methods that ultimately will be required for effective use of ecological risk-assessment methodology. Despite their limitations, simple laboratory tests, such as the Ceriodaphnia dubia and Pimephales promelas tests described in this paper, are likely to be relied on more and more. This is because the need for data that can be used for ecological risk assessments grows much faster than the rate at which regulatory agencies approve new methods for assessing the environment.

Conclusions

Standardized tests designed to estimate the toxicity of effluents to aquatic biota can, with minimal modification, also be used to assess ambient water-quality conditions in receiving streams. However, ambient tests should not be analyzed statistically in the same way as effluent tests. Site and test-period combinations, rather than effluent concentrations, serve as the principal unit of assessment for ambient toxicity test results. In addition, the results of ambient tests are in many cases more appropriately compared with the results of reference-site tests than with negative controls, which are commonly included with effluent tests. These considerations shape the strategy for cost-effective use of ambient toxicity testing. The value of ambient toxicity testing increases if the tests are used to support a broader-based, long-term biological monitoring program; conducted frequently with one sensitive species, rather than more often with two or more species; and accompanied by a diagnostic ("experimental") toxicity testing program. Data pruning by date can be used to help identify sites where water quality is suspect, and a representative suite of reference sites should be included in every ambient testing program to help place suspect sites into appropriate perspective. Specific linkages between ambient toxicity test results and chemical conditions at the test site are extremely desirable and can be revealed using methods such as PCA or logistic regression. The long-term prognosis is that in situ testing will replace the ambient toxicity testing procedures now in use. However, requirements for data that can be used in ecological risk assessments are likely to grow much faster than the rate of approval for in situ test methods for regulatory purposes. Thus, the next decade is likely to bring a marked increase in ambient testing with EPA-approved static-renewal laboratory procedures using organisms such as Ceriodaphnia and fathead minnow larvae.

Acknowledgments

This paper was improved through reviews and comments provided by T. L. Ashwood and T. L. Phipps and was made possible by technical contributions from members of the Biomonitoring Group, including L. A. Kszos, T. L. Phipps, L. F. Wicker, P. W. Braden, G. W. Morris, B. K. Beane, L. S. Ewald, J. R. Sumner, K. J. McAfee, and W. S. Session. Oak Ridge National Laboratory is managed for the U.S. Department of Energy by Lockheed Martin Energy Research Corp. un-



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