Species Sensitive Distribution For Aquatic Biota Exposed To Chlorpyrifos

Species sensitivity distribution (SSD) is an extrapolation method, which is based on the single-species toxicity test. In recent years, the applications of SSD in the derivation of environmental quality criteria and ecological risk assessment for contaminated ecosystems have become the research hotspots. The increasing pollutions of organophosphorus pesticides (OP) in water have been of concerns. Taking the widely used chlorpyrifos as the object, the SSD models were developed on the median effective concentrations (EC50) of chlorpyrifos to aquatic species at various trophic levels. Four different distributions, i.e., log-logistic, log-normal, log-triangular and Weibull, were chosen to fit the SSD. Then, all models were tested and evaluated using probability plots and good-of-fit tests. The uncertainty analysis for model parameters, hazardous concentration for 5% of the species (HC5), and potentially affected fraction (PAF) were performed through parametric bootstrap simulations. The ways of reducing the uncertainty of SSD through the model selection were discussed in the paper. Besides, the development of SSD based on the small data sample and the minimum sample size for constructing a valid SSD were also studied. The present study will facilitate the environmental regulatory agency to perform ecological risk assessment to a polluted aquatic environment, and will offer the scientific evidences and technical supports in derivation of water quality criteria. The main results are as follows:(1) The results of probability plots and good-of-fit tests showed that SSDs for aquatic biota exposed to chlorpyrifos were well fitted with all the specific probability distributions. Based on the Anderson-Darling test, the fitted log-triangular distribution was preferred, which was determined by three parameters, a=-2.2260, b=3.8306, and c =2.9032, with standard error 0.4016,0.1588 and 0.0401,respectively.(2) According to the built log-triangular SSD model, the HC5 and the criteria maximum concentration (CMC) of chlorpyrifos were 0.1048μg/L and 0.0524μg/L, respectively. In addition, according to the reported data and log-triangular SSD model, the PAF of species exposed to chlorpyrifos in the Laizhou Bay (Bohai Sea, China) area, Iporanga River, Preto River and Piloes River were 0.01%,4.84%,11.01% and 12.35%, respectively. These PAF values could simply characterize that the ecological risk of chlorpyrifos in Laizhou Bay area was very low; the ecological risk of chlorpyrifos in Iporanga River was lower, but the strengthening monitoring was a necessary measure to prevent a higher risk harm happening; the ecological risks of chlorpyrifos Preto River and Piloes River were higher, and chlorpyrifos should be banned.(3) The results of uncertainty analysis for model parameters indicated the high robustness of each model. Obvious differences of the estimation results of HC5 and PAF among all SSD models showed that the model selection led to the uncertainty of SSD.(4) In view of reducing the uncertainties of SSD through the model selection, the nonparametric bootstrap method was introduced. The median estimation of the HC5 and CMC of chlorpyrifos were 0.0700μg/L and 0.0350μg/L, respectively. The PAF of species exposed to chlorpyrifos in Iporanga River, Preto River and Piloes River were 7.00%,16.00% and 16.00%, respectively. The PAF of species exposed to chlorpyrifos in the Laizhou Bay areawas below the lower tail of Bootstrap distribution, and the corresponding risk might not be significant. The nonparametric bootstrap method gave the more conservative results when compared to the log-triangular SSD model.(5) Based on the parametric bootstrap method, a sample with size 9 was minimum requirement for a valid and valuable SSD,while the nonparametric bootstrap method gave a sample size with 10 that was the minimum for constructing a valid SSD.