| Abstract
| - Aquatic nontarget organisms are exposed to fluctuatingconcentrations or sequential pulses of contaminants, so weneed to predict effects resulting from such patterns ofexposure. We present a process-based model, the ThresholdDamage Model (TDM), that links exposure with effectsand demonstrate how to simulate the survival of the aquaticinvertebrate Gammarus pulex. Based on survival experimentsof up to 28 days duration with three patterns of repeatedexposure pulses and fluctuating concentrations of twopesticides with contrasting modes of action (pentachlorophenol and chlorpyrifos) we evaluate the new model andcompare it to two approaches based on time-weightedaverages. Two models, the Threshold Damage Model andthe time-weighted averages fitted to pulses, are able tosimulate the observed survival (mean errors 15% or less,r2 between 0.77 and 0.96). The models are discussed withrespect to their theoretical base, data needs, and potentialfor extrapolation to different scenarios. The ThresholdDamage Model is particularly useful because its parameterscan be used to calculate recovery times, toxicokineticsare separated from toxicodynamics, and parameter valuesreflect the mode of action.
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