Phototaxis index of Daphnia carinata as an indicator of joint toxicity of copper, cadmium, zinc, nitrogen and phosphorus in aqueous solutions

An orthogonal design method was used for examining the responses of Daphnia carinata to 16 combination mixtures of copper, cadmium, zinc, nitrogen and phosphorus in synthetic solutions. Phototaxis index (Ip), mobilization inhibition percentage (MIP) and mortality percentage (MP) were used as test endpoints for toxicity assessment. The results show that Cu and Cd were the dominant toxicants that affected the D. carinata in the test solutions. However, the contribution of nitrogen, relative to other components, to the overall toxicity to D. carinata could increase over time. It was found that three toxicity parameters (i.e. Ip, MIP and MP) used in this study were significantly interrelated (at p < 0.003 level). Cluster analysis also indicates that there was a general agreement between the toxicity indicated by Ip and the universally accepted test endpoints (MIP or MP); MIP and MP represent the universally accepted test endpoints, i.e. mobilization inhibition and mortality. These results suggest that the phototaxis index of D. carinata is an appropriate test endpoint for toxicity testing of environmental samples. This has implications for developing rapid, non-destructive bioassays for ecotoxicological assessment of aquatic environments.