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with a linear Trend (p< 0.01) from Station I to Station 4. Once again Stations I and <br />• 2 were statistically different from Stations 3 and 4. Further decrease in density and <br />biomass in spring at the downstream stations was due to the effects of runoff from <br />the surrounding watershed (i.e. lowland runoff). Station I exhibited an increase in <br />density and biomass in spring due to The high numbers of midge larvae (chironomids) <br />associated with the expansive growths of Hydrurus foetidus (yellow-green algae) <br />found at this station at this time of year. The stations were significantly different <br />in April (p < 0.05) although the effects of lowland runoff changed the relationship <br />between stations from linear to o significant quadratic trend (p< 0.01). The lower <br />densities at Station 2 and especially Station 3 were different from stations I and 4 <br />(p < 0.05) and led to the quadratic trend observed. <br />The seasonal influence on invertebrate numbers and biomass in Oak Creek were <br />similar at both stations (Exhibits 2.7-IS and 2.7-19). Lower density in August was <br />due primarily to the effects of spring runoff and emergence of aquatic insects. <br />Nonetheless, density of Station I was significantly higher than Station 2 (p< 0.01). <br />The increase at Station I in October was due to increased density and biomass of <br />Diptera (primarily midges), Ephemeroptera, Oligochaeta and To o lesser extent <br />Trichoptera and Diptera. The winter density values at Station I were even higher <br />• despite decreased numbers of oligochaetes, mayflies and caddisflies due primarily to <br />increased abundance of chironomids. These small larvae, however, did not counter <br />the loss of biomass aT this station. Stotion I was statistically different from Station <br />2 during this month (p<0.01) because of these high numbers of midges. Extremely <br />low standing crop in June was The result of a severe spring runoff. <br />The invertebrate data from Oak Creek presented above indicates the benthic <br />community at Station is affected by mild organic enrichment. Typically, mild <br />organic enrichment tends to increase density of invertebrates and algae without a <br />drastic change in species composition (Hynes 1960). As noted in Table 2.7-14, density <br />of invertebrates was three times greater at Station I Than oT Stotion 2. Although <br />algal populations were not quantified, visual observations indicated attached algae <br />were much more abundant at Station I than at Station 2. This increased density is <br />accompanied by decreased diversity at Station I, which fits the pattern of mild <br />organic enrichment. The source of at least some of this enrichment appears to be <br />the Oak Creek sewage treatment plant a short distance upstream of Station. <br />Although the plant generally meets effluent standards when operating, it has <br />problems coping with the high flows of spring runoff which tends to overload the <br />• system and has no facilities for continued Treatment while bypassing units for <br />2.7-55 <br />