Environmental Toxicology and Chemistry

In previous work, the authors of the present study intensively studied the acute responses of the cladoceran Ceriodaphnia dubia to major geochemical ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl^–, SO₄^2–, and HCO₃^–/CO₃^2–), culminating in the development of models to predict acute toxicity from ionic composition. To quantitatively evaluate whether the toxicological behavior of major ions observed for C. dubia extends to chronic toxicity, the authors conducted 58 chronic toxicity tests with individual major salts and binary mixtures thereof. Chronic responses paralleled those demonstrated previously for acute exposure. Acute-to-chronic ratios were fairly consistent among salts, with values of approximately 1.8 for acute 50% lethal concentration (LC50) to chronic 50% effect concentration (EC50) and 2.8 for LC50/EC20 when expressed on an activity basis. Adjusting the previous acute toxicity model for acute-to-chronic ratios yielded chronic models that predict chronic toxicity within the range of intertest variability. Because these models are informed by a wide range of ion mixtures, they should provide robust assessment tools for natural waters enriched with major ions.