Predictive value of laboratory tests with aquatic invertebrates: influence of experimental conditions

Considering the difficulty of making meaningful extrapolations of laboratory bioassay data to real world situations, short-term tests have been carried out in a factorial pattern to determine the magnitude of effect variation resulting from changes in experimental abiotic conditions.Three selected zooplankton species (the rotifer Brachionus plicatilis, the brine shrimp Artemia salina and the waterflea Daphnia magna) have been exposed to increasing concentrations of two chemicals (one inorganic and one organic) in different combinations of two major environmental variables.For the brackish water rotifer B. plicatilis the acute toxicity of potassium dichromate and sodium laurysulphate was determined in 16 different combinations of temperature and salinity (10-17-24-31°C and 5-20-35-50 pro mille). For the marine crustacean A. salina, the acute toxicity of the same two chemicals was determined in 20 temperature-salinity combinations (10-15-20-25-30°C and 5-20-35-50 pro mille) and for the freshwater crustacean D. magna, 16 combinations of temperature and water hardness (7-14-21-28°C and 80-320-560-800 mg/l CaCO₃) were assayed.The entire study comprised nearly 300 complete toxicity tests. 24-h LC₅₀ values (for Artemia and Brachionus) and 24-h EC₅₀ values (for Daphnia) revealed that the variation in toxicity resulting from changing environmental conditions, is both species- and chemical-specific and (within the limits of this study) ranged from a minimum of a factor 2.5 to a maximum exceeding a factor of 100. The necessity to take variations into consideration in predictive hazard assessment studies is underlined.