Toxicity of cadmium, copper, and zinc and their binary mixtures to Daphnia magna in laboratory and field-collected waters

Many surface waters are impacted by elevated concentrations of multiple metals resulting from acid mine drainage and industrial inputs. Because these elevated concentrations of metal mixtures sometimes can be toxic to aquatic life, there is a need to better understand and predict the toxicity of metal mixtures in the environment. Laboratory toxicity tests are important for providing data to develop multi-metal toxicity models. However, many laboratory toxicity tests do not closely mimic the aqueous geochemistry at contaminated sites. Therefore, it is difficult to accurately predict the toxicity of metal mixtures in the field. I investigated differences in the toxicity of Cd, Cu, and Zn to neonates of a freshwater invertebrate (Daphnia magna) in laboratory waters and in waters from the Clear Creek Superfund Site, a Colorado stream impacted by acid mine drainage. I exposed Daphnia magna neonates to individual metals and binary combinations of metals. I also used the Biotic Ligand Model to evaluate how accurately the model can predict the toxicity of Cu and Zn in field water compared to standard laboratory waters. The Cu-Zn binary combinations tested provided evidence of response-additive to slightly more-than-additive toxicity in field water, and the Cd-Zn binary combinations tested provided evidence of less-than-additive toxicity in field water. The toxicity of Cu and Zn in field water was not accurately predicted by the Biotic Ligand Model. This study highlights the importance of water chemistry when extrapolating results of metal-mixture toxicity tests from the laboratory to the field, and it emphasizes the need for statistical models to quantify the differences in toxicity of metal mixtures between varying water types.