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algae also relatively important at Station 3. This can partially be explained by <br />• examining the seasonal changes in relative abundance of the four stations for diatoms <br />(Exhibit 2.7-I I), blue-green algae (Exhibit 2.7-12) and other algae (Exhibit 2.7-13). <br />Diatoms dominated all sites during the summer and fall seasons. However, blue- <br />green algae increased greatly in importance in January, especially at Stations I and <br />3. This was due to increased representation by the blue-green algae Oscillatoria spp. <br />as well as the decreased importance of the diatoms Nitzchia ap lea and Surirello <br />tenius. Diatom abundance recovered in spring at Stations 3 and 4. However at <br />Station 2 and especially Station I another algae dominated the periphyton. This <br />algae, Hydrurus foetidus, a yellow-green algae (Chrysophyta), was not present during <br />the other three seasons. The species lives only in cold, turbulent streams (Prescott, <br />1970) and becomes very abundant in the spring at the upstream sites in Trout Creek. <br />Apparently, it cannot withstand either The abrasion of ice during winter or the <br />warmer temperatures in summer at the lower stations. <br />Benthic Invertebrates -The benihic invertebrates are organisms That live <br />on and in the substrate of the stream. In This study, only those organisms inhabiting <br />the riffle areas were collected. In streams like Trout and Oak Creeks, the riffle <br />invertebrates are primarily the immature forms of aquatic insects. The majority of <br />• These insects have one year life cycles (Merritt and Cummins 1978). The adult <br />emerges, eggs are laid, adults die, larvae hatch and grow for 10 months or so and <br />the cycle repeats itself. However, several sioneflies (Plecoptera) found in streams <br />similar to Trout Creek take two to three years to complete their life cycle. Such <br />factors as stream temperature and day-night periods determine when the adult <br />emerges, in fact, increases in temperature of as much as 5oC may cause adults to <br />emerge months ahead of schedule. The timing of this emergence plus the duration <br />of life cycles of the insects exert a strong influence on the seasonal changes in <br />benihic density and biomass. The general trend for Colorado streams is for low <br />numbers in late spring -early summer due to emergence of adults and the effects <br />of scour from spring runoff, followed by an increase through the summer and fall as <br />larvae hatch from the eggs laid during the summer. Maximum densities are often <br />encountered in late fall followed by a general decline through the winter due to the <br />severe effects of ice formation with IiTt{e replacement (Hynes 1970). Biomass may <br />not follow this pattern as loss of organisms is offset by the growth of surviving <br />idividuals, producing a maximum for biomass in early spring prior to emergence and <br />runoff. Goodnight (1973) indicates several reasons for using macroinvertebrales as <br />• indicators of water quality: I) they collectively show a wide range of tolerance to <br />2.7-45 <br />