Rapid Bioassessment Protocols (RBPs) have been widely used to assess the ecological health of aquatic ecosystems. Specific aims of RBPs for wadeable streams are to indicate the ecological condition of a stream using low-cost protocols to allow long-term and widespread routine monitoring. Our study was part of an ongoing effort to test and standardize a protocol using benthic macroinvertebrates as indicators of the water quality of wadeable streams in southeast Brazil. One of the most controversial issues during RBP development is deciding the taxonomic resolution that should be used. We evaluated how well genus-, family-, and order-level taxonomic resolution detected a gradient of impairment. All 3 taxonomic resolutions statistically discriminated reference, intermediately impaired, and impaired sites based on assemblage structure, water-quality classification, and biotic index responses. Analysis at the genus level was more effective than analysis at other levels of taxonomic resolution for discriminating sites that varied in degradation conditions, especially when considering biotic index responses, but the lack of comprehensive taxonomic keys and information about the ecology of those genera hinder their widespread use in bioassessments. On the other hand, analyses at the order level had lower discriminating power to separate reference sites from intermediately impaired sites when considering biotic index responses. Analyses at the family level gave results similar to results at the genus level, and we support its use in a RBP program for this region, at least until better keys and autoecological knowledge are available.
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