tration of the effluent that causes a statistically significant effect on survival, growth, reproduction, or behavior, compared with a control.

Initial EPA-approved effluent bioassays were acute toxicity tests (Peltier, 1978). Beginning in 1985, EPA introduced guidelines for chronic toxicity tests. Chronic toxicity tests are important tools because they enable the detection of toxic effects that although sublethal may have important consequences for individuals exposed to the toxins. For example, a toxin could destroy an adult organism's ability to reproduce without killing the organism itself. Acute toxicity tests would fail to detect such effects. As the worst cases of environmental pollution have been detected and addressed, chronic tests have become increasingly important. The EPA has approved chronic toxicity tests that measure growth rates of algae populations, growth rates of larval fishes, and reproduction of small freshwater crustacean zooplankton (Homing and Weber, 1985). Because effluent bioassays have become the primary tool for ensuring the protection of ecosystems from toxic effluents, it is important to evaluate whether they achieve this goal.

Regulatory Appeal of Effluent Bioassays

From an administrative viewpoint, regulations based on end-of-the-pipe measurements hold distinct advantages. These measurements (based on samples collected from pipes as they leave corporate or government facilities) require no information on the composition of the effluent or on when various constituents are added to the effluent. In addition, it is inherently easier to study the toxicity of effluents by working with the effluents themselves rather than with water from the receiving ecosystems after the addition of the effluents in question and any other inputs.

Acute bioassays have particular appeal because they are discrete, well-designed tests with well-established statistical procedures for characterizing dose-response effects (Bliss, 1934a,b; Finney, 1952a). If technicians follow required protocols carefully, they can perform acute tests accurately with minimal training. Chronic tests are more complicated. Because the performance of the test individuals in chronic tests is particularly sensitive to subtleties of culture conditions, food sources, and interactive effects of foods and toxins, technicians require more training and experience to perform chronic toxicity tests than acute toxicity tests. As a result, chronic tests are about three times as costly.

The National Pollution Discharge Elimination System (NPDES) administers the effluent bioassay program in individual states. The NPDES effluent toxicity standards are based on established state or federal water-quality criteria for compounds in effluents. Current recommended test species for freshwater effluents include the alga Selenastrum capricornutum , the crustacean zooplankton Daphnia magna and Ceriodaphnia dubia, and the fathead minnow, Pimephales promelas. Although other taxa can be used in effluent bioassays, most states

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