• Ecological;
  • Risk;
  • Mixture;
  • Method;
  • Striped bass


The method developed here provides a quantitative, objective measure of ecological risk for natural populations exposed to mixtures of chemical contaminants. It is founded on generally accepted risk assessment concepts: use of toxic units to assess the joint toxic effects of mixtures and expression of ecological risk as a relationship between toxicological end points and estimated environmental concentrations. Toxicologicai end points may be regulatory levels with zero variance and species-dependent concentrations with estimates of variance. Risk is the probability that a linear combination of toxic units exceeds 1, which expresses the probability that a measurement end point (e.g., 50% mortality in 96 h) will occur. Computations have three variations. One addresses concentration addition, in which chemicals act independently to produce similar biological effects. For noninteractive joint action with no addition, in which the biological response to the mixture is not significantly different from the response to the most toxic component, the method reduces to an analysis of extrapolation error. For other noninteractive joint action — antagonism, partial addition, and supra-addition — a correction factor similar to Konemann's mixture toxicity index is applied. An initial validation using published data indicated that increased in situ striped bass mortality was generally associated with elevated risk estimates. The method is applicable to many organisms and toxicant mixtures.