Toxicity of selected pesticides to freshwater shrimp, Paratya australiensis (Decapoda: Atyidae): use of time series acute toxicity data to predict chronic lethality.

Toxicity of six pesticides (carbaryl, chlorpyrifos, cypermethrin, dimethoate, diuron and fenarimol) to the freshwater shrimp, Paratya australiensis was assessed after 96 h exposures. Of the six pesticides tested, alpha cypermethrin was the most toxic to the shrimp followed by chlorpyrifos, carbaryl, dimethoate, fenarimol and diuron. Regression methods for predicting chronic toxicity (lethality) from acute lethality data with shrimp were developed and compared, and it was found that the log-log model gives the most reliable predictions of the probability of death as a function of extended exposure times. Based on this model, chronic toxicity (21 days) to P. australiensis was estimated as 0.0058 microg/L for alpha cypermethrin, 4.9 microg/L for carbaryl, 0.004 microg/L for chlorpyrifos, 89 microg/L for dimethoate, 240 microg/L for diuron and 1500 microg/L for fenarimol. Acute LC(10) values were also useful predictors of the chronic lethality. The log-log model was used to derive extrapolated chronic values that were compared to measured experimental chronic values for two fish species. The predictions of chronic toxicity based on acute toxicity data were found to give credible results for both fish species. These predictions of chronic toxicity can therefore be used in ecological risk assessments to fill in gaps with reasonable confidence where no measured estimates of chronic toxicity are available.