The impact of larval and juvenile fish on zooplankton and algal dynamics

The impact of young-of-the-year (YOY) fish predation on zooplankton dynamics and cascading effects down to phytoplankton were studied in lake enclosures using different densities of larval and juvenile European perch (Perca fluviatilis). Two experiments, one in June and one in August, were performed in Lake Dagstorpssjon, a slightly eutrophic lake in Scania, southern Sweden. In our experiments, we found that in August, there was a negative relation between YOY perch density and cladoceran biomass and between YOY perch density and cladoceran size. Moreover, in August, both rotifer and algal abundance increased with increasing YOY density, and species composition of algae was affected. At high YOY densities, there was also a negative effect on water transparency. In June, when growth of larval perch was slow and daphnid abundance low, larval perch was not the major structuring force on either zooplankton or algal communities. In early summer, the likelihood of YOY perch having an impact on cladoceran biomass is dependent on the timing of many factors. One factor is the timing between larval occurrence and the increase in large cladoceran biomass that will enhance larval growth. In late summer, however, a high enough predation pressure of juvenile fish will prevent recovery of large herbivores after the midsummer decline in zooplankton abundance and will drive the algal community toward a late summer species composition of small algal cells and blue-green algae.