THE PRINCIPAL REGULATING MECHANISM ON THE STRUCTURE OF THE PHYTOPLANKTON COMMUNITY IN A HIGH-MOUNTAIN LAKE

The changes in the structure of the phytoplankton community in an oligotrophic high mountain lake are analysed, highlighting the effects of nutrient availability, predation by herbivores and fungal parasitism. The phytoplankton community (made up of green algae and chrysophyceae) reached a biovolume-density of 206 . 10(3) mum3 ml-1 at the beginning of the ice-free period, followed by a continual decrease. This community developed in limiting nutrient conditions (mainly P.). The population of green algae suffered a fungal epidemic after the ice melt which lasted approximately three weeks. Oocystis lacustris CHOD, showed a loss rate due to parasitism between 0.002 and 0.42 d-1, which amounts to up to 58 % of the total losses of this population, whereas the losses of Chlorella sp. were unimportant (> 3.5 . 10(-3) d-1). The zooplankton reached a maximum value of biomass (200 mg m-3) at the end of the studied period. The filtration rates oscillated between 0.003 (July) and 0.412 d-1 (September) coinciding with the maximum development of the population of Daphnia pulicaria HRBACEK.