POTENTIAL ROLE OF FISH PREDATION AND NATURAL-POPULATIONS OF ZOOPLANKTON IN STRUCTURING A PLANKTON COMMUNITY IN EUTROPHIC LAKE WATER

During April to September, plankton community structure was monitored in large enclosures (60 m3) in the presence and absence of planktivoro us fish. Weekly sampling included inorganic nutrients, oxygen, pH, and transparency as well as biomass of phytoplankton, bacteria, heterotro phic flagellates, rotifers, and macrozooplankton. In fishless enclosur es, concentrations of inorganic nutrients and transparency were higher than in enclosures with fish, whereas oxygen, pH, and biomasses of ph ytoplankton, picoplankton and rotifers were lower. As an average for t he entire period, Ch1 a was 14 mug liter-1 in the enclosures without f ish and 46 mug liter-1 in enclosures with fish. Macrozooplankton bioma ss was significantly lower in enclosures with fish than in enclosures without fish. Averaged over the entire period macrozooplankton biomass was 625 mug C liter-1 in enclosures with fish and 1, 149 mug C liter- 1 in those without fish. Cladocerans dominated the macrozooplankton co mmunity in fishless enclosures and controlled the biomass of bacteria and heterotrophic flagellates. The rotifer community was depressed in enclosures without fish, which probably was a combined effect of mecha nical interference with Daphnia and food competition. The initial buil dup of a high macrozooplankton biomass and the dominance of cladoceran s in the enclosures without fish, prevented cyanobacteria from forming large colonies. In contrast, cyanobacteria (Microcystis, Aphanizomeno n, and Anabaena) bloomed in the enclosures with fish. The observed cha nges in the plankton communities support the theory of cascading troph ic interactions.