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 planktivorous fish. Weekly sampling included inorganic nutrients, oxygen, pH, and transparency as well as biomass of phytoplankton, bacteria, heterotrophic flagellates, rotifers, and macrozooplankton. In fishless enclosures, concentrations of inorganic nutrients and transparency were higher than in enclosures with fish, whereas oxygen, pH, and biomasses of phytoplankton, picoplankton and rotifers were lower. As an average for the entire period, Ch1 a was 14 mug liter-1 in the enclosures without fish and 46 mug liter-1 in enclosures with fish. Macrozooplankton biomass 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 community 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 mechanical interference with Daphnia and food competition. The initial buildup of a high macrozooplankton biomass and the dominance of cladocerans in the enclosures without fish, prevented cyanobacteria from forming large colonies. In contrast, cyanobacteria (Microcystis, Aphanizomenon, and Anabaena) bloomed in the enclosures with fish. The observed changes in the plankton communities support the theory of cascading trophic interactions.