To evaluate ecosystem response to and recovery from toxic contamination, the authors added phenolic compounds to a series of experimental ponds. Toxicants were added repeatedly in a temporally staggered sequence to evaluate the influence of seasonal factors and previous exposure history on the responses to toxicant stress. The authors hypothesized that seasonal changes in ecosystem structure, e.g. shifts in the relative importance of 'to-down' and 'bottom-up controls on energy flow (Bartell et al. 1988), would influence the system-level responses to the toxicant. Information from these experiments is being incorporated into models that predict ecological risk and system-level behavior under toxicant stress. The study focuses on the responses of zooplankton populations to toxicants and factors which may affect the apparent severity of toxic effects.