Phosphorus nutrition alters herbicide toxicity on Daphnia magna

We examined the effects of algal phosphorus (P) content on the toxicity of a common herbicide, WeatherMAX Roundup (WMR), to Daphnia magna. The growth, reproduction, and survival of D. magna were assessed with animals consuming different food P content and exposed to different concentrations of WMR. While the effects of WMR on mortality increased with time of exposure, we found no interactive effects between food P content and WMR on daphnid survival over any of time periods examined (4, 10, or 20 days). In contrast, we found interactive effects of WMR and dietary P content on Daphnia juvenile growth (measured after 6 days) with the greatest effects of WMR on animals consuming P-rich food. Interactive effects of WMR and food P content were also found on some aspects of Daphnia's reproduction (number of broods and total offspring production) with P-deprived animals most affected by WMR exposure. Our results demonstrate that P-nutrition can alter the toxicity of WMR on key life-history traits of D. magna but that the nature and strength of these effects differ among the traits examined. The effects of P-nutrition on WMR-toxicity likely reflect changes in the exposure to and/or incorporation of WMR into animal bodies associated with changes in growth resulting from poor nutrition and the ability of animals to repair ensuing damage. Given the widely variable nutritional state of animals in nature, this differential toxicity of WMR with food quality warrants further study and should be incorporated in future risk assessments of this widely used chemical.