SUMMER MAXIMA OF PHYTOPLANKTON IN THE RIMOV RESERVOIR IN RELATION TO HYDROLOGIC PARAMETERS AND PHOSPHORUS LOADING

The Rimov Reservoir (Czech Republic) is a eutrophic, stratified, deep- valley reservoir with large inter-seasonal fluctuations in the occurre nce of summer algal blooms. Its phytoplankton is moderately to strongl y limited by phosphorus during summer period. The results of ten-year monitoring of phytoplankton, phosphorus loading, and related hydrologi c and climatic factors were evaluated in order to elucidate principles of phytoplankton growth in this reservoir. The study revealed that av erage summer concentrations of chlorophyll-a and fresh mass in the lac ustrine part were linearly and positively correlated with the dissolve d phosphorus load into the epilimnion, caused by surface discharges. T his type of load did not result in a signifi cant increase of total ph osphorus concentration in the lacustrine part of the reservoir. Howeve r, it caused shifts in the structure of the phytoplankton community to wards the prevalence of net phytoplankton species >40 mu m which were able to accumulate in large biomasses at low demands for phosphorus. T he dominant large-sized species were represented mainly by cyanobacter ia (Microcystis aeruginosa, Aphanizomenon flos-aquae), except for 1993 , when a green alga (Staurastrum planctonicum) dominated.