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).