What is the relationship between whole effluent toxicity and instream biological condition?

We compiled a database of 250 dischargers across the United States and exam ined relationships between standardized Ceriodaphnia dubia and Pimephales p romelas (fathead minnows), whole effluent toxicity (WET) test endpoints, an d instream biological condition as measured by benthic macroinvertebrate as sessments. Sites were included in the analysis if the effluents were not ma nipulated before testing (e.g., dechlorination), and standardized biologica l and physical habitat assessment methods were used upstream and directly d ownstream of the discharge. Several analyses indicated that fish endpoints were more related to instream biological condition than Ceriodaphnia WET en dpoints. Dischargers that failed <25% of their tests had less than or equal to 15% chance of exhibiting instream impairment. Effluent dilution was the strongest factor affecting relationships between WET and observed biologic al conditions. Effluents that comprised >80% of the stream under low-flow c onditions exhibited better relationships between WET and instream condition than effluents with greater dilution. Effluents that comprised <20% of the stream had a low probability of exhibiting impairment, even if several WET test failures were observed over a 1-year period. Fish acute and chronic W ET information could predict instream biological conditions; however. WET c ompliance, based on 7Q10 stream flow, was consistently conservative. Our re sults indicate that WET was more predictive of instream biological conditio n if several tests were conducted, more than one type of test was conducted , and endpoints within a test were relatively consistent over time.