The effects of Zn-contaminated diets on Daphnia magna reproduction may be related to Zn-induced changes of the dietary P content rather than to the dietary Zn content itself

The effect of dietary zinc (Zn) exposure to Daphnia magna fed living algae remains unsure as existing experimental data exhibit considerable inconsistency. In this study, we examined if Zn-induced changes in nutritional quality (i.e., the molar carbon to phosphorus ratio (C:P) and concentrations of essential omega-3-poly-unsaturated fatty acids (ω3-PUFA)) may contribute to the reproductive effects of dietary Zn exposure to D. magna. We prepared 8 different algal diets differing in Zn content, C:P ratio and ω3-PUFA, by varying the culture conditions (i.e., exposure duration) and culture medium (i.e., Zn concentration and mineral composition). These diets were representative for the diets typically used in published dietary metal toxicity bioassays. The algal diets were offered to D. magna during a standard chronic bioassay, using reproduction as endpoint. A generalized linear model (GLM) was used to determine which algal characteristics significantly explained the observed variability in D. magna reproduction. The most parsimonious GLM resulting in the best prediction of the first brood size had the molar C:P ratio as the sole predictor. The 21-day reproduction was also predicted best by the molar C:P ratio, whereas the contribution of other variables (notably Zn and ω3-PUFA content of the diet) to enhanced predictability was only marginal. In addition, our GLM, which only uses C:P as a predictor, could accurately predict reproduction in an independent (previously published) chronic bioassay with dietary Zn and D. magna. Furthermore, this GLM also accurately predicted the observed effects of algal C:P ratio shifts on D. magna reproduction as reported in ecological literature. Our analysis highlights that the reproductive effects of dietary Zn exposure in D. magna, as observed in previous studies, are probably not caused by direct toxicity of Zn in the diet, but may rather be related to Zn-induced shifts of the dietary C:P ratio. Our study thus seems to resolve inconsistencies among results from different previous studies and has important implications for the experimental design of future dietary metal toxicity research.