Effect of temperature on nickel uptake and elimination in Daphnia magna

It is well known that temperature can affect the ecotoxicity of chemicals (including metals) to aquatic organisms. It was recently reported that nickel (Ni), a priority substance under the European Water Framework directive, showed decreasing chronic toxicity to Daphnia magna with increasing temperature, between 15 and 25 °C. We performed a toxicokinetic study to contribute to an increased mechanistic understanding of this effect. More specifically, we investigated the effect of temperature on Ni uptake and elimination in D. magna (in 4 clones) using an experimental design that included Ni exposures with different stable isotopic composition and using a one‐compartment model for data analysis. Both Ni uptake and elimination were affected by temperature, and some clear interclonal differences were observed. On average (across all clones), however, a similar pattern of the effect of temperature was observed on both Ni uptake and elimination, that is, the uptake rate constant (kᵤ) and elimination rate constant (kₑ) during 72 h of Ni exposure were lower at 25 than at 19 °C, by 2.6‐fold and 1.6‐fold, respectively, and they were similar at 19 and 15 °C. This pattern does not correspond to the effects of temperature on chronic Ni toxicity reported previously, suggesting that Ni compartmentalization and/or toxicodynamics may also be affected by temperature. The data gathered with our specific experimental design also allowed us to infer that 1) the kᵤ was up‐regulated over time, that is, the kᵤ after 2 d of Ni exposure was significantly higher than the initial kᵤ, by 1.5‐ to 2.3‐fold, and 2) the kₑ decreased significantly when the external Ni exposure was stopped, by 1.2‐ to 1.9‐fold. These 2 findings are in contrast with 2 commonly used assumptions in toxicokinetic models, that is, that kᵤ is constant during exposure and kₑ is independent of external exposure. We suggest that future toxicokinetic studies consider these factors in their experimental designs and data analyses. Overall, our study contributes to the growing body of evidence that temperature affects toxicokinetics of metals (and chemicals in general), but at the same time we emphasize that knowledge of toxicokinetics alone is not necessarily sufficient to explain or predict temperature effects on (chronic) toxicity. Environ Toxicol Chem 2019;38:784–793. © 2019 SETAC