Water quality criteria for toxic substances published by the U.S. Environmental Protection Agency (EPA) are established to protect aquatic life and human health. An aquatic life criterion specifies the concentration of metal, pesticide, or other toxicant below which ecosystem integrity is maintained. However, most criteria are based on toxicity evidence from only a few sensitive aquatic species, such as Daphnia magna, Pimephales promelas, and Salmo spp. Single-species tests usually use organisms of uniform age and size and therefore lack the heterogeneous responses of a natural population. Other factors that affect the natural ecosystem balance are not necessarily reproduced in the laboratory, such as nutrition, physiological interaction, substrate, current, and light.