Contaminant concentrations in aquatic systems are seldom constant. Erratic inputs such as industrial discharges, rain water flushing, and random spills may cause concentrations to increase rapidly. Environmental processes may contribute through the dispersive actions of tides and currents, adsorptive losses to or release from resuspended sediments, and contaminant losses due to photo-degradation and volatilization. Despite such variability in contaminant concentrations, environmental guidelines are derived from toxicity test data using continuous exposure, where contaminant concentrations at the beginning of the exposure are assumed to remain relatively constant over the test duration. Responses of organisms exposed to fluctuating contaminant concentrations may differ from those exposed continuously to contaminants, even for equal contaminant loads. The current knowledge gap regarding the differing responses of organisms to contaminants from continuous and pulsed exposures is impeding decision making processes of both regulatory bodies and discharging industries. We have investigated the effects of continuous and pulsed copper exposure on the growth of the copper-sensitive microalga Phaeodactylum tricornutum as an indicator of ecosystem health.