Critical analysis of factors affecting the sensitivity of zooplankton and the reproducibility of toxicity test results

Abstract

This paper concerns the variables that affect the sensitivity of test zooplankton and the reproducibility of results obtained from repeated toxicity tests. Among the variables considered are: (a) the nutrition of the test zooplankton, their health and diet; (b) culturing techniques, in particular, the effect of the ambient medium on demographic variables and testing results; (c) physical and chemical characteristics of the test compound; (d) purity of the test compound, and finally (e) the variation in results obtained with nominal and measured concentrations of the test compound. Critical factors addressed include the effects of inadequate cobalamine or selenium concentrations in the diet, adversely affecting fertility, which may result in misleading interpretation of short-term toxicity test results involving Ceriodaphnia or chronic studies using Daphnia. Other factors considered are, for example, the problems of poor health, as measured by fertility or by the reproducibility of static acute toxicity test results, and their effect on zooplankton sensitivity to toxicants. The observation that diets of several algae result in greater tolerance of zooplankton to toxicants than do diets based on synthetic food are discussed. It is also noted that organisms reared in clean, frequently replenished and renewed habitats and fed axenic cultures of algae, are healthier than animals maintained in static environments. The observation that the results of interlaboratory comparisons of zooplankton response to toxicants are more uniform and exhibit smaller standard deviations when the test animals are sustained on algal foods rather than on synthetic diets is emphasized. Variability due to test compound characteristics include the observation that test compounds of variable purity often are responsible for poor test results reproducibility, and that the reproducibility of the toxic response is often variable due to physical and chemical characteristics of the test chemical. Finally, poor interlaboratory comparisons may result when nominal concentrations are the only ones considered. Data are selected from the published literature to illustrate the above discussion.