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Approaches to Deriving Reference Condition

Ideally, every water body segment will have its own reference condition. In order to describe reference condition in a way that will not change over time due to further human activity, several general methods have been developed through practical experience. These methods include sampling the biota at sites with little or no indication of stressors associated with human disturbance (i.e., minimally disturbed reference sites). In altered landscapes, where such sites are few or absent, reference conditions are determined through a combination of methods: (1) sampling biota from least disturbed sites (reference sites), (2) interpreting historical records to deduce which biological characteristics occurred at times with substantially less human disturbance, (3) developing models that incorporate the best ecological knowledge, and (4) using best professional judgment.

1. Reference Sites. The selection and characterization of reference sites that are minimally or not disturbed by human activities have been the basis for defining reference conditions that approximate biological integrity. However, reference sites have also been identified as “the best of what’s left” and as such are used to estimate a least disturbed condition. The approach used to select reference sites may be similar, regardless of whether the sites are classified as minimally disturbed or least disturbed. The process and considerations involved in selecting reference sites is discussed in further detail in the following pages.

2. Historical Reconstruction. The role of historical reconstruction is to use available data to describe a range of water body or riparian conditions that existed at an earlier time. Historical reconstruction estimates a minimally disturbed condition rather than a least disturbed condition. Benefits of this approach include the following:

3. Empirical modeling. When the number of representative reference sites is low and historical information is not sufficient to reconstruct reference condition, predictive modeling can be used to construct and calibrate a model reference condition. This approach effectively leverages a smaller number of sites from the region or water body type than is needed for the typical spatially intensive reference site approach. However, it does require reliable data from representative sites.

Absent such data, this approach reverts to a best professional judgment approach with its inherent shortcomings of subjectivity. This approach is limited also by the data used in the model, and therefore, inferences beyond those data must be undertaken with great caution.

Predictive modeling approaches that show promise include the following:

4. Best Professional Judgment. Best professional judgment (BPJ) should be incorporated into all decisions; however, comparisons with a stressor gradient are also needed to accurately identify the position of the reference sites. The natural variability in biological attributes for a certain biological condition (e.g., biological integrity) dictates that biological data alone should not be used to develop reference conditions. Although BPJ is a critical part of biological assessments, great care must be taken to ensure that the development of reference conditions and selection of reference sites are well documented and include objective procedures that can be reproduced easily by others.

Material above taken from Best Practices for Identifying Reference Condition in Mid-Atlantic Streams.

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Biological Indicators | Aquatic Biodiversity | Statistical Primer

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