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

The impact of young-of-the-year (YOY) fish predation on zooplankton dynamic s and cascading effects down to phytoplankton were studied in lake enclosur es using different densities of larval and juvenile European perch (Perca f luviatilis). Two experiments, one in June and one in August, were performed in Lake Dagstorpssjon, a slightly eutrophic lake in Scania, southern Swede n. In our experiments, we found that in August, there was a negative relati on between YOY perch density and cladoceran biomass and between YOY perch d ensity and cladoceran size. Moreover, in August, both rotifer and algal abu ndance increased with increasing YOY density, and species composition of al gae was affected. At high YOY densities, there was also a negative effect o n water transparency. In June, when growth of larval perch was slow and dap hnid abundance low, larval perch was not the major structuring force on eit her zooplankton or algal communities. In early summer, the likelihood of YO Y perch having an impact on cladoceran biomass is dependent on the timing o f many factors. One factor is the timing between larval occurrence and the increase in large cladoceran biomass that will enhance larval growth. In la te summer, however, a high enough predation pressure of juvenile fish will prevent recovery of large herbivores after the midsummer decline in zooplan kton abundance and will drive the algal community toward a late summer spec ies composition of small algal cells and blue-green algae.