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i BENTHIC INVERTEBRATES <br />I Benthic invertebrates are organisms living on and in the substrate of <br />streams. In streams like Fish Creek and Middle Creek they are primarily the <br />immature forms of aquatic insects. The majority of these insects have one <br />year life cycles (Merritt and Cummins, 1978) -- the adult emerges and lays <br />1 eggs, larvae hatch and grow for approximately 10 _months and the cycle . <br />repeats. The timing of the adult emergence and duration of life cycles of <br />I these aquatic insects exert a strong influence on seasonal changes in benthic <br />invertebrate density. The general seasonal trend in Colorado streams is for <br />low numbers of organisms in late spring-early summer due to emergence of <br />I adults and the effects of scour from spring runoff, followed by an increase <br />in density through summer and fall as larvae hatch from the eggs laid during <br />the summer. Maximum densities are usually encountered in fall, followed by <br />a general decline through the winter due to mortality from ice formation <br />1 with little replacement (Hynes 197G). Benthic invertebrates can be valuable <br />1 indicators of water quality. They show a wide range of tolerence to various <br />stresses, their life cycles are long enough to reflect temporary stresses <br />undetectable with periodic chemical sampling, they have low motility and <br />they occupy a central position in the food web (Goodnight, 1973). <br />~~ <br />Middle Creek <br />} A total of 27 invertebrate taxa were collected from Middle Creek, <br />l although only 12 were found at all three stations (Table 1). As is typical <br />j of rocky-bottomed cold water streams, aquatic insects dominated the benthic <br />f density in both riffles and pools. There was a slight decrease in density <br />from Station 1 to Station 2, followed by a substantial increase in density <br />at Station 3. <br />Riffle invertebrates at Station 1 were dominated by net-spinning caddis- <br />flies (Hydropsyche and Cheumatopsyche) although mayflies and chironomids <br />were also abundant. Station 2 was dominated by Chironomidae Farm 1 with <br />caddisflies again abundant. The caddisflies Ochrotrichia and Cheumatopsyche, <br />the blackfly Simulium arcticum and the tubificid worm Limnodrilus were the <br />} dominant members of the benthos at Station 3. Mayflies were reduced greatly <br />in both density and numbers of taxa at Stations 2 and 3. Stoneflies <br />(Plecoptera) were conspicuously absent from all stations. <br />-10- <br />