A physiologically based biotic ligand model for predicting the acute toxicity of waterborne silver to rainbow trout in freshwaters

An early silver-gill binding model using conditional equilibrium binding constants (K) was fitted to actual toxicity data for rainbow trout (Oncorhynchus mykiss) and subsequently modified to produce a mechanistically based acute toxicity model for predicting silver toxicity. The model used an "off the shelf" aquatic geochemistry software program (MINEQL(+)) and physiologically based log K values to predict the acute effects of waterborne silver in rainbow trout. The final version of the model does not predict total gill-silver loading, as the early model did, but rather predicts the binding of Ag+ to key toxic sites on the gill and incorporates the effects of cation competition at these sites. The acute toxicity model for Ag+ provided the best fit to toxicity data when a log K value for the affinity of these sites was 7.6 with cationic competition log K values for Na+ and Ca2+ of 2.9 and 2.3, respectively. A log K for Ag-DOM of 9.0 was used representing strong Ag+ binding to dissolved organic matter. The model we present is easy to use and provides a good match with previously published acute AgNO3 toxicity data for rainbow trout from 31 data sets in 10 studies. The modified model is now ready for full verification with a greater range of laboratory and natural waters.