Joint acute toxicity of diazinon and copper to Ceriodaphnia dubia

Diazinon and copper are two contaminants that are widely found in urban streams and in municipal wastewater effluents. Because these contaminants may be found concurrently, the potential for their joint toxicity is of interest, particularly with regard to toxicity testing of wastewater effluents and the ecological implications of simultaneous exposures in urban streams. Although interactions between metals are well studied, relatively little is known about interactions between metals and organophosphate compounds such as diazinon. In this study, the interaction between copper and diazinon was evaluated using cladoceran (Ceriodaphnia dubia) in 48‐h static tests within laboratory water. Using toxic units derived from concurrently established 48‐h median lethal concentration values (LC50) of test organisms of each individual toxicant, the effects of the mixture of copper and diazinon on the survival of C. dubia were shown to be generally nonconcentration additive (LC50 significantly greater than one toxic unit). However, evaluation of the dose‐response relationship across the entire range of effect levels revealed that the mortality induced by the mixture of copper and diazinon supported concentration additivity at higher effect levels.