H. Helminen et J. Sarvala, RESPONSES OF LAKE PYHAJARVI (SOUTHWESTERN FINLAND) TO VARIABLE RECRUITMENT OF THE MAJOR PLANKTIVOROUS FISH, VENDACE (COREGONUS-ALBULA), Canadian journal of fisheries and aquatic sciences, 54(1), 1997, pp. 32-40
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.