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<br />velocity are important factors. It has been noted that <br />the faster the current, the more the loosely attached <br />species will be washed downstream. Velocities greater <br />than about 5 m/sec erode all but the most tenacious <br />periphyton. Depth is important as it interacts with <br />turbidity and available light to limit photosynthetically <br />active light at the stream bottom (Whitton, 1975). Some <br />benthic algae are very tolerant of shade. Whitton (1972) <br />found that a quarter of the species taken from the River <br />Wear, England, and incubated in the dark for two months, <br />were able to grow when given favorable conditions. On the <br />other hand, Whitton (1973) has shown some species to be <br />tolerant only of high light conditions. The shallow, <br />lower velocity, high light areas in the White River are <br />always higher in chlorophyll a than the deeper, faster <br />areas (Figure 10). <br /> <br />C\l50 <br />:e <br />......... <br />~40 <br />caI <br /> <br /> <br />70 <br /> <br />TRANSECT 18 <br /> <br />60 <br /> <br />D <br /> <br />Figure 10. The temporal change in periphyton <br />biomass at a pool and a riffle transect in the <br />White River at Southam Canyon. <br /> <br />In preliminary experiments, oil shale leachate has <br />been experimentally introduced into P/R chambers in situ. <br />Low-level leachate additions either depressed or <br />stimulated the community responses depending upon the <br /> <br />279 <br />