A COMPARISON OF LAKES AND LAKE ENCLOSURES WITH CONTRASTING ABUNDANCES OF PLANKTIVOROUS FISH

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 × 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. © 1990 Oxford University Press.