The long
stratification period and high light availability led to high phytoplankton biomass (110.8
fresh-weight mg L1) in the epilimnion during summer. The reservoir showed P-limitation
for phytoplankton growth in this period. All functional groups included one or more
species (X2-Rhodomonas spp.; Y-Cryptomonas spp.; F-Oocystis lacustris; K-Aphanocapsa spp.)
selected by resources, especially phosphorus. Species of Cryptomonas (group Y) dominated
during the mixing period (winter season) in conditions of low light and relatively high
availability of dissolved nutrients. Increases in water-column stability during spring
stratification led to phytoplankton biomass increases due to the dominance of small
flagellate functional groups (X2 and X3, chrysophyceans). The colonial chlorophycean O.
lacustris (group F) peaked during the mid-summer stratification, when the mixed epilimnion
was clearly depleted in nutrients, especially SRP. High temperature and increases in
nutrient concentration during the end-summer and mid-autumn resulted in a decrease of
green algae (group F) and increase of Aphanocapsa spp. (cyanobacteria, group K) and
dinoflagellates (group Lo). The study also revealed the important role of physical processes
in the seasonal gradient, in selecting phytoplankton functional groups, and consequently
in the assessment of ecological status.