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Criteria for Reference Sites
The goal when establishing criteria is really to define explicitly the reference or acceptable “healthy” ecosystem for the context, so that there can be common decision rules emphasizing expectations of minimal exposure to the stressors of concern (Bailey et al. 2004). Abiotic characteristics and attributes should be the principal criteria for selecting candidate reference sites. Non-biological factors are 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 serve 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 that are thought to have been minimally disturbed by stressor(s) of concern are thought to be in a state known as reference condition (Bailey et al. 2004, Stoddard et al. 2006). Sites are eliminated if they have undergone direct human modification, especially to riparian zones and instream habitat (Bryce et al. 1999). However, Angradi et al. (2009) stated that great river assessment based on empirical “least disturbed” conditions represents progress because the best extent conditions in great rivers set the lower bounds on “best attainable” conditions—the condition of the river ecosystem after all socially-acceptable flow naturalization and channel form rehabilitation is achieved (Angradi et al. 2009). Angradi et al. (2009) contend that as river naturalization goals are attained, reference expectations based on least disturbed conditions can be reset to provide benchmarks for future bioassessments.
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 are useful as secondary and tertiary filters for selecting reference sites (McCormick et al. 2001; Bryce and Hughes 2002; Harris et al. 2006 [reference condition brochure]); however, these data are only available for sites that have been visited previously, and are likely lacking from a multitude of candidate sites.
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 the criteria is uses to select reference sites and how the sites are used 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).
Some Frequently Asked Questions
Question: How do criteria for reference site inclusion influence the calibration of a biological index or indicator and our ability to derive biocriteria?
Answer: To test this question, we performed analyses on the affect of altering reference criteria on a biological indicator (Figure below). Relaxing reference criteria to allow a higher level of stressors reduced the quality of the reference for both biological metrics and index score and decreased the ability to distinguish impairment. Tightening (making the criteria more stringent) the reference criteria did not dramatically affect metric or index distributions for Mississippi or West Virginia (two test cases), suggesting that initial reference criteria were adequate and that stricter criteria do not help in improving regional reference conditions. The level of rigor is likely to be more affected by reducing reference criteria than by tightening. A structured and systematic process results in documented characteristics for reference site membership and deters inclusion of subpar conditions.
Question: How do we deal with situations where criteria have to be relaxed to obtain least-disturbed sites to serve as reference? For example, in middle America row crops and grain farming are the primary landscape, and no unaffected waterbodies exist.
Answer: There seem to be two ways to deal with the issue of subpar sites improperly influencing biocriteria: 1) by restricting membership in the reference site population as much as is feasible while providing an adequate number of reference sites are in the population; or 2) establishing a biological condition gradient (BCG) framework whereby the reference condition is modeled to be a target at a level above your population of least disturbed sites (See elements 10 and 11 for more detail). If relaxed reference site criteria (for membership) are used in some bioregions to allow subpar conditions to influence biocriteria, then a higher percentile of reference (e.g., ≥ 25%) is used to set an appropriate threshold for interpreting impairment.
See Figure on left. Analyses performed on the effects of altering membership criteria for reference sites on (top) a benthic index, and (bottom) a benthic metric. “Reference” is the primary set of criteria with “subreference” lessening the criteria and 4 levels of tightening the criteria.
References
Angradi, T.R., D.W. Bolgrien, T.M. Jicha, M.S. Pearson, B.H. Hill, D.L. Taylor, E.W. Schweiger, L. Shepard, A.R. Batterman, and M.F. Mofett. 2009. A bioassessment for mid-continent great rivers: the Upper Mississippi, Missouri, and Ohio (USA). Environmental Monitoring and Assessment 152(1):425-442.
Bailey, R.C., R.H. Norris, and T.B. Reynoldson. 2004. Bioassessment of Freshwater Ecosystems Using the Reference Condition Approach. Kluwer Academic Publishers, Dordrecht, The Netherlands. 170 pp.
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.
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.
Harris, J.A., R.J. Hobbs, E. Higgs, and J. Aronson. 2006. Ecological restoration and global climate change. Restor. Ecol. 14:170-176.
McCormick, F.H., R.M. Hughes, P.R. Kaufmann, D.V. Peck, J.L. Stoddard, and A.T. Herlihy. 2001. Development of an index of biotic integrity for the mid-Atlantic Highlands region. Transactions of the American Fisheries Society 130:857-877.
Stoddard, J., D.P. Larsen, C.P Hawkins, R.K. Johnson, and R.H. Norris. 2006. Setting expectations for the ecological condition of streams: the concept of reference condition. Ecological Applications 16:1267-1276.
Learn more about reference conditions by reading
- Stoddard, J.L. et al. (2006). Setting Expectations for the Ecological Condition of Streams: The Concept of Reference Condition (PDF). Ecological Applications 16(4):1267-1276) (10pp, 231K,About PDF)
- 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)