Functional ecosystem tests should reflect the hazards of toxic chemicals, as well as stimulation by nutrients, by measuring a single flux of phytoplankton to the dominant members of the community. The flux of phytoplankton and detritus to zooplankton is reflected by the filtering rates of individual organisms, expressed as millilitres per animal per hour. The authors used common particle counting techniques to measure such fluxes in the waters of Lake Erie. They then examined the impact of complex effluents on the filtering rates. These effluent effects are scored as inhibition or stimulation of filtering by the dominant herbivores in the Lake Erie ecosystem. In the River Raisin, a tributary to Lake Erie, specific effluents usually inhibited grazing by the herbivores Daphnia. Diaptomus, and Cyclops, although one effluent was stimulatory. These results were directionally consistent and probably depended on the characteristics (especially the concentrations of metals) of the effluents.