TOP-DOWN EFFECTS OF UNDERYEARLING FISH ON A PHYTOPLANKTON COMMUNITY

1. The influence of underyearling (0+) fish on planktonic algal biomas s and composition was examined during June-August in eight large enclo sures (40 m(3)). Averaged for the entire period, Daphnia densities, tr ansparency and phosphate concentrations were significantly higher (P < 0.05) and chlorophyll a values significantly lower in fishless control s than in enclosures with fish (bleak (Alburnus alburnus), roach (Ruti lus rutilus) or perch (Perca fluviatilis)). 2. Enclosures without fish demonstrated a more heterogeneous algal community structure than encl osures with fish. Desmidiales and dinoflagellates were more abundant i n controls, whereas green algae dominated the phytoplankton community in enclosures with fish. Larger grazing-resistant algae occurred most frequently in enclosures without fish. 3. The importance of cascading trophic interactions was demonstrated even under strong eutrophic cond itions. The strongest effects on Daphnia densities, phytoplankton biom ass and nutrients were observed in enclosures with perch, with weaker effects in enclosures with cyprinids. Differences in initial fish size and species accounted for the top-down effects observed. The results of this study emphasize the importance of 0+ perch as a key predator i n structuring the plankton community in lakes.