A comparative approach to determining the role of fish predation in structuring limnetic ecosystems

Strong levels of fish piscivory and planktivory are hypothesized to have pervasive influences on all lower trophic levels of limnetic ecosystems. We tested this hypothesis with a detailed comparison of the ecosystems of two small, oligotrophic, brown-water lakes located in south-central Ontario. One lake (Mouse Lake) had no primarily piscivorous fish species, while the other lake (Ranger Lake) had a large population of large- and small-mouth bass. These differences had persisted for approximately 30 years. For two years (1991 and 1992) we used standard limnological techniques to collect detailed data on populations of fish, benthos, zooplankton, and phytoplankton in both lakes, as well as data on the lakes' physical and chemical characteristics. Compared to Mouse Lake (low piscivore), Ranger Lake (high-piscivore) had (1) almost the same physical and chemical characteristics, (2) a much smaller biomass of adult fish planktivores, (3) approximately the same biomass of large-bodied invertebrate planktivores (Chaoborus), (4) a lower biomass of herbivorous zooplankton, of which a higher percentage were Daphnia spp., and (5) about the same biomass and species composition of phytoplankton. Our results show that large differences between the lakes in the biomasses and body size of components at the top of the food web are diminished at lower trophic levels. We suggest that this attrition in the impacts of top-down control is due to compensatory mechanisms of prey populations. Since these compensatory mechanisms develop over time scales of years, short-term studies of the effects of fish predation may be biased towards detection of strong top-down control.