Suitability of test media containing EDTA for the evaluation of acute metal toxicity to Daphnia magna straus.

The acute toxicity of six metals [Hg, Cd, Cu, Zn, Cr(II), and Cr(VI)] to Daphnia magna neonates was evaluated using three different test media (Elendt M7, a complex medium containing EDTA; ASTM hard water and EEC, two simple media free of chelators). The EC50 values, at both 24 and 48 h, obtained for Zn, Cr(II), and Cr(VI) were similar in all of the media tested. Hg was more toxic in ASTM than in M7 and in EEC media. The toxicity of Cd and Cu was similar in ASTM and EEC media and higher when evaluated in M7 medium. Thus, M7 should be used only carefully for the toxicity evaluation of mixtures and effluents containing metals. It is recommended, however, that it be excluded from use in tests evaluating samples of unknown composition, or those known to contain Cu and Cd. For the metals tested in this study, a factor of five applied to each 48-h EC50 would be sufficient in order to attain the respective acute NOECs for immobility.