Biology-based Stressor Thresholds

The objective of this memo is to describe how biological assessment data (such as those collected using Surface Water Ambient Monitoring Program (SWAMP) bioassessment protocols) can be used to develop biologically-relevant stressor thresholds. This memo describes an objective approach for setting regulatory stressor thresholds based on the association between biological condition and stressor intensity gradients. While these relationships show a large amount of variability typical of biological data, there are clear thresholds that indicate levels below or above which stream health is maintained or degraded.