Two small, adjacent kettle lakes in southern Ontario were sampled during spring and summer 1987. The data comprised weekly samples of zooplankton and water chemistry, monthly diel assessments of the densities of pelagic fish and zooplankton found at 1-m depth intervals in the water column, and an annual mark and recapture assessment of the entire fish population. The two lakes had very different community structures. Haynes Lake was characterized by high piscivore numbers, few planktivores, a relatively large assemblage of large bodied zooplankton, low chlorophyll a concentrations, and clear water. Lake St. George had a lower piscivore to planktivore ratio, smaller zooplankton, more chlorophyll a, and murkier water. Comparisons of trophic level biomasses for the two lakes suggested that in both communities, the relationships between piscivores and planktivores and between planktivores and zooplankton were strongly correlated with predator abundances. In the more oligotrophy community (Haynes Lake) this influence extended weakly to the phytoplankton, but in the more eutrophic system, little of the variability in chlorophyll a with respect to total phosphorus could be explained by total zooplankton (or Daphnia) abundance. This suggests that for freshwater pelagic communities, top-down effects may be stronger in more oligotrophic systems.
Toward linking ocean models to fish population dynamics
