Environmental parameters
In summer, precipitation and inflow were lower than in late autumn (Table 2). The air temperature was consistent with the temperate region, wind was low and cloud cover variable with no or limited precipitation in summer. In autumn, low precipitation was observed before the sampling campaigns (Table 2). The water level decreased by approximately 1.0 m in Klíčava and Římov and by 2.2 m in Žlutice between the summer and autumn sampling campaigns (Table 2). In all reservoirs, stable thermal stratification was established in summer when the surface water temperature exceeded 20 °C. In late autumn, surface water temperature was on average 6 °C and thermal stratification eroded (Supplementary Figure 1). The vertical distribution profiles of oxygen during summer differed between the three reservoirs. In Klíčava, a positive heterograde profile with a maximum oxygen concentration in the metalimnion (middle layer) and anoxic hypolimnion (deep layer) was observed. In contrast, a negative heterograde profile with a metalimnetic minimum and anoxic deep layers characterized oxygen stratification in Římov. In Žlutice, a clinograde oxygen profile with the maximum in the epilimnion (surface layer) and anoxic deep strata was observed (Supplementary Figure 2). In late autumn, the hypolimnetic layers remained anoxic in Klíčava and Římov, where weak thermal stratification persisted. Klíčava had the lowest ChlA concentration with summer maxima in the metalimnion (Supplementary Figure 3). In the other two reservoirs, inflowing nutrients resulted in high summer ChlA concentrations in the upstream parts (Supplementary Figure 3). The longest water age was in Klíčava (˃2 years), with little water exchange and complete mixing of inflow within the reservoir in the tributary zone (Supplementary Figure 4). The shortest water age was in Římov, where river water was directed above the metalimnetic layer and mixed with the epilimnion in summer, but in autumn river water was vertically mixed into the whole water column supplying flowing fresh water along the longitudinal reservoir profile (Supplementary Figure 4). The water age in Žlutice shows that the inflow mixed extensively with the epilimnetic layer in summer and the reservoir was mostly homogeneous in autumn (Supplementary Figure 4).