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

Raw data and model script from journal article with the same title published in Environmental toxicology and Chemistry (ET&C). Abstract of paper:

Ecological interactions and abiotic stress factors may significantly affect species sensitivities to toxicants and these are not incorporated in standard single species tests. This study tests the hypothesis if 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-day daphnid population size was considerably reduced by increased Zn and by decreased P supply with a significant (p<0.001) interaction among both 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’s found at the highest pH and high P treatments. For both experiments, Zn toxicity to D. magna was correctly predicted within a factor of two for EC50-values and are 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. This study illustrated 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.

Included files:
- Script_population_model.m: Model script (written in matlab) with equation numbers as described in paper.
- raw_data.xlsx: Excel file with the raw data from Expt. 1 and Expt. 2, including final D. magna population size, algal concentration and P/Zn measurements.