Simulating toxicity of carbaryl to Gammarus pulex after sequential pulsed exposure.

Aquatic nontarget organisms are typically exposed to sequential pulses of contaminants with fluctuating concentrations. We use the semimechanistic threshold damage model (TDM) to simulate survival of the aquatic invertebrate Gammarus pulex after sequential pulsed exposure to carbaryl and compare itto a simpler model based on time-weighted averages (TWA). The TDM is a process-based model and we demonstrate how to parametrize it with data from an uptake and elimination experimenttogether with data from a survival experiment with sequential pulses. The performance of the two models is compared by the fit to the first survival experiment and the simulation of another, independent survival experiment with different exposure patterns. Measured internal concentrations in the first survival experiment are used to evaluate the toxicokinetic submodel of the TDM. The TDM outperforms the TWA model, facilitates understanding of the underlying ecotoxicological processes, permits calculation of recovery times (3, 15, and 25 days for pentachlorophenol, carbaryl and chlorpyrifos respectively) and enables us to predict the effects of long-term exposure patterns with sequential pulses or fluctuating concentrations. We compare the parameters of the TDM for carbaryl, pentachlorophenol and chlorpyrifos and discuss implications for ecotoxicology and risk assessment.