Effect of age on acute toxicity of cadmium, copper, nickel, and zinc in individual-metal exposures to Daphnia magna neonates.

In previous studies, variability was high among replicate acute cadmium (Cd) Daphnia magna lethality tests (e.g., >10-fold range of median effect concentrations [EC50s]), less among zinc (Zn) tests, and relatively low for copper (Cu) and nickel (Ni) tests. Although the US Environmental Protection Agency's (USEPA's) protocol includes starting toxicity tests with neonates less than 24 h old, the authors hypothesized that age-related differences in sensitivity to metals might occur even within that relatively narrow age range. Daphnia magna neonates were collected during 3 age windows (0-4 h, 10-14 h, and 20-24 h old) and immediately exposed to each of the 4 metals for 48 h using the standard USEPA protocol. In repeated sets of tests during different weeks, the Cd EC50 of the youngest neonates was approximately 10-fold greater than the EC50 of the oldest neonates (i.e., Cd was less toxic to the youngest neonates) and the EC50 of neonates aged 10 h to 14 h was intermediate. Age-related differences were negligible in Cu, Ni, and Zn tests. Therefore, variability in toxicity of Cd may partly be caused by temporal variability in neonate age at the start of toxicity tests. Decreasing the age range of D. magna used in toxicity tests could help to improve the accuracy and precision of toxicity models, particularly for metal mixtures. Environ Toxicol Chem 2017;36:113-119.