AbstractThe reference-condition approach to bioassessment often uses the observed/expected (O/E) ratio to indicate anthropogenic alteration of aquatic macroinvertebrates, fish, or periphyton assemblages. Given a list of taxa found at ≥1 minimally disturbed reference sites, E is the number of those taxa that would expected in a sampled assemblage if the sampled stream were in reference condition, and O is the number of those taxa observed in the sample. An O/E value significantly <1.0 indicates that a stream has lost taxa relative to its reference-condition expectation, possibly because of anthropogenic stress. However, the O/E index can be insensitive to stress-induced shifts in taxonomic composition that leave assemblage richness unchanged. As an alternative to O/E, I propose using BC, an adaptation of Bray–Curtis distance, to measure the compositional dissimilarity between an observed and expected assemblage directly. I compared the performance of BC and O/E at 5685 stream and lake sites throughout the ...