The WHAM-F-rox model quantifies the combined toxic effects of protons and metal cations towards aquatic organisms through the toxicity function (F-rox), a linear combination of the products of organism-bound cation and a toxic potency coefficient for each cation. We describe the application of the model to predict an observable ecological field variable, species richness of pelagic lake crustacean zooplankton, studied with respect to either acidification or the impacts of metals from smelters. The fitted results give toxic potencies increasing in the order H+