Abstract. Methane (CH4) emissions from reservoirs are
responsible for most 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 anoxic sediments by methanogenic archaea. However, the
origin of the recurrent and ubiquitous CH4 supersaturation in oxic
waters (i.e., the methane paradox) is still controversial. Here, we
determined the dissolved CH4 concentration in the water column of
12 reservoirs during summer stratification and winter mixing to explore
CH4 sources in oxic waters. Reservoir sizes ranged from 1.18 to 26.13 km2. We found that dissolved CH4 in the water column varied
by up to 4 orders of magnitude (0.02–213.64 µmol L−1), and all oxic depths
were consistently supersaturated in both periods. Phytoplanktonic sources
appear to determine the concentration of CH4 in these reservoirs
primarily. In anoxic waters, the depth-cumulative chlorophyll a
concentration, a proxy for the phytoplanktonic biomass exported to
sediments, was correlated to CH4 concentration. In oxic waters, the
photosynthetic picoeukaryotes' abundance was significantly correlated to the
dissolved CH4 concentration during both the stratification and the
mixing. The mean depth of the reservoirs, as a surrogate of the vertical
CH4 transport from sediment to the oxic waters, also contributed
notably to the CH4 concentration in oxic waters. Our findings suggest
that photosynthetic picoeukaryotes can play a significant role in
determining CH4 concentration in oxic waters, although their role as
CH4 sources to explain the methane paradox has been poorly explored.