Development and validation of a chronic Pb bioavailability model for the freshwater rotifer Brachionus calyciflorus

The univariate effects of Ca, pH, and dissolved organic carbon (DOC) on chronic (48-h) Pb toxicity to the freshwater rotifer Brachionus calyciflorus were investigated. High pH (8.2) and higher concentrations of DOC were protective against filtered Pb toxicity, whereas Ca was not. However, expressed as the free Pb²⁺ ion, Pb²⁺ toxicity increased with increasing DOC concentration, indicating that Pb-fulvic acid (FA) complexes may be bioavailable and may contribute to toxicity. Two different bioavailability models were developed. The pH effect was modeled as a single-site competition effect by H⁺ (log K_HBL = 7.14). In the first model, only this H⁺ effect was considered; in a second model, a log-linear fulvic acid bioavailability effect (S_FA = 0.602) was also incorporated. Both models predicted chronic Pb toxicity for most waters used for model development within a 2-fold error. To validate the predictive capacities of the models, chronic Pb toxicity to B. calyciflorus was tested in 5 natural waters and a reference water. Both models consistently underestimated Pb toxicity in the natural waters, as a result of a shift in rotifer sensitivity to Pb between the development and the validation test series. However, optimizing the intrinsic sensitivity in the models specific for the validation test series resulted in reasonable predictions of Pb toxicity in the natural waters. The second, more complex model predicted chronic Pb toxicity most accurately. The protective effects of water chemistry on Pb toxicity to Brachionus are comparable to those observed for Ceriodaphnia. However, the developed Brachionus models were not able to accurately predict chronic Pb toxicity to the rotifer Philodina rapida. It remains unclear why the influence of water chemistry on chronic Pb toxicity appears to be different between 2 rotifer species. Environ Toxicol Chem 2016;35:2977-2986. © 2016 SETAC.