Predicting the joint algal toxicity of multi-component s-triazine mixturesat low-effect concentrations of individual toxicants

Herbicidal s-triazines are widespread contaminants of surface waters. They are highly toxic to algae and other primary producers in aquatic systems. T his results from their specific interference with photosynthetic electron t ransport. Risk assessment for aquatic biota has to consider situations of s imultaneous exposure to various of these toxicants. In tests with freshwate r algae we predicted and determined the toxicity of multiple mixtures of 18 different s-triazines. The toxicity parameter was the inhibition of reprod uction of Scenedesmus vacuolatus. Concentration - response analyses were pe rformed for single toxicants and for mixtures containing all 18 s-triazines in two different concentration ratios. Experiments were designed to allow a valid statistical description of the entire concentration-response relati onships, including the low concentration range down to ECL Observed effects and effect concentrations of mixtures were compared to predictions of mixt ure toxicity. Predictions were calculated from the concentration-response f unctions of individual s-triazines by applying the concepts of concentratio n addition and independent action (response addition) alternatively. Predic tions based on independent action tend to underestimate the overall toxicit y of s-triazine mixtures. In contrast, the concept of concentration additio n provides highly accurate predictions of s-triazine mixture toxicity, irre spective of the effect level under consideration and the concentration rati o of the mixture components. This also holds true when the mixture componen ts are present in concentrations below their individual NOEC values. Concen trations statistically estimated to elicit non-significant effects of only 1% still contribute to the overall toxicity. When present in a multi-compon ent mixture they can co-operate to give a severe joint effect. Applicabilit y of the findings obtained with s-triazines to mixtures of other contaminan ts in aquatic systems and consequences for risk assessment procedures are d iscussed. (C) 2001 Elsevier Science B.V. All rights reserved.