Factors other than toxicants can affect fathead minnow survival and growth and Ceriodaphnia survival and reproduction in ambient waters. Growth of minnow larvae in laboratory tests, for example, is affected by concentrations of common salts, and survival of the larvae in ambient waters from relatively pristine streams can be low and variable due to the presence of pathogenic microorganisms (Kszos et al., 1997). Ceriodaphnia reproduction is commonly greater in ambient water than in diluted mineral-water controls, due to the nutritional benefits they derive from consuming naturally occurring particulate matter, but some naturally occurring algae can be toxic (Reinikainen et al., 1994). These situations make it inadvisable to compare the results of ambient tests only with diluted mineral water controls to determine if an ambient site is toxic or nontoxic. Comparison with an appropriate suite of reference sites is critical to derive the correct answer for the correct reasons. In ambient toxicity testing, and in biological monitoring generally, one must be constantly alert to the difference between biological importance and statistical significance (Cairns and Smith, 1994; Yoccoz, 1991).
New and potentially useful ambient assessment procedures are being developed at a rapid pace; innovations in biological monitoring occur more slowly; slower still is the rate at which field-validated bioassessment methodology is being incorporated and used in a regulatory framework (see, for example, Hart, 1994). Examples of rapid progress in bioassay development can be found in both the water- and the soil-assessment arenas. A 3-day laboratory test that uses snail feeding rate to evaluate water quality appears to be about as sensitive as a 7-day Ceriodaphnia test, at least for some kinds of contaminants (R. L. Hinzman, Environmental Sciences Division, Oak Ridge National Laboratory, unpublished data). Procedures for estimating the toxicity of sediments with laboratory tests using invertebrates are nearing readiness for regulatory use (American Society for Testing and Materials, 1991). Methods for laboratory tests designed to estimate the toxicity of soils are being revised, calibrated, and field validated (L. F. Wicker, Environmental Sciences Division, Oak Ridge National Laboratory, unpublished data).
The increasing use of ecological risk assessment methodology for regulatory purposes drives the need not only for faster and more cost-effective laboratory tests, but also for data that accurately reflect exposure regimes and reveal ecological effects in the field. In situ test procedures using caged or noncaged organisms are in various stages of development and validation for terrestrial (Callahan et al., 1991; Menzie et al., 1992) and aquatic environments (Napolitano et al., 1993). Aquatic (Graney et al., 1994) and terrestrial (Gunderson et al., 1997; Parmalee et al., 1993) mesocosm studies are key to the development of in situ test