Limnol. Oceanogr., 44(7), 1999, 1655–1666

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 Dagstorpssjön, 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. Most YOY fishes are planktivorous and may reach high abundances in summer. Therefore, YOY fish are potentially important regulators of zooplankton communities (Whiteside et al. 1985; Cryer et al. 1986; Mills et al. 1987; De Vries and Stein 1992; Qin and Culver 1995), and their effects may cascade down to the phytoplankton (Mills et al. 1987; Kurmayer and Wanzenböck 1996). Bioenergetic models predict that in several species, including percids and cyprinids, YOY fish have a higher mass specific consumption rate than older fish (Post 1990; Hewett and Johnson 1992; Karås and Thoresson 1992) and that the mass specific consumption rate is at its highest in larval fish and then decreases during the first summer (Post 1990; Karjalainen et al. 1997). However, few and to a large extent contradicting data exist regarding the size and density at which YOY fish have an impact on zooplankton and if this effect cascades down to phytoplankton. Pond and enclosure experiments have shown that high densities of larval walleye (Stizostedion vitreum), juvenile gizzard shad (Dorosoma cepedianum), and European perch (P. fluviatilis) may affect daphnid zooplankters in early and midsummer (DeVries and Stein 1992; Qin and Culver 1995, 1996; Hülsmann and Mehner 1997). Long-term enclosure experiments have shown that YOY fish continue to affect daphnids throughout the summer (Post and McQueen 1987; Kurmayer and Wanzenböck 1996). In field studies, similar impacts of YOY fish on zooplankton have been observed in late summer (Cryer et al. 1986; Mills et al. 1987; but see Mehner et al. 1995) but not in early summer (Treasurer