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Abstract-Toxicity bioassays were conducted to quantify water quality conditions under which silver, as silver nitrate, is toxic to Oncorhynchus mykiss, Pimephales promelas, and Daphnia magna. Bioassays for P. promelas and D. magna were conducted as static replacement tests, whereas a flow-through bioassay system was modified and used for O. mykiss. Results from 96-h toxicity bioassays for O. mykiss indicated that chloride, hardness, and dissolved organic carbon (DOC) protected against silver toxicosis, with DOC affording the highest protective effects. For P. promelas and D. magna, little protection was provided by increased CaCO 3 alone, whereas DOC had a major ameliorating influence on measured silver toxicity. Lower concentrations of chloride (Ͻ20 mg/L) had little effect on reducing silver nitrate toxicity. Dissolved organic carbon was more important than hardness for predicting the toxicity of ionic silver in natural waters to O. mykiss, P. promelas, and D. magna. Similarly, DOC significantly reduced silver nitrate toxicity to trout, whereas Cl Ϫ and hardness had only a minor protective effect. However, Cl Ϫ /DOC mixtures showed a greaterthan-additive protective effect. Thus, we suggest that incorporating an organic carbon coefficient into the silver criterion equation will enhance the criterion values for site specificity.

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