The Apparent Thresholds of Toxicity to Daphnia Magna for Chlorides of Various Metals When Added to Lake Erie Water

The apparent threshold concentrations of toxicity to Daphnia magna are presented for 25 cations when added to Lake Erie water. Various factors such as the specific toxic actions of the cations, high acidities, excessive osmotic pressures, and precipitates operated to kill daphnids when the salts used were above the thresholds. When factors other than the specific toxic actions may have been responsible for death at threshold concentrations they are described. The thresholds presented are compared with those found by other investigators for Daphnia and other animals, especially fish. In general the Daphnia and related forms are more susceptible to cations than are fish. Ecdysis is a critical period in the life of Daphnia, particularly in the presence of certain salts. Various explanations are advanced to account for this fact. Since daphnids are more susceptible at molting it is essential that exposure periods be long enough to provide sufficient time for all the experimental animals to molt when threshold concentrations of toxicity are determined. This finding is expected to apply in toxicity experiments with other arthropods. That the maximum safe concentrations in which wastes may be permitted to enter natural waters are not likely to exceed the threshold concentrations to Daphnia is discussed.