Response of Daphnia magna to pulsed exposures of chlorpyrifos

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Abstract
Because aquatic organisms can be exposed to contaminants in an episodic manner, it is necessary to determine whether standard toxicity tests adequately simulate the toxicity of short‐lived compounds, such as the organophosphate insecticide chlorpyrifos (CPF). We conducted experiments to evaluate the effect of binary combinations of concentration, duration, and interval of CPF exposures to Daphnia magna. Organisms were monitored for changes in survival and/or reproduction after pulsed exposure to CPF at 0.12, 0.25, 0.5, or 1.0 μg/L. The exposure duration resulting in a 50% response in survival was 6.5 h at 1.0 μg/L, 12.2 h at 0.5 μg/L, and 48 h at 0.25 μg/L. Daphnids exposed to two 12‐h pulses of CPF at 0.5 μg/L responded similarly (≥85% mortality) regardless of the pulse interval (0, 3, 7, 14 d). However, daphnids were able to survive a 12‐h exposure of CPF at 0.5 μg/L if the exposure regime was separated into two 6‐h pulses with a minimum interval of 3 d. Further experiments suggested that the interval studies may have been potentially confounded, because organisms exposed on day 3, 7, or 14 seemed more sensitive than those exposed as neonates. Additionally, no latent effects were measured for daphnids that survived the initial exposure.