Narrative and numeric biocriteria

NARRATIVE BIOLOGICAL CRITERIA should include specific language about aquatic community characteristics that (1) must exist in a waterbody to meet a particular designated aquatic life use, and (2) are quantifiable. Narrative criteria need to be supported by numeric measures that define some of the ambiguous language that one finds in them, such as “detrimental,” “as naturally occurs,” “minor changes,” etc.

NUMERIC BIOLOGICAL CRITERIA are measures that quantify what a healthy biological community should look like for different designated aquatic life uses. Several specific measures (i.e., abundance, diversity, presence or absence of key indicator types and tolerance to pollution) are often combined into indices. The benefit of an index is that it can compensate for the weakness of one measure by combining it with the strengths of others.

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