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Criteria for Reference Sites
Abiotic characteristics and attributes should be the principal criteria used in selecting candidate reference sites. Using non-biological factors is necessary to avoid circularity in defining the biological characteristics that become the basis of reference biological datasets. In addition, using water quality and physical habitat data as primary filters also results in some circularity for reference sites, which are serving as a background for assessment of ecological condition. Factors to be considered in selecting reference sites include human population density and distribution, road density, and the proportion of mining, logging, agriculture, urbanization, grazing, or other land uses. Candidate reference sites are evaluated for these factors to determine the degree of human modification that has occurred. Sites are eliminated if they have undergone direct human modification, especially to riparian zones and instream habitat (Bryce et al. 1999).
Candidate sites can be selected from either probabilistic sampling (a posteriori determination) or sites identified primarily from data files that do not require site-specific sampling (a priori selection). A phased process for selecting reference sites is recommended to ensure consideration of sites and catchments that may not have been sampled previously. The first phase would consist of compiling and evaluating remote sensing data for land use patterns. Subsequent phases would require site reconnaissance and data acquisition. Chemical and physical characteristics of sites should be used only as secondary and tertiary filters for selecting reference sites (McCormick et al. 2001; Bryce and Hughes 2002). It is very important that staff biologists verify the current conditions of candidate reference sites in the field. A candidate site should be eliminated if local conditions preclude its suitability to serve as a reference for high-quality water. A reference site should be natural or minimally disturbed while maintaining essential attributes. When reference sites are used to establish reference conditions, the State needs to document how it selects reference sites (by what criteria) and how it uses them to define regional reference conditions (e.g., by aggregating sites in a regional reference population distribution, or through other approaches such as a paired watershed or upstream/downstream design).
Learn more about reference conditions by reading
- Best Practices for Identifying Reference Condition in Mid-Atlantic Streams (EPA-260-F-06-002, August 2006) (8 pp, 1.49MB About PDF)
- Reference condition concept (from Best Practices)
- Reference condition types (from Best Practices)
- Approaches to Deriving Reference Conditions (from Best Practices)
- Reference site selection (from Best Practices)
- Reference condition benchmarks (from Best Practices)
- Reference sites did not always meet criteria for reference condition in Developing Biological Indicators: Lessons Learned from Mid-Atlantic Streams
- Regional Reference Concept (from Monitoring Web site)
- Great Rivers Reference Condition Workshop (from Biocriteria Web site)
- Aquatic Life Use Support - Reference Condition (from Biocriteria Web site)
- Basic Reference Condition and Classification Techniques (from Biocriteria Web site)
- Reference Condition Case Studies (from Biocriteria Web site)
- Advanced Reference Condition Techniques, Special Circumstances and Problem Solving (from Biocriteria Web site)
References
Bryce, S.A., and R.M. Hughes. 2002. Variable assemblage responses to multiple disturbance gradients: case studies in Oregon and Appalachia, USA. Pp. 539-560 in T.P. Simon (ed.) Biological response signatures: indicator patterns using aquatic communities. CRC Press. Boca Raton, FL.
Bryce, S.A., E.P. Larsen, R.M. Hughes, and P.R. Kaufmann. 1999. Assessing relative risks to aquatic ecosystems: a mid-Appalachian case study. Journal of the American Water Resources Association 35:23-36.