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 ecotoxicological mode of action of pollutants. It was demonstrated that by exposing Daphnia females over a single instar to three toxic substances (3,4-dichloroaniline, cadmium and fluoranthene) it was possible to successfully measure both lethal effects on egg and adult stages and nonlethal effects on food acquisition and production rates. Dichloroaniline exposure reduced egg survival during development at concentrations below those which affected adult survival or production rates. For cadmium, however, concentration effects on production were almost an order of magnitude lower than those which affected adult survival, and no reductions in egg survival were observed. Responses to fluoranthene showed a different pattern again, with egg survival during development and production equally affected at concentrations which affected adult survival. Thus the three pollutants chosen showed contrasting dominant ecotoxicological modes of action, in terms of the relative importance of production and mortality effects, and this could be easily assessed using 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 production rates and concentration can be determined.