Seasonal differences and potential biological drivers of the methane paradox in two peri-Alpine lakes

Seasonal variations and the biological drivers underlying the methane paradox in freshwater lakes are poorly understood. Here, we investigated the relationship between subsurface methane inventories and phytoplankton in two peri-Alpine lakes across different seasons. Surface waters of both lakes were consistently saturated in methane, with maxima reaching 570 and 205 nmol L^-1 during summer in the metalimnion of lakes Mondsee and Attersee, respectively. Methane concentrations were positively correlated with phytoplankton abundance in meso-oligotrophic lake Mondsee but not in ultra-oligotrophic lake Attersee. However, although phytoplankton peaked in abundance at the methane maxima in Mondsee, incubation experiments with ¹³C-labeled bicarbonate revealed negligible methane production from primary productivity. Instead, our results suggest that phytoplankton might only be indirectly involved in methane production through alternative pathways, or by providing precursor compounds to other members of the microbial community in these oligotrophic lakes.