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Reference Condition

The issue of reference conditions is critical to the interpretation of biological data. Ideally, reference conditions represent the highest biological conditions found in waterbodies undisturbed by anthropogenic stressors. Recognizing that pristine habitats are rare or non-existent (even remote lakes and streams are subject to atmospheric deposition), resource managers must decide on an acceptable level of disturbance to represent an attainable or existing reference condition. Acceptable reference conditions will differ between geographic regions and States and will depend on the aquatic life use designations incorporated into State water quality standards to set benchmarks. Reference condition can be derived from reference sites, an empirical model of expectations that may include knowledge of historical conditions, or a model extrapolated from ecological principles. Usually, data from sites tha t represent best attainable conditions (i.e., least disturbed) of a waterbody are used (see Technical Element 5, Criteria for Reference Sites).

Generally, USEPA recommends the use of a regional reference condition based on an aggregate of sites that allows for broader application in State water resource programs than site-specific conditions. There must be a sufficient number of reference sites to capture regional stratification and the range of natural variations in biological assemblages due to geology, climate, and other natural physicochemical differences. When such reference sites are not available (e.g., in large ecosystems such as large rivers, estuaries, near-shore coastal areas, and significantly altered watersheds such as urban centers and intensive agricultural regions), a disturbance gradient can be constructed to extrapolate to an appropriate reference condition. Knowledge and use of historical archives and information is essential in this process.

Using site-specific reference sites to determine reference condition typically involves measuring conditions upstream of a point source discharge or in a “paired” watershed design. Regional reference conditions, on the other hand, are based on measurements from populations of least disturbed sites within a relatively homogeneous region with respect to ecotype and other non site-specific factors. Although both site-specific and regional reference sites represent conditions without the influence of a particular stressor, they may not yield equivalent results. While site-specific reference conditions represented by the upstream/downstream or paired site approach are useful, they are limited in terms of regional representativeness. They typically possess limited capacity for extrapolation, allow only limited variance estimates, and involve a substantial assessment effort when considered on a statewide basis. The use of regional reference conditions, on the other hand, has strengthened the assessment of biological condition by establishing a means of estimating and partitioning the natural variability within a region. Ecological regionalization (see Technical Element 3, Natural Classification) takes into account factors that control spatial patterns, such as climate, topography, and mineral availability (soils, geology), and factors that respond to or integrate these controlling factors, such as vegetation and land use.

Determining the appropriate number of reference sites can only be done on a regional basis because of differences in reference site heterogeneity among regions. In a highly heterogeneous region, where natural conditions are highly variable among streams, either 1) a much larger reference pool may be necessary to accurately represent reference conditions or 2) further partitioning the natural variability through classification analysis is needed. As illustrated in the Figure, the variability in reference quality is reduced as the number of reference sites increases to estimate the overall condition of reference.

 

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Biological Indicators | Aquatic Biodiversity | Statistical Primer

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