Repeated and multiple stress (exposure to pesticides) on aquatic organisms
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A systematic literature search identified publications relevant to repeated (94 papers) and mixture exposure (152 papers) of aquatic organisms. Limited data are available on pulsed exposure of pesticides and reports on repeated pulse or fluctuating exposures are even rarer. Although there has been recognition of the need for more realistic testing scenarios these lack a systematic approach. Among the reports on pesticide mixtures most of the plant studies evaluated interactions between mixtures of herbicides or fungicides / algicides. A small number of reports evaluated the effects on invertebrates of mixtures of pesticides with the same mode of action and these showed additive toxicity. More studies in invertebrates assessed interactions between pesticides with different modes of action including herbicide/ insecticide combinations some of which showed toxicity greater than additive. The majority of studies in fish have investigated the toxicity of insecticide mixtures. The toxicity of most combinations was at most additive, with a small number antagonistic. A small number of papers reported mixture effects of pesticides in amphibians; more than additive effects were observed with atrazine combinations. Simple models can be used to determine the additive toxicity of pesticide mixtures based on concentration addition or independent action. More complex models to determine the scale of synergy are species- and dose-dependent and rely on an understanding of the interactions involved. Further work is required to develop a greater understanding of the species, dose and time dependence of exposure to pesticide mixtures in which more complex interactions occur. In the interim, in the majority of cases the increase in toxicity is no greater than 3 fold when compared to additive, the deviation from additivity decreases with the increasing number of components in the mixture, and where no known interactions have been reported this may be an appropriate factor to include in risk assessments.