The effect of piscivorous fish and nutrient availability on the biomass and community size structure of phytoplankton and zooplankton in 29 temperate lakes.

Abstract

It has been hypothesized that the trophic structure of aquatic ecosystems is controlled by two sets of processes, often referred to as bottom-up and top-down. The bottom-up hypothesis postulates that nutrient limitation, and various physical-chemical factors directly or indirectly regulate the biomass and community structure of organisms at all levels of the food web. The top-down hypothesis postulates that predation by upper levels of the food web (specifically piscivorous fish) strongly influence the biomass, community size structure and productivity of organisms in the lower trophic categories. In this study we attempt to determine the strength of both bottom-up and top-down control on lower trophic level biomass and community size structure, in lakes of varying trophic status. During summer stratification, 29 temperate lakes of varying trophic status and fish species composition were sampled in Quebec and Ontario. This study found a strong relationship between nutrient availability and phytoplankton biomass, and between nutrient availability and zooplankton size structure. Bottom-up effects were quite strong at both the phytoplankton and zooplankton levels. Neither piscivorous fish presence nor indices of piscivorous fish abundance had a detectable effect on most indices of phytoplankton and zooplankton biomass and community size structure. Only cladoceran biomass and the variance in individual dry weight were higher in lakes where piscivorous fish were present. Piscivorous fish predation-effects were not evident at the phytoplankton level and were relatively weak at the zooplankton level. Total zooplankton biomass, crustacean biomass and cladoceran biomass were significant in explaining residual variation in the TP-Chl a model and improved this model's predictive ability by 24%, 18% and 22%, respectively. This study found that the effect of piscivorous fish did not depend on the concentration of total phosphorus in the 29 lakes. No evidence of interaction between trophic status and predation was determined. (Abstract shortened by UMI.)