EFFECTS OF INVADING VENDACE (COREGONUS-ALBULA L.) ON SPECIES COMPOSITION AND BODY-SIZE IN 2 ZOOPLANKTON COMMUNITIES OF THE PASVIK RIVER SYSTEM, NORTHERN NORWAY

Species composition and body-size distribution were studied in the cru stacean zooplankton communities of two limnologically similar lake loc alities situated 50 km apart in the Pasvik River System, northern Norw ay. A recent invasion and successive downstream expansion of vendace ( Coregonus albula), a specialized zooplanktivorous fish, allowed compar isons between sites with different predation pressures. Vendace had es tablished a high population density and was the dominant fish species in the pelagic of the upper locality, but had just invaded the lower l ocality with a small number of individuals. Whitefish (Coregonus lavar etus), a closely related but less specialized zooplanktivore species, dominated the native fish community of both lakes. The zooplankton com munity of the upper locality was dominated in June and August by Bosmi na longirostris, the smallest zooplankton species represented in the w ater course, and in September by Daphnia cristata. The lower locality was dominated by the larger Holopedium gibberum and Eudiaptomus gracil oides in June, by D.cristata in August, and by D.cristata and B.longir ostris in September. The mean body size of the three most abundant cla doceran species was significantly smaller in the upper locality, compa red to the lower locality. It was concluded that the invasion and esta blishment of a dense vendace population in the upper locality had incr eased the predation pressure in the pelagic, resulting in a reduction of body size and a shift towards smaller species in the zooplankton co mmunity.