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Biological Significance <br />Toxicity of copper and zinc to olfactory receptors is <br />affected by water hardness, pH, alkalinity, and suspended and <br />dissolved materials (Sprague 1985; Klaprat et al. 1992). <br />Equations from USEPA water quality criteria predict average <br />concentrations of copper (USEPA 1985) and zinc (USEPA 1980) for <br />protection of freshwater aquatic organisms based on water <br />hardness. For dilution water used in our experiments, calculated <br />criteria values are greater than, or fall within the 950 <br />confidence intervals for ECls for all exposures except the 96-h <br />zinc exposure for which there was no significant concentration <br />response (Table 3). The .criteria value for copper is <br />particularly high compared to the 24-h EC1. It is about 22 times <br />greater than the EC1 and falls outside of the 95o confidence <br />interval suggesting that the difference is significant. Criteria <br />values for other exposures fall within the EC1 confidence <br />intervals; however, confidence intervals are relatively broad, <br />probably because of a lack of data with > 50% probability of <br />inhibition. Broad confidence intervals may also reflect the <br />general statistical property of binomial data of greater <br />variability at extreme proba3~ilities compared to the median <br />(Zar 1984). This is a disadvantage of using EC1, but despite <br />broadening confidence intervals at the to level, the predicted <br />value should, on the average, be an accurate estimate of the <br />concentration-response relation (Stephan and Rogers 1985). <br />19 <br />