A comparison of the lethal and sublethal toxicity of organic chemical mixtures to the fathead minnow (Pimephales promelas).

The joint toxic effects of known binary and multiple organic chemical mixtures to the fathead minnow (Pimephales promelas) were defined at both the 96-h 50% lethal effect concentration (LC50) and sublethal (32-d growth) response levels for toxicants with a narcosis I, narcosis II, or uncoupler of oxidative phosphoralation mode of toxic action. Experiments were designed to define the degree of additive joint toxicity for mixtures of specific xenobiotics that are believed to act through a similar or different primary mode of toxic action. Our results support the general conclusion that concentration addition is expected for the joint toxicity of similarly acting toxicants. When chemicals were thought to act by a dissimilar mechanism, the combined effects we observed at both of the response levels tested were less than predicted by concentration addition, but usually more toxic than that predicted by the independent action/response addition model. It was demonstrated in multichemical mixtures that several toxicants can act together in a nearly additive fashion to produce effects even when they are present at concentrations below their individual no-observed-effect concentration. Concentration-response relationships for test chemicals at both the lethal and sublethal responses were defined for each of the three modes of toxic action studied. When normalized for potency, it was observed that one relationship could be defined to predict lethality to juvenile fathead minnows when exposed to individual chemicals with either a narcosis I, narcosis II, or uncoupler mode of toxic action. These sublethal relationships were similar for the narcosis I and narcosis II test chemicals, but a steeper response was observed for tests conducted with uncouplers.