The need for multiple lines of evidence for predicting site-specific ecological effects

The goal of this paper is to illustrate the value and importance of the "we ight of evidence" approach (use of multiple lines of evidence from field an d laboratory data) to assess the occurrence or absence of ecological impair ment in the aquatic environment. Single species toxicity tests, microcosms, and community metric approaches such as the Index of Biotic Integrity (IBI ) are discussed. Single species toxicity tests or other single lines of evi dence are valuable first tier assessments that should be used as screening tools to identify potentially toxic conditions in a effluent or the ambient environment but these tests should not be used as the final quantitative i ndicator of absolute ecological impairment that may result in regulatory ac tion. Both false positive and false negative predictions of ecological effe cts can occur due to the inherent variability of measurement endpoints such as survival, growth and reproduction used in single species toxicity tests . A comparison of single species ambient toxicity test results with field d ata showed that false positives are common and likely related to experiment al variability or toxicity to selected test species without measureable eff ects on the ecosystem. Results from microcosm studies have consistently dem onstrated that chemical exposures exceeding the acute or chronic toxicity c oncentrations for highly sensitive species may cause little or no ecologica lly significant damage to an aquatic ecosystem. Sources of uncertainty iden tified when extrapolating from single species tests to ecological effects w ere: variability in individual response to pesticide exposure; variation am ong species in sensitivity to pesticides; effects of time varying and repea ted exposures; and extrapolation from individual to population-level endpoi nts. Data sets from the Chesapeake Bay area (Maryland) were used to show th e importance of using "multiple lines of evidence" when assessing biologica l impact due to conflicting results reported from ambient water column and sediment toxicity tests and biological indices (benthic and fish IBIs). Res ults from water column and sediment toxicity tests with multiple species in tidal areas showed that no single species was consistently the most sensit ive. There was also a high degree of disagreement between benthic and fish IBI data for the various stations. The lack of agreement for these biologic al community indices is not surprising due to the differences in exposure a mong habitats occupied by these different taxonomic assemblages. Data from a fish IBI, benthic IBI and Maryland Physical Habitat Index (MPHI) were com pared for approximately 1100 first through third-order Maryland non-tidal s treams to show the complexity of data interpretation and the incidence of c onflicting lines of evidence. A key finding from this non-tidal data set wa s the need for using more than one biological indicator to increase the dis criminatory power of identifying impaired streams and reduce the possibilit y of "false negative results". Based on historical data, temporal variabili ty associated with an IBI in undisturbed areas was reported to be lower tha n the variability associated with single species toxicity tests.