Abstract. CH4 emissions from reservoirs are responsible for the majority of the atmospheric climatic forcing of these aquatic ecosystems, comparable to emissions from paddies or biomass burning. Primarily, CH4 is produced during the anaerobic mineralization of organic carbon in the anoxic sediments by methanogenic archaea. However, the origin of the recurrent and ubiquitous CH4 supersaturation in oxic waters (i.e., methane paradox) is still controversial. Here, we determined the dissolved CH4 concentration in the water column of twelve reservoirs during the summer stratification and the winter mixing. We obtained that the dissolved CH4 concentration varied up to four orders of magnitude (0.02–213.64 μM), and all depths were consistently supersaturated (710–7082234 %) in both periods. Phytoplanktonic sources of carbon appear to determine the concentration of CH4 in the reservoirs. In the anoxic waters, the depth-cumulative chlorophyll-a concentration, a proxy for the total phytoplanktonic biomass exported to sediments, determined the CH4 concentration. In the oxic waters, the photosynthetic picoeukaryotes abundance significantly determined the dissolved CH4 concentration both during the stratification and the mixing. The mean depth of the reservoirs, as a surrogate of the CH4 transport from sediment to the oxic waters, also contributed in shallow systems. Our findings suggest that photosynthetic picoeukaryotes can have a significant role in determining the CH4 concentration in oxic waters and, in comparison to cyanobacteria, have been poorly explored as CH4 sources.