Determining the ecotoxicological mode of action of chemicals from measurements made on individuals: results from instar-based tests with Daphnia magna Straus

A short-term Daphnia toxicity test design was used to assess the ecotoxicol ogical mode of action of pollutants. It was demonstrated that by exposing D aphnia females over a single instar to three toxic substances (3,4-dichloro aniline, cadmium and fluoranthene) it was possible to successfully measure both lethal effects on egg and adult stages and nonlethal effects on food a cquisition and production rates. Dichloroaniline exposure reduced egg survi val during development at concentrations below those which affected adult s urvival or production rates. For cadmium, however, concentration effects on production were almost an order of magnitude lower than those which affect ed adult survival, and no reductions in egg survival were observed. Respons es to fluoranthene showed a different pattern again, with egg survival duri ng development and production equally affected at concentrations which affe cted adult survival. Thus the three pollutants chosen showed contrasting do minant ecotoxicological modes of action, in terms of the relative importanc e of production and mortality effects, and this could be easily assessed us ing a novel toxicity test design. These results have important implications for risk assessment since with a relatively short, and thus cost-effective test, the functional relationship between effects on lethality and product ion rates and concentration can be determined. (C) 2000 Elsevier Science B. V. All rights reserved.