The US Environmental Protection Agency's stressor identification guidance: A process for determining the probable causes of biological impairments

The United States Environmental Protection Agency (USEPA) has ongoing programs to encourage the evaluation of stream conditions based on biological indicators. Bioassessments reveal impairments but do not identify causes of impairments, a necessary step in the restoration of aquatic life. Furthermore, changes in biological condition are often caused by multiple stressors (chemical, physical, biological). To address this need, the USEPA has developed guidance to identify probable causes of biological impairments in aquatic ecosystems and to provide a structure for organizing the scientific evidence to make a credible case. The concepts and organizational structure of the process have potential applications to most ecoepidemiological investigations.