| Summary: | The results of acute test on suppression of Daphnia magna filtration activity for one toxicant (K2Cr2O7, ZnSO4 and MnSO4) were applied to carry out acute binary tests for 3 binary solutions: “Cr-Mn”, “Cr-Zn” and “Cr-Zn”, where the daphnids were exposed for 1, 2, 3, 4 and 5 hs. While the level of toxicity manifested by the “Cr-Mn” binary systems was lower than “additive”, the results that were observed in the binary solution of “Cr-Zn”, especially for the shorter exposure time of 1–3 hours, showed toxicity above “additive”. A nearly “additive effect” was observed in the binary solution of “Zn- Mn”. We supposed that the theory of metals competing for binding to biotic ligands or any yet unidentified physiological interactions of the metals could explain the toxicity effect on filtration activity of D. magna exposed in three binary solutions. So, the more-than-additive effect for the “Cr-Zn” binary system may indicate that Zn (II) demonstrated its high ability building up protein complexes due to competition with Cr (VI). The less-than-additive toxicity for the “Cr-Mn” solution likely revealed the fact that bioavailability of Cr (VI) to D. magna was reduced in the presence of Mn (II).
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