showed that there are no methodological issues with the models when appropriate boundary conditions are used.
While OMP mechanisms need further investigation, the study showed that light and photoautotrophs may play a significant role. Consequently, future changes in light availability and temperature may induce positive feedbacks by promoting algal species capable of producing methane. Although the contribution of OMP to total diffusive emissions from inland waters is not well constrained, the study showed that it can be a dominant source from pre-alpine lakes where climatic changes are occurring at higher rates than the global average. It is therefore important to continue quantifying the contribution of Pnet from various aquatic systems and identify the main drivers of OMP to better understand the impact of OMP on the global methane cycle and how to predict and mitigate its impact in a changing climate.
Ordóñez, C., DelSontro, T., Langenegger, T., Donis, D., Suarez, E. L., McGinnis, D.F. Evaluation of the methane paradox in four adjacent pre-alpine lakes across a trophic gradient. Nature Communications 14, 2165 (2023). https://doi.org/10.1038/s41467-023-37861-7
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