Phytoplankton of the Slapy reservoir: seasonal changes and sedimentation losses

Seasonal development of phytoplankton in the central part of the eutrophic canyon-shaped Slapy Reservoir was studied in 2003-2007. Weekly sampling interval of the euphotic layer (0-4 m) revealed detailed information on the timing and shape of phytoplankton peaks, not clearly detected by previous monitoring carried out in three-week intervals. The phytoplankton biomass (expressed as chlorophyll a concentration) formed usually three sharp peaks during the vegetation period. In winters with little or no ice cover, the winter minimum was followed by an exponential increase under conditions of almost continuous mixing. After the onset of thermal stratification the increase continued due to markedly improved light conditions in the epilimnetic layer, until the first seasonal maximum was reached. After the sudden decline of the spring peak, the clear-water phase occurred in late May or early June. The pattern of the two summer peaks was especially in 2006 more complex, due to their dependence on a set of less predictable factors. Concerning species composition of peaks, a regular pattern was also observed. Typically, the spring maximum was formed by fast growing cryptomonad flagellates (Plagioselmis spp., Cryptomonas spp.) and small centric diatoms from various genera. The other two peaks were less..