PHYTOPLANKTON AND PHYSICAL-CHEMICAL FEATURES OF THE SHALLOW LAKE MIKRI-PRESPA, MACEDONIA, GREECE

The structure and dynamics of phytoplankton assemblages in the shallow Lake Mikri Prespa were studied during the period from May 1990 to September 1992. Biomass values varied between 1.2 and 38.0 g m-3 and revealed one autumnal and one vernal peak. Cyanophytes, the most important algal group (mean annual contribution was 78.6% and 49.8% in 1990-1991 and 1991-1992, respectively) remained dominant almost throughout the year. Diatoms, the second abundant group (mean annual contribution was 15.3% and 34.9% in the successive years, respectively) dominated in winter and spring. All other classes contributed to a small degree to the total biomass. The periodicity of the phytoplankton and the height of annual peaks resembled those of many temperature eutrophic lakes but the absence of a summer maximum was opposed to the PEG model statement that eutrophic systems are characterized by increased summer biomass. Temperature, dissolved inorganic nitrogen and dissolved silica appeared to be the main of the studied factors controlling the development of cyanophytes and diatoms. Changes in weather conditions, especially rainfall, seemed to have a strong impact on the species seasonality.