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<br />Materials and Methods <br />Dilution water <br />Two sources of dilution water were used. The source of dilution water for algae cultures, <br />rotifer cultures, and acute toxicity tests was tap water (City of Fort Collins, Fort Collins, <br />Colorado) treated by vigorous aeration for at least 24 h while being heated to 18°C (room <br />temperature). The treatment process reduced total residual chlorine concentration to less than <br />the detection limit of 0.02 mg/L. Water was adjusted using procedures for preparing "very hard" <br />water (USEPA 1991) to approximate water-quality characteristics of the Colorado River near the <br />Colorado-Utah state line (U.S. Geological Survey records; gage 09163500). Water used for <br />culture of algae and rotifers was pasteurized at 70°C for 1 h to prevent contamination of cultures <br />with undesirable biological organisms. Previous investigations have demonstrated that <br />pasteurization does not influence selenium concentration or chemical composition of water <br />treated with this procedure (Beyers and Sodergren 1999; Hoff and Snell 1989). <br />Water used for dissolved exposure of razorback and flannelmouth sucker in the 28-d <br />exposure was supplied by a well on the Colorado State University campus. Well water was <br />treated by vigorous aeration for approximately 8 h while being heated to a test temperature of <br />20 + 1 °C. Water from this source was known to contain dissolved selenium. However, it was <br />concluded that the benefits of flow-through exposure using this source outweighed the <br />disadvantages associated with trace selenium concentrations. Average selenium concentration of <br />the water was 6.16 µg/L which does not cause reduced growth or survival of fish (Beyers and <br />Sodergren 1999). <br />3 <br />