On the basis of these facts about freshwater fish and invertebrates: (i) the Na{sup}+ turnover is a physiological process associated with the gill membranes; (ii) the key mechanism of acute silver toxicity consists of reduction in Na{sup}+ uptake by blockade of gill Na{sup}+,K{sup}+-ATPase; (iii) the massspecific surface area of the gills depends on animal body mass; and (iv) the gill surface is also the major site of Na{sup}+ loss by diffusion, we hypothesized that whole body Na{sup}+ uptake rate (i.e., turnover rate) and secondarily body mass would be good predictors of acute silver toxicity. Results obtained from toxicological (LC{sub}50 of AgNO{sub}3) and physiological (22{sup left}Na uptake rate) tests performed on juvenile fish (rainbow trout, Oncorhynchus mykiss), early juvenile and adult crayfish (Cambarus diogenes diogenes), and neonate and adult daphnids (Daphnia magna) in moderately hard water of constant quality support the above hypothesis. Therefore, sensitivity to AgNO{sub}3, in terms of either total measured silver or free Ag{sup}+, was reliably predicted from the whole body Na{sup}+ uptake rate in animals with body mass ranging over 6 orders of magnitude (from micrograms to grams). A positive log-log correlation between acute AgNO{sub}3 toxicity and body mass of the same species was also observed. Furthermore, the whole body Na{sup}+ uptake rate was inversely related to body mass in unexposed animals. The combination of these last two results explains why the small animals in this study were more sensitive to Ag{sup}+ than the larger ones. Taken together, these results clearly point out the possibility of incorporating the Na{sup}+ uptake rate into the current version of the Biotic Ligand Model to improve the predictive capacity of this model. In the absence of information on Na{sup}+ uptake rate, then body mass may serve as a surrogate.
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