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a variety of stresses, 2) their relatively long life cycles can reflect temporary <br />• stresses difficult to detect with periodic chemical sampling, 3) their relatively low <br />mobility keeps Them in the area, 4) they occupy a central position in the food web, <br />and 5) they are easily adapted to laboratory study. <br />A Total of 71 invertebrate taxa were collected from Trout Creek, with less than <br />SO% of these occurring aT all four stations (Appendix 2.7-C -Table 2). As is common <br />in streams in The Rocky Mountains, aquatic insects dominated both faunal density and <br />biomass (Table 2.7-13). There were steady increases in both density and biomass <br />from Station I to 5tatlOn 4. This linear trend was statistically significant (p~ .05). <br />This was similar to the downstream trend observed in Trout Creek in 1975-1976 by <br />Canton and Ward (1978). This downstream trend can be accounted for primarily by <br />the changes in the density and biomass of the aquatic insects. C addisflies <br />(Trichoptera) dominated all stations in numbers (Exhibit 2.7-14) and all but Station <br />in biomass (Exhibit 2.7-15). The large increase in biomass at Station 4 with only <br />slightly increased numbers was due to an increase in The larger caddisfly species (e.g. <br />Hydropsyche) and a reduction in importance of the smaller caddisflies (e.g. <br />Glossosoma) at this station. Increased amounts of suspended organic matter are <br />apparNntly present at the lower stations, which favors the filter-feeding caddisflies. <br />• both density and biomass of dipterous decreased from Station I to Station 3, followed <br />by sharp increases at Station 4 due to large numbers of midge larvae (Chironomidoe). <br />Midges can often complete many generations in one year in the wormer, lower <br />sections of streams such as the area of Station 4. They may maintain their high <br />numbers at this station through continuous recruitment of larvae during the summer <br />and fall as a result of multiple adult emergence. 5toneflies (Plecoptera) showed <br />little change in either biomass or numbers between the sites. Predators generally <br />maintain constant, low numbers of individuals over a gradient of habitats and <br />stoneflies are primarily predators. <br />Table 2.7-13 also presents the number of species and species diversity (D) for <br />the stations on Trout Creek. Number of taxa also increased downstream, due mainly <br />to increased numbers of species of mayflies and caddisflies at the lowest station <br />(Appendix 2.7-C -Table 2). Species diversity was used as another tool to measure <br />the quality of the environment of Trout Creek. EPA (Weber 1973) encourages the <br />use of this index (D) to determine the effects of stress on The invertebrate <br />community. The use of a species diversity index is based on the premise that <br />undisturbed environments will support communities that possess large numbers of <br />• taxa with no one taxo dominating. In other words, there would be a few toxo with <br />2.7-47 <br />