Review of toxicological effects caused by episodic stressor exposure

Water quality monitoring tools that rely on data from stress-response tests with continuous exposure at constant concentrations are not always appropriately protective when stressor exposure in the field is episodic in nature. The present study identifies various approaches that have attempted to account for episodic stressor exposure, describes the development of a toxicological effects database of episodic stressor exposure collated from published scientific literature, and discusses whether any discernible trends are evident when these data are reviewed. The episodic stressor exposure literature indicated that few generalizations can be made regarding associated biological responses. Instead, when attempting to characterize the hazard of a certain episodic pollution event, the following situation-specific information is required: the specific species affected and its age, the specific stressor and its concentration, the number of exposures to the stressor, the duration of exposure to the stressor, and the recovery time after each exposure. The present study identifies four main challenges to the inclusion of episodic toxicity data in environmental water quality management: varying stressor concentration profiles, defining episodic stressor concentration levels, variation resulting from routes of exposure and modes of action, and species-specific responses to episodic stressor exposure. The database, available at http://iwr.ru.ac.za/iwr/download, could be particularly useful for site-specific risk assessments related to episodic exposures.