Figure 5 shows the relationships between the various habitat classifications, relative biological integrity, and the biological index values used to express the quantitative biological criteria. Narrative ratings of biological community performance are given opposite the habitat classifications. The highest biological index values coincide with the highest degree of biological integrity and the designation of exceptional warm-water habitat. Lower biological index values coincide with lesser degrees of biological integrity and poorer habitat classifications. As a matter of policy, only the criteria for warm-water habitat and exceptional warm-water habitat are considered consistent with the biological integrity goal of the Clean Water Act. However, states may designate poorer classifications such as modified warm-water habitat or limited resource waters if they can demonstrate that attempts to attain better biological integrity would cause substantial adverse socioeconomic impacts.
Once biological criteria are derived and codified in state regulations, they are ready to use in water-quality management. Biological criteria need not be included in the water-quality standards to use them as an assessment tool, but in our experience, adopting them in water-quality standards significantly broadens and