RESPONSES OF LAKE PYHAJARVI (SOUTHWESTERN FINLAND) TO VARIABLE RECRUITMENT OF THE MAJOR PLANKTIVOROUS FISH, VENDACE (COREGONUS-ALBULA)

We studied the functional relationships between water quality, phytopl ankton, zooplankton, and planktivorous fish (vendace, Covegonus albula ) in Lake Pyhajarvi (southwestern Finland) in 1979-1992. The observed average values of water quality were typical of a mesoeutrophic system : total phosphorus (P) concentration, 16 mg.m(-3); chlorophyll a, 5 mg .m(-3); and Secchi depth, 3.3 m. The average total biomass of crustace an zooplankton in August was 62 mg carbon (C).m(-3), and large Daphnia spp. (>1.2 mm) were totally missing. The correlation between the late summer (August-September) chlorophyll a concentration and the total P concentration in the lake water was weak. The year-class strength of the vendace in autumn correlated significantly with late summer values of chlorophyll a. The August mean zooplankton biomass was negatively correlated both with the year-class strength of the vendace in autumn and with the August-September values of chlorophyll a, indicating the role of zooplankton as an intermediary in the trophic cascade of venda ce feeding through the food web. We suggest that the reduced grazing p ressure by zooplankton enabled the observed increase in phytoplankton biomass in years with a strong vendace stock.