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e e EPA Rapid Bioassessment Protocols III The primary difference between the u.s. Environmental Protection Agency's Rapid Bioassessment Protocols (RBP) II and III is RBP II invertebrates are identified in the field to family and for RBP III invertebrates are identified in the laboratory to genus or species (82). The degree of impairment at a site is based on the relative differences observed for eight primary metrics calculated between a sampling site and a control or reference site. In many cases reference data are unavailable, nevertheless these metrics can be used to evaluate general water quality and they can be used when reference data becomes available (Table 6). The eight metrics used in RBP II and III are: 1. Species richness 2. Modified Hilsenhoff Biotic Index 3. Ratio of scraper to collector-filterer + scraper functional feeding groups - This ratio reflects the riffle/run community food base and can provide insight into the nature of potential disturbance factors. The proportion of the two feeding groups is important because predominance of a particular feeding type may indicate an unbalanced community responding to an abundance of a particular food source. The predominant feeding strategy reflects the type of impact detected. Scrapers increase with increased abundance of diatoms and decrease as filamentous algae and aquatic mosses increase. Filamentous algae and aquatic mosses provide good attachment sites for collector-filterers and the organic enrichment often responsible for high abundance of filamentous algae provides FPOM utilized by filterers. Filterer-collectors are also sensitive to toxicants bound to fine particles and may decrease in abundance when exposed to such sources. The scraper:collector-filterer + scrapers ratio may not be a good indicator of organic enrichment if adsorbing toxicants are present. 4. Ratio of EPT to Chironomidae + EPT abundances - This ratio evaluates the relative abundance of these indicator groups as a measure of community balance. Good biotic condition is reflected in communities having a fairly even distribution among all four major taxonomic groups and with substantial representation in the sensitive groups Ephemeroptera, Plecoptera, and Trichoptera. Skewed populations having a disproportionately high number of the generally tolerant Chironomidae relative to the more sensitive groups may indicate environmental stress (82). In general, Chironomids tend to become increasingly dominant along a gradient of increasing enrichment or heavy metal concentration. 5. Percent contribution of dominant taxon - A community dominated by few taxa indicates environmental stress (82). Values range from 0-1. Lower values indicate a more balanced community balance and better water quality. 6. EPT richness - The total number of taxa within the insect orders Ephemeroptera, Plecoptera, and Trichoptera (EPT). These orders are considered to be sensitive to pollution. EPT generally increases with increasing water quality (53). 7. Ratio of shredder functional feeding group and total number of individuals collected - This metric evaluates potential impairment as indicated by the relative presence of shredders. Shredders are sensitive to riparian impacts and are good indicators of the availability of CPOM and toxic effects. The degree of toxicant effects on shredders versus filterers depends on the the toxicant, the size of the particle it is attached to, and its organic particle adsorption efficiency. Toxicants of a terrestrial source (e.g., pesticides, herbicides) accumulate on CPOM prior to leaf fall and may have a substantial effect on shredders.