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1,102 organisms/ftz which occurred in September. March and June had the <br />lowest total mean densities with 553 organisms/ftz and 119 organisms/ftz, <br />respectively. Spring run-off probably contributed to the relatively low <br />densities is March and June by scouring and increased turbidities. <br />Caddisflies (Trichoptera) and mayflies (Ephemeroptera) were the <br />- dominant taxonomic orders at the majority of stations and sampling <br />periods. True flies (Diptera) were always the third most abundant <br />taxonomic group, except in March when this group was dominant at <br />Station 3. <br />Caddisfly Larvae accounted for approximately 54 percent of all <br />organisms identified from this survey. Caddisflies were most abundant <br />in September with a mean relative abundance of 74 percent and the least <br />abundant is Harch with a mean relative abundance of 16 percent. The <br />total annual mean density was high in September with 819 organisms/ft.z <br />and low in March with 88 organisms/ft2. Caddisflies were generally low <br />in numbers during the March sampling. This is probably the result of <br />emergence and scouring effect of spring run-off. No consistent numerical <br />difference was noted between stations. <br />The family Hydropsychidae usually represented the most abundant <br />caddisfly group. The two dominant genera were Cheumatopsyche sp. and <br />Hydropsyche sp. Both of these genera are coLlectors-filterers that feed <br />on particles of algae, detritus, and some animals that drift down the <br />river (Wiggins 1978). <br />Cheumatopsyche sp. and Hydropsyche sp. were the dominant and sub- <br />dominant taxa aG all sites in September and November. Hvdropsyche sp. <br />was the most abundant organism at Stations 2 and 3 in June and the <br />second most abundant [axon at Station 1. Cheumatopsyche sp. was <br />subdominant at Station 2. <br />Mayflies were the second most abundant group on all four sample <br />dates. Mayflies accounted for approximately 24 percent of the macro' <br />invertebrates collected in this survey. <br />Mayfly mean densities were variable with a maximum of 248 organisms/ <br />ft2 in March and a minimum of 46 organisms/f t2 in June. Of the four <br />. sample dates, the highest mean relative abundance, 49 percent, occurred <br />in March, while the lowest value, 12 percent, was found is September. <br />There were no significant differences between mayfly numbers above and <br />below the silo. <br />7 <br />