Toxic chemicals can alter ecosystem structure and ecological processes. The functional redundancy of systems - an assumption that biotic communities have several members that perform similar functions - is used to explain how communities maintain processes such as nutrient cycles and production-respiration patterns under stress. We tested community functional redundancy by analyzing the uncoupling of multispecies processes. We used continuously collected pH values in replicate laboratory microcosms to compare changes in the pH spectrum (an analog of diurnal net primary production and respiration) to changes in community structure (species richness, composition, standing crop). Spectral analyses identified the repeating frequencies in continuous pH records and showed that severe uncoupling of diurnal production and respiration patterns in microcosms treated with copper 9.9-205 ug Cu/L) occurred at concentrations near or below numerical water quality criteria (ca. 20 ug Cu/L). Changes in the pH spectrum were coincident with the loss of species from the microcosms. Changes in spectral density and community structure were indicative of adverse ecological effects, and these changes occurred at lower concentrations than detectable changes in nutrient cycles and standing crop biomass (protein, chlorophyll). Results of these experiments suggest that functional redundancy in communities does not extend to continuous measures of community metabolism. As such, the concept of functional redundancy is questioned: the failure to detect changes in processes belies alterations in process dynamics. The concept of functional redundancy is probably a result of insufficient statistical power rather an inherent ecosystem property.