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Reference Condition

Reference Condition

Text Box: • Reference condition for biotic integrity - pristine condition. The intent of the CWA. • Minimally disturbed condition - condition in the absence of significant human disturbance. • Least disturbed condition - best available conditions today given the state of the landscape. • Best achievable condition - expected condition with best possible management practices. The issue of reference condition is critical to the interpreting biological data. Ideally, reference conditions represent the highest biological conditions found in waterbodies undisturbed by anthropogenic stressors.  However, different assumptions of what constitutes reference condition will have important effects on the final assignment of sites into condition classes; so bioassessments should be consistent in describing the definitions and methods used to set expectations (Stoddard et al. 2006).  A 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.  Typically, water quality agencies have derived data from sites that represent best achievable conditions (i.e., least disturbed) of a waterbody and used these as reference (see Technical Element 4, Criteria for Reference Sites).  USEPA recommends the use of a regional reference condition based on an aggregate of sites considered to be minimally disturbed and 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.  However, reliable ways must be developed to adjust for variation in reference-site quality, especially given that the best conditions in some regions are far from historical conditions (Herlihy et al. 2008).

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.  Modeling of continuous environmental gradients, as is done in RIVPACS-type modeling, is perhaps the most direct approach to specifying site-specific conditions (Herlihy et al. 2008).  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.

Some Frequently Asked Questions

Question:  How is the BCG used in the absence of minimally disturbed sites?
Answer:  Using the ecological attributes as a guide (See Element 10), an agency can identify fauna from historical records or ecological knowledge of the area to be representative of the attributes.  Supplementing this process with external technical experts helps to validate the characterization of the ecological attribute matrix.  Working with the BCG guidelines (Davies and Jackson 2006), the agency can then describe the applicable BCG in terms that place current “least disturbed” conditions along the gradient.

Question:  How does the number of reference sites (N) affect characterization of biological expectations?
Answer: 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 very 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 below, the variability in reference quality is reduced as the number of reference sites increases to estimate the overall condition of reference.

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.
Standard deviations of 25th percentile fish assemblage IBI scores estimated by randomly drawing reference sites at a given sample size (x-axis) five times for wading sites in the HELP and EOLP ecoregions of Ohio.

References

Davies, S.P. and S.K. Jackson.  2006.  The biological condition gradient:  a descriptive model for interpreting change in aquatic ecosystems.  Ecological Applications 16(4):1251-1266.

Herlihy, A.T., S.G. Paulsen, J.V. Sickle, J.L. Stoddard, C.P. Hawkins, and L.L. Yuan.  2008.  Striving for consistency in a national assessment: the challenges of applying a reference-condition approach at a continental scale.  Journal of the North American Benthological Society 27: 860-877.

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.

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Biological Indicators | Aquatic Biodiversity | Statistical Primer

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