Lack of a top-down effect on the zooplankton community after a cyprinid reduction

Abundance, biomass and sizes of crustaceans were followed before, during an d after a fish reduction in the three basins of Lake Ringsjon, southern Swe den. Zooplankton were sampled over the summer season (May-September) for ei ght years. Although the fish reduction occurred in different years in the t hree basins, the zooplankton community did not differ between basins. Betwe en-year differences in biomass occurred in all basins, but these were not c orrelated with the time before and after the fish reduction, with the excep tion of the Eubosmina species. Analyses were performed on total biomass, as well as on biomass of Daphnia galeata, D. cucullata, Bosmina longirostris, Eubosmina coregoni, E. crassicornis, calanoid and cyclopoid copepods. Both calanoid copepods and Daphnia cucullata were larger before, than after the fish reduction. The same was true for a proportional length measurement of all grazers (=cladocerans+calanoid copepods). We found no evidence of a to p-down effect on zooplankton biomass or length following the reduction in p lanktivorous fish. The possibility of an 'indirect bottom-up effect' due to cyprinid fish directly affecting nutrient availability is discussed.