Species composition and seasonal cycles of phytoplankton with special reference to the nanoplankton of Lake Mikri Prespa

The phytoplankton of Lake Mikri Prespa was studied at monthly or biweekly intervals during the period May 1990-September 1992. Its species composition, consisting of a great number of cyanophytes and a very small number of chrysophytes and desmids, may reflect the eutrophic character of the lake. Moreover, the mean annual biomass values (15.0 and 3.2 g m(-3) in the two years, respectively) and the maximum biomass (38.1, 6.4 and 9.6 g m(-3)), classify Mikri Prespa as a eutrophic lake. A tendency towards a double-peaked pattern of biomass distribution in time with one peak in autumn, composed mainly of cyanophytes, and another in spring made up of diatoms, was observed. This pattern contrasts with the standard pattern in eutrophic, stratified temperate lakes, which exhibit a third biomass maximum in summer. Cyanophytes were the mast important group in terms of biomass and were dominated by the species Microcystis aeruginosa, Microcystis wesenbergii, Anabaena lemmermannii var. minor and Aphanocapsa elachista var. conferta. Diatoms constituted the second most important group, with main representative the species Cyclotella ocellata. Cyanophytes, diatoms, chlorophytes and dinophytes revealed annual periodicity whereas the other algal groups did not show any seasonality at all. The nanoplankton constituted an important part of algal biomass (38.9 and 49.9% in the two years, respectively) and revealed annual periodicity with maximum values in winter and spring, mainly composed of diatoms and cryptophytes. Low temperature, increased rainfall and high DIN concentrations seemed to be the main factors influencing the seasonality. Although the percentage contribution of nanoplankton decreased with the increase in total biomass, justifying the classification of Lake Mikri Prespa among the eutrophic lakes, the nanoplankton biomass did not correlate significantly with total phytoplankton biomass.