Potential drivers and seasonal comparison of the methane paradox in three Austrian peri-Alpine lakes

Methane, traditionally thought to be produced only under anoxic conditions, is widely observed in oxic surface layers of freshwater lakes-a phenomenon known as "methane paradox". The methane paradox results from a complex interaction of biotic and abiotic processes which could vary substantially across different lacustrine systems. The variability in biological and geochemical characteristics of lakes can influence methane production and transport, limiting our understanding of the main drivers sustaining elevated methane concentrations in oxic surface waters.

In this study, we investigated the methane paradox in three Austrian peri-alpine lakes differing in size and trophic state, and compared the factors controlling oxic methane production in these lakes during different seasons. Two of the studied lakes, Mondsee (14.2 km²) and Attersee (49.5 km²), are located within the same catchment area. Lake Mondsee is mesotrophic and lake Attersee ultra-oligotrophic. Lake Lunzsee is oligotrophic, and the smallest lake studied (0.7 km²). Elevated methane concentrations were observed in all three lakes during both summer and autumn seasons indicating year-round occurrence of the methane paradox in the lakes. Subsurface methane concentrations ranged from 100 to 400 nM which was substantially higher than the atmospheric equilibrium (~3 nM), indicating oxic methane production as a potential, yet unaccounted, source of methane to the atmosphere. Positive correlations of methane concentrations with chlorophyll-a and ammonium concentrations suggested a link with biological activity. Additionally, high phytoplankton abundances coincided with the methane maximum, further indicating that primary productivity was one of the main drivers associated with oxic methane production. Methane concentrations were the highest in mesotrophic lake Mondsee, which was dominated by cyanobacterial phytoplankton. In contrast, the phytoplankton composition in lake Attersee and Lunzsee was mainly composed of eukaryotic species.

Our findings indicate that the magnitude of subsurface methane concentrations in peri-Alpine lakes is influenced by nutrient availability, which is one of the key factors determining phytoplankton taxonomic composition. Our results demand a further investigation of oxic methane production pathways associated with different phytoplankton taxa to better understand how future eutrophication events might affect methane dynamics in peri-Alpine lakes.