Numeric Biological Criteria

"Numeric biological criteria" are quantitative values that describe the desired conditions of the aquatic community for different designated uses. Usually, these numeric values describe the characteristics of the aquatic community, such as abundance; diversity; presence or absence of key indicator types; the relative abundance of different types; tolerance to pollution; and relative abundance of feeding groups that would exist in unimpaired waters.

Numeric criteria are sometimes expressed in the form of a single index value which summarizes all these characteristics. For example, the State of Ohio - an early leader in the development of biocriteria - uses an Invertebrate Community Index (ICI), which is a single number that incorporates various structural and functional aspects of the benthic macro-invertebrate community. Ohio breaks the state into various habitat types (for example, warm water or cold water habitat) and specifies ICI values that should be achieved for each habitat.

Narrative Biological Criteria

"Narrative Biological Criteria" describe the desired conditions of an aquatic community in qualitative, no quantitative terms. An example is, "aquatic life shall be as naturally occurs."

Another, more detailed example comes from State of Connecticut rules for Class B waters: "Water quality shall be sufficient to sustain a diverse macro-invertebrate community of indigenous species. All functional feeding groups and a wide variety of macro-invertebrate taxa shall be present, however, one or more may be disproportionate in abundance. Waters which currently support a high quality aquatic community shall be maintained at that high quality."

Narrative biological criteria for higher water quality classifications tolerate few deviations from natural conditions. For example, they do not allow some taxa to be "disproportionate in abundance."

Cumulative and Synergistic Effects

What are "cumulative" and "synergistic" effects?

A cumulative effect is one that is equal to the sum of the effects of several different stressors. A synergistic effect is even greater than a cumulative effect because of the interaction of the stressors or their effects. The EPA specifically defines "synergism as " the characteristic property of a mixture of toxicants that exhibits a greater-than-additive total toxic effect."