J. N. Am. Benthol. Soc., 2006, 25(3):712–727 Ó 2006 by The North American Benthological Society Evaluating a novel Test Site Analysis (TSA) bioassessment approach Michelle F. Bowman 1 Department of Zoology, University of Toronto, 25 Harbord St., Toronto, Ontario, Canada M5S 3G5 Keith M. Somers 2 Dorset Environmental Science Centre, Ontario Ministry of the Environment, 1026 Bellwood Acres Road, Dorset, Ontario, Canada P0A 1E0 Abstract. We compared the performance of Test Site Analysis (TSA) to 2 existing bioassessment methods: the BEnthic Assessment of SedimenT (BEAST), and the multimetric approach. In TSA, simple spreadsheet calculations are used to: 1) derive an overall multivariate measure of dissimilarity between a potentially impaired test site and a reference benchmark using a number of biological summary metrics, 2) classify each site as impaired, potentially impaired, or in reference condition with formal hypothesis tests, and 3) identify the metrics important in distinguishing a significantly impaired test site from the reference benchmark. We used data collected as part of the Fraser River Action Plan (British Columbia, Canada) to compare the biological condition of 15 potentially impaired test sites known to be exposed to agriculture, logging, or mining to the condition of 61 minimally disturbed reference sites with comparable habitat. When TSAwas used, the false-positive (i.e., Type-1 error) and false-negative (i.e., Type-2 error) rates were as low as or lower than when the other methods were used. Using TSA enhances our ability to identify impairments because it accounts for correlations among metrics and uses a statistically defined potentially impaired category. TSA integrates formal hypothesis testing with the strengths of many existing approaches, does not require specialized software, and enhances our ability to detect and diagnose biological impairments. Key words: Test Site Analysis (TSA), bioassessment, multimetric, multivariate, stream macroinverte- brates, reference condition approach, Fraser River. It possible to say water has lost its natural qualities only when pollution is severe enough to cause changes that exceed the boundaries of normal variation (Hynes 1960). In the absence of biological standards, the Reference Condition Approach (RCA) (Hughes et al. 1986, Bailey et al. 2004) often is used to define the empirical boundaries of normal variation in biological communities. The RCA is based on the premise that the biological composition of minimally disturbed reference sites can be predicted using physical and chemical attributes. However, many of the important physical and chemical qualities in which natural waters differ are the same qualities that are altered by most pollutants, so pollution may merely change one type of natural water into another (Hynes 1960). Thus, in the RCA, only features not likely to be affected by anthropogenic activities (e.g., stream order) are used to match an exposed test site with physically and chemically comparable reference sites. Criteria for selecting appropriate reference sites can be set a priori using physiochemical and biogeographical knowledge or can be derived empirically using a posteriori biological clustering and associated discriminant func- tions analysis (DFA; Barbour et al. 1999). Both reach- scale and larger-scale landscape features generally are needed to predict the composition of freshwater fauna accurately (Hawkins et al. 2000). In the final step of a RCA-type bioassessment, the biological community from a test site is compared to the range of communities at appropriate reference sites. Assuming suitable reference sites are used, differences in the biological attributes of a test site relative to those at reference sites imply the test site has been impaired (i.e., is different from reference). The RCA has become the basis for national benthic biomonitoring programs worldwide (Bailey et al. 2004), but the specific methods used to analyze data have 1 E-mail addresses: michelle.bowman@ene.gov.on.ca 2 To whom correspondence should be addressed. E-mail: keith.somers@ene.gov.on.ca 712