A Novel Approach to Determining a Population-Level Threshold in Ecological Risk Assessment: A Case Study of Zinc

ABSTRACT

A novel approach to population-level assessment was applied in order to demonstrate its utility in estimating and managing the risk of zinc in a water environment. Much attention has been paid to population-level risk assessment, but there have been no attempts to determine a “safe” population-level concentration as an environmental criterion. Based on the published results of toxicity tests for various species, we first theoretically derived a threshold concentration at which a population size is unchanged due to the adverse effects of zinc exposure. To derive a zinc concentration that will protect populations in natural environments, we adopted the concept of species sensitivity distribution. Assuming the threshold concentrations of a set of species are log-normally distributed, we calculated the 95% protection level of zinc (PHC₅ :population-level hazardous concentration of 5% of species), which is 107 μg/L. Meanwhile, the 95% protection criterion (HC₅) based on conventional individual-level chronic toxicity, was calculated to be 14.6 μg/L. The environmentally “safe” concentration for a population-level endpoint is about 7 times greater than that for an individual-level endpoint. The proposed method provides guidance for a pragmatic approach to population-level ecological risk assessment and the management of chemicals.

Key Words:

• ecological risk assessment
• SSD
• HC₅
• zinc
• PSD
• PHC₅
• extinction

ACKNOWLEDGMENTS

This research is financially supported by the New Energy and Industrial Technology Development Organization. We thank Dr. Yap for providing original data from his study, as well as Dr. Takehiko Hayashi and Dr. Yao-Bin Meng for reading our earlier manuscript and for their helpful comments, Dr. Noriyuki Yamaguchi for the technical assistant for statistics, and anonymous reviewers for helpful comments. All nonlinear fitting and statistical analyses were done with Mathematica (Wolfram Research, Inc.) version 5.2 on an Apple G5 computer. We used ImageJ software (http://rsb.info.nih.gov/ij/) for reading graphs on hard copy, and we thank the software developers.