WHAM-F_TOXβ - 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-F_TOX model, which combines the calculated binding of cations by the organism with toxicity coefficients (α_H, α_M) to estimate the variable F_TOX, a measure of toxic effect; the key parameter α_M,max (applying at infinite time) depends upon both the metal and the test species. In our new model, WHAM-F_TOXβ, values of α_M,max are given by the product α_M* × β, where α_M* has a single value for each metal, and β a single value for each species. To parameterise WHAM-F_TOXβ, we assembled a set of 2182 estimates of α_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 log₁₀α_M,max values with α_M* and β values (a total of 91 parameters). The resulting model accounted for 72% of the variance in log₁₀α_M,max. The values of α_M* increased markedly as the chemical character of the metal changed from hard (average α_M* = 4.4) to intermediate (average α_M* = 25) to soft (average α_M* = 560). The values of log₁₀β were normally distributed, with a 5-95 percentile range of -0.73 to +0.56, corresponding to β values of 0.18 to 3.62. The WHAM-F_TOXβ model entails the assumption that test species exhibit common relative sensitivity, i.e. the ratio α_M,max / α_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.