Effects of binary mixtures on the life traits of Daphnia magna.

The environment is constantly exposed to a cocktail of contaminants mainly due to human activities. Because polluted ecosystems are characterized by an amalgam of chemical compounds, the objective of the present study was to assess the joint effect of chemical mixtures to the life--history traits of Daphnia magna Straus. For that a binary mixture of two neonicotinoid insecticides, imidacloprid and thiacloprid, and another one of imidacloprid with nickel chloride were tested. Theoretical models have been developed and applied in studies with chemical mixtures, predicting toxicity based on their modes of action: concentration addition (CA) and independent joint action (IA) models. Still there are cases where deviations are observed (e.g. synergistic or antagonistic behaviors, dose ratio or level dependency). In this study, the effects of the individual compounds and their mixtures were studied in a chronic test where reproduction, survival and body length were evaluated in D. magna. Regarding single compound effects, it was observed that the most toxic was nickel chloride followed by thiacloprid and imidacloprid. For the mixture exposure of imidacloprid and thiacloprid, a synergistic pattern was observed in the sublethal doses used for the number of neonates produced, while for the body length the best fit was shown with the CA model. In the mixture exposure of imidacloprid and nickel, no deviation from the IA was observed for the neonate production data; for the body length parameter, a synergistic pattern was observed in low doses of the chemicals while an antagonistic pattern was observed.