complicate a diagnosis. Thus, the endpoint for the diagnostic process includes both the definition 
of the primary causes of impairment as well as the allocation of observed effects among multiple 
potential stressors, and the assessment of potential interactive effects among stressors. 
To narrow the number of realistic stressors of concern, an approach based on the Toxicity 
Identification Evaluation (TIE) procedures will be developed. In the TIE process, toxic 
chemicals are first considered in broad classes. As the evaluation proceeds, the focus moves 
towards specific chemicals. In this way, large numbers of insignificant chemicals are excluded 
from further evaluation. For example in a sediment TIE, sediment may be classified as toxic due 
to organic chemicals, then narrowed to pesticides, and finally to DDT. Analogously, in 
diagnosing causes of impairment, an approach will be developed which starts with broad stressor 
classes (i.e., habitat alteration, nutrients, suspended and bedded sediments and toxic chemicals), 
then unimportant stressors will be disregarded, and ultimately, a specific stressor(s) will be 
selected as the cause of impairment. 
In developing this plan, we considered and evaluated the States’ implementation stages from 
monitoring through diagnosis to restoration. Implementation stages were then linked to 
associated uncertainties, research needs, and desired research products. From these efforts, 
APGs, their accompanying APMs, and the critical path for diagnostic research were developed, 
and are presented in the next two subsections. The evolution of a combined TMDL/Restoration 
Path from the current parallel paths for State/Tribal assessments, TMDL, and watershed 
restoration processes is described below. 
Goals 
There are four primary goals for this diagnostics research: 
• Provide a framework for interpreting cause-and-effect relationships, including: 
• Conceptual ecosystem models based on appropriate mechanisms of action that can 
be used to improve the accuracy of impairment decisions; 
• Conceptual models to define ecosystem and watershed natural conditions and 
driving factors to use as a basis to quantify degree of impairment and to set 
restoration goals; and 
• Classification frameworks that explain variation in the response of individuals, 
populations, communities, and ecosystems at regional, watershed, water body, and 
habitat scales. 
• Develop single-stressor diagnostic methods and models to determine the primary source 
of biological impairment of aquatic ecosystems. 
• Develop methods and models to allocate causality among multiple stressors and/or to 
diagnose interactions among them. 
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