Effects of invading vendace (Coregonus albula L.) on species composition and body size in two zooplankton communities of the Pasvik River System, northern Norway

Species composition and body-size distribution were studied in the crustacean zooplankton communities of two limnologically similar lake localities situated 50 km apart in the Pasvik River System, northern Norway. A recent invasion and successive downstream expansion of vendace (Coregonus albula), a specialized zooplanktivorous fish, allowed comparisons between sites with different predation pressures. Vendace had established a high population density and was the dominant fish species in the pelagic of the upper locality, but had just invaded the lower locality with a small number of individuals. Whitefish (Coregonus lavaretus), a closely related but less specialized zooplanktivore species, dominated the native fish community of both lakes. The zooplankton community of the upper locality was dominated in June and August by Bosmina longirostris, the smallest zooplankton species represented in the water course, and in September by Daphnia cristata. The lower locality was dominated by the larger Holopedium gibberum and Eudiaptomus graciloides in June, by D.cristata in August, and by D.cristata and B.longirostris in September. The mean body size of the three most abundant cladoceran species was significantly smaller in the upper locality, compared to the lower locality. It was concluded that the invasion and establishment of a dense vendace population in the upper locality had increased the predation pressure in the pelagic, resulting in a reduction of body size and a shift towards smaller species in the zooplankton community.