The results of acute test on suppression of Daphnia magna filtration activity for one toxicant (K 2 Cr 2 O 7 , ZnSO 4 and MnSO 4 ) 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).
The present study focused on observation of effect of potassium dichromate as standard toxicant on feeding activity of D. magna. Isolated feeding suppression was measured as fluorescence intensity of ingested microbeads by daphnids and expressed as its feeding rate (EC50FI). We calculated the sub-lethal feeding suppression endpoints for 1, 2, 3, 4 and 5 hours exposure in potassium dichromate and compared these values with EC50 concentration of immobilization test of D. magna. The most close to immobilization (EC50)result was obtained as for 4 as well for 5 hours exposure time; however 4 hs exposure was chosen as more adequate exposure time for our design of feeding suppression bioassay.
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