In the period 1990-1994 the average biomass of phytoplankton increased from 2.9 to 9.9 mg dm-3. In 1990 Cryptophyceae dominated (29% of total biomass), in 1991-1993 Bacillariophyceae (20.5-35%), and in 1994 Cyanophyta (39%). On the turn of winter and the beginning of spring a mass development of the diatoms Asterionella formosa Hass., Cyclotella sp., and Stephanodiscus hantzschii Grun. occurred every year. The diatom density was over 4000 ind. cm-3. On the turn of summer and the beginning of autumn cyanobacterial blooms were observed of Woronichinia naegeliana (Unger) Elenkin and Microcystis aeruginosa Kütz. with a density of 950 colonies cm-3.