Temperature is known to alter the sensitivity of Daphnia magna to metals. In a population experiment, it was investigated if the temperature effect on nickel (Ni) sensitivity, as observed in toxicity tests, was also present in D. magna populations. Surprisingly, independent of temperature, no significant population-level effects of Ni were observed at concentrations that reduced apical reproduction.
A mechanistic model was able to confirm the unexpected absence of Ni effects at the population level at three temperature levels (15, 20 and 25°C). In the model, Ni mortality was compensated by reduced food starvation, resulting in less intra-specific competition and higher resource efficiency. Predicted population-level effect concentrations were higher than the concentrations imposed in the experiment. In addition, predicted population-level EC50 values showed less variation between temperature treatments (1.9-fold variation) compared to the individual level (6.5-fold variation).
Our research shows the potential of mechanistic models as a tool for effect extrapolation, for experimental design optimization, as well as population-level risk assessment.