Age as a factor influencing results in the acute daphnid test with Daphnia magna Straus

Contrary to assumptions which have been current up to the present, juvenile daphnids of the species D. magna Straus exhibit a very varied sensitivity towards toxic chemicals between the ages of 0–24 h. During this period the greatest susceptibility towards potassium dichromate is reached between the ages of 22–24 h; younger daphnids are up to four times less sensitive. For this reason it is impossible to arrive at a reliable prognosis with potassium dichromate in defined concentration as reference substance using test groups of unknown age structure. However, positive controls for verifying correct testing procedure rely on comparable sensitivity amongst the test organisms and are essential for validating the test results. The range of 0.6–1.7 mg/l for the 24 h-EC50 of potassium dichromate, which the ISO norm 6341 propagates as validity criterium, does not take sufficient account of the connection between dosage and effectiveness of this substance and thereby masks age related inequalities in the test animals. Therefore it cannot substitute the positive control which is missing in the test specifications. As the toxicity of potassium dichromate is pH-dependent a specific carbonate hardness for the test medium should be determined.