Zinc toxicity to Daphnia magna in a two-species microcosm can be predicted from single-species test data: The effects of phosphorus supply and pH.

Abstract

Ecological interactions and abiotic stress factors may significantly affect species sensitivities to toxicants, and these are not incorporated in standard single-species tests. The present study tests whether a model, calibrated solely on single-species data, can explain abiotic stress factors in a two-species microcosm, a test applied to the effects of nutritional stress (phosphorus [P] limitation) on zinc (Zn) toxicity to Daphnia magna. A population model was developed based on P- and Zn-dependent algal and daphnid growth. Two separate two-species (phytoplankton Pseudokirchneriella subcapitata and consumer D. magna) microcosm experiments with P × Zn factorial combinations and a different pH (7.3 and 7.8) were set up to validate the model. The 21-d daphnid population size was considerably reduced by increased Zn and by decreased P supply, with a significant (p < 0.001) interaction between the 2 factors. The observed median effective concentration (EC50) of Zn on D. magna population size varied 12-fold (25 to 310 μg Zn L-1 ), with the lowest EC50 values found at the highest pH and high P treatments. For both experiments, Zn toxicity to D. magna was correctly predicted within a factor of 2 for EC50 values, and this is explained by the model through 1) a higher phytoplankton Zn sensitivity at higher pH, affecting food supply to D. magna, and 2) an increased algal P content at higher Zn, offering a nutritional benefit to daphnids that counteracts direct Zn toxicity under P limitation. The present study illustrates that indirect effects of Zn via producer-consumer relationships can outweigh the direct toxic effects and that models calibrated solely on single-species test data can help with interpreting these results in two-species systems. Environ Toxicol Chem 2018;37:2153-2164. © 2018 SETAC.