WHAM-FTOX?-An aquatic toxicity model based on intrinsic metal toxic potency and intrinsic species sensitivity

We developed a model that quantifies aquatic cationic toxicity by a combination of the intrinsic toxicities of metals and protons and the intrinsic sensitivities of the test species. It is based on the WHAM-FTOX model, which combines the calculated binding of cations by the organism with toxicity coefficients (alpha H, alpha M) to estimate the variable FTOX, a measure of toxic effect; the key parameter alpha M,max (applying at infinite time) depends upon both the metal and the test species. In our new model, WHAM-FTOX beta, values of alpha M,max are given by the product alpha M* x beta, where alpha M* has a single value for each metal, and beta a single value for each species. To parameterise WHAM-FTOX beta, we assembled a set of 2182 estimates of alpha M,max obtained by applying the basic model to laboratory toxicity data for 76 different test species, covering 15 different metals, and including results for metal mixtures. Then we fitted the log10 alpha M,max values with alpha M* and beta values (a total of 91 parameters). The resulting model accounted for 72% of the variance in log10 alpha M,max. The values of alpha M* increased markedly as the chemical character of the metal changed from hard (average alpha M* = 4.4) to intermediate (average alpha M* = 25) to soft (average alpha M* = 560). The values of log10 beta were normally distributed, with a 5-95 percentile range of-0.73 to +0.56, corresponding to beta values of 0.18 to 3.62. The WHAM-FTOX beta model entails the assumption that test species exhibit common relative sensitivity, i.e. the ratio alpha M,max / alpha M* is constant across all metals. This was tested with data from studies in which the toxic responses of a single organism towards two or more metals had been measured (179 examples for the most-tested metals Ni, Cu, Zn, Ag, Cd, Pb), and statistically-significant (p < 0.003) results were obtained.

Automated summary

We developed a model that measures aquatic cationic toxicity based on the toxicities of metals and protons and the sensitivities of the test species. The WHAM-FTOX beta model combines the binding of cations by organisms with toxicity coefficients to estimate the variable FTOX. The model accounted for 72% of the variance in log10 alpha M,max.