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.