EFFECTS OF INVADING VENDACE (COREGONUS-ALBULA L.) ON SPECIES COMPOSITION AND BODY-SIZE IN 2 ZOOPLANKTON COMMUNITIES OF THE PASVIK RIVER SYSTEM, NORTHERN NORWAY
T. Bohn et Pa. Amundsen, EFFECTS OF INVADING VENDACE (COREGONUS-ALBULA L.) ON SPECIES COMPOSITION AND BODY-SIZE IN 2 ZOOPLANKTON COMMUNITIES OF THE PASVIK RIVER SYSTEM, NORTHERN NORWAY, Journal of plankton research, 20(2), 1998, pp. 243-256
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.