JOURNAL OF PLANKTON RESEARCH | VOLUME 12 | NUMBER 1 | PAGES 109-124 | 1990
© Oxford University Press
research-article |
A comparison of lakes and lake enclosures with contrasting abundances of planktivorous fish
Department of Biology, University of Waterloo Waterloo, Ontario N2L3G1 1Department of Biology, York University North York, Ontario M3J 1P3 2National Water Research Institute PO Box 5050, Burlington, Ontario L7R 4A6, Canada
Received on February 13, 1989; accepted on August 30, 1989 Experimental manipulations of planktivorous fish in large enclosures produced plankton communities comparable to those in lakes with contrasting abundances of planktivorous fish. Total epilimnetic phosphorus (TP), its distribution among five size-classes of dissolved (<0.2 µm) and paniculate phosphorus (PP 0.2–1, 1–20, 20–200 and >200 µm), phosphate turnover time, water clarity (Secchi depth) and phytoplankton biomass (chlorophyll a) were measured for two summers in eight large enclosures where planktivorous fish (1 + yellow perch) and nutrients (N and P) were added in a 2 x 2 factorial design. These parameters were also measured in two meso-eutrophic kettle lakes, Lake St George and Haynes Lake, containing low and high abundances of planktivorous fish, one of which was the lake (Lake St George) containing the enclosures. Comparable data were also collected from three oligo-mesotrophic lakes in central Ontario. In both the enclosures and the lakes, intense planktivorous fish predation was associated with increased TP, decreased abundance of larger zooplankton and mesoplanktonic PP (>200 µm), increased pico- and nanoplanktonic PP (1–20 µm), increased phosphate limitation (faster turnover time), increased chlorophyll a and reduced water clarity. Slope parameter, an index of plankton size-spectrum, was correlated with phosphate turnover time and Secchi depth among enclosures, and the data from all five lakes conformed to these empirical relationships. Fertilization of enclosures produced increased variability in the relationship among the variables. Our 2 years of experiments produced qualitatively similar treatment effects, but the magnitude of the effects was not similar for all parameters. We suggest that the responses of plankton communities and associated parameters to planktivore predation that we observed in large experimental enclosures are basically similar to those in the lakes we studied, and that enclosures are an important tool in understanding complex interactions in aquatic systems.