John Wiley & Sons, Ltd.

Testing the individual effective dose hypothesis

The assumption of the individual effective dose is the basis for the probit method used for analyzing dose or concentration ‐ response data. According to this assumption, each individual has a uniquely innate tolerance expressed as the individual effective dose (IED) or the smallest dose that is sufficient to kill it. An alternative to IED, stochasticity suggests that individuals do not have uniquely innate tolerance; deaths result from random processes occurring among similar individuals. While the probit method has been used extensively in toxicology the underlying assumption has not been tested rigorously. The goal of this study was to test which assumption, IED or stochasticity, best explained the response of Daphnia magna exposed to multiple pulses of copper sulfate (CuSO4) over 24 days. Daphnia magna were exposed to subsequent age‐dependent 24‐h LC50 copper (Cu) concentrations. Age‐dependent 24‐h LC50 values and Cu depuration test were determined prior to the 24‐d bioassay. The LC50 values were inversely related to organism age. The Cu depuration of D. magna did not depend on age or Cu concentration and five days was sufficient recovery time. Daphnia magna were exposed to four 24‐h Cu exposures, surviving organisms after each exposure were transferred to Cu‐free culture media for recovery before the next exposure. Stochasticity appropriately explained the survival and reproduction response of D.magna exposed to Cu. Environ Toxicol Chem 2013;9999:XX–XX. © 2013 SETAC

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