Effect of pulse frequency and interval on the toxicity of chlorpyrifos to Daphnia magna

Due to the episodic nature in which organisms are exposed to non-point sour ce pollutants, it is necessary to understand how they are affected by pulse d concentrations of contaminants. This is essential, as standard toxicity t ests may not adequately simulate exposure scenarios for short-lived hydroph obic compounds, such as chlorpyrifos (CPF), a broad-spectrum organophosphat e insecticide. Studies were conducted with 7-day old Daphnia magna for 7 da ys to evaluate the effect of pulse frequency and interval among multiple CP F exposures. Daphnids were exposed to a total exposure of either 12 h at 0. 5 mug/l or 6 h at 1.0 mug/l nominal CPF, respectively, in all studies. For interval studies, D. magna were exposed to two pulses of CPF at each concen tration, with 0-96-h intervals between pulses. For frequency studies, D. ma gna were exposed to each CPF concentration altering the pulse scheme by dec reasing the exposure duration but increasing the number of pulses, keeping the total exposure time the same. The pulse interval between multiple pulse s in these experiments was 24 h. Our results suggest that D. magna can with stand an acutely lethal CPF exposure provided that there is adequate time f or recovery between exposures. (C) 2001 Elsevier Science Ltd. All rights re served.