Elisabet Perez-Coronel, Stephen C. Hart, J. Michael Beman

Freshwater lakes are important but poorly constrained sources of methane to the atmosphere due to high, but variable, rates of methane production, as well as limited and inconsistent measurements worldwide. High-elevation lakes have been particularly overlooked—despite their large numbers in mountain ranges around the world, and despite methane dynamics at high elevations may be altered by rapid increases in temperature due to climate change. We examined variations in surface methane concentrations and diffusive fluxes, temperature, dissolved organic matter (DOC), and inorganic nutrients in five montane lakes spanning multiple elevations in the Sierra Nevada of California. Over two years, we found strong and consistent seasonality in methane concentrations in lakes; higher concentrations were typically observed in the warmest months and lower concentrations in fall. Changes in methane concentrations were significantly
related to temperature in the majority of the individual lakes (r2 = 0.43-0.81) and related to elevation (r2 = 0.39) and DOC (r2 = 0.30) across lakes. Methane concentrations in lakes at elevations <3000 m were strongly related to temperature, nitrite concentrations and elevation (r2 = 0.90), whereas at elevations >3000 m, methane correlated with dissolved inorganic
nitrogen to dissolved inorganic phosphorus ratios and elevation (r2 =0.48). Our results expand on our understanding of temporal variations in methane and demonstrate substantial seasonality in methane concentrations and diffusive fluxes in freshwater lakes—suggesting that temporal variation should be considered in large-scale estimates, and may be a predictable function of elevation, temperature, organic matter, and nutrients.