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Selecting Reference Sites

Selecting reference sites involves applying screening criteria to a set of sites from the region or water body type of interest. These criteria will identify sites that are most likely to be minimally or least disturbed by human activities. The result is a set of candidate reference sites, qualified as such because depending upon the protectiveness that the screening criteria represents, some of the sites selected may not represent the desired reference condition (Figure below).

This schematic shows the steps involved in developing reference condition using reference sites. Ateach step, it is important to consider the coarseness of he filter, which influences the confidence that the quality of the reference sites accurately represents the intended quality of the reference condition.This schematic shows the steps involved in developing reference condition using reference sites. Ateach step, it is important to consider the coarseness of he filter, which influences the confidence that the quality of the reference sites accurately represents the intended quality of the reference condition.

Criteria useful for screening sites include stressors, indicators of stressor sources, and indicators along the pathway from source to exposure. The goal is to evaluate as many of these indicators as is practical, efficient, and relevant to the water body type and region of interest. Only those sites that meet the criteria of minimal disturbance or those that are the least disturbed among the set of sites are considered candidate reference sites.

Applying Reference Screening Criteria
Screening criteria should apply to potential reference sites at three levels: landscape or watershed, reach or riparian corridor, and site. While watershed and reach scale analysis using maps and/or GIS technology can identify candidate reference areas, there are many human activities that can only be revealed by collecting data at the site level. However, it is essential that the final list of candidate sites identified through this screening be evaluated against an objective set of stressor thresholds obtained from sampling data. Brief descriptions of the Mid-Atlantic Highlands Streams Assessment and the Maryland Biological Stream Survey illustrate approaches to reference site selection using objective criteria. Having too few reference sites to analyze each region or water body type is a common constraint in indicator development and may lead to relaxation of reference site criteria to obtain more sites.

The development of a fish IBI for MAHSA (Davis and Scott 2000) addressed this situation by evaluating metric performance against three different reference definitions: (1) least restrictive criteria based on chemical thresholds and the mean RBP habitat score (producing 46 reference sites with good geographic coverage); (2) moderately restrictive criteria based on chemical criteria, watershed land use, road density, and quantitative habitat filters (producing 23 reference sites with good geographic coverage); and (3) most restrictive criteria based on the moderately restrictive criteria plus the watershed condition class (Bryce et al. 1999; producing 12 reference sites with limited geographic coverage). If less restrictive criteria are used to define reference sites, then the lower quality may result in conditions that correspond to lower positions along the Biological Condition Gradient (e.g., Position 3 rather than 2, below).

Evaluating Quality and Representativeness
Because reference criteria vary among water quality monitoring programs, it is essential to evaluate the selected reference sites for whether they are truly minimally disturbed and whether they are representative of the water bodies of interest. The Maryland Biological Stream Survey used a set of water chemistry, physical habitat, and land use reference criteria that produced reference sites with considerably higher benthic macroinvertebrate IBIs than all sites sampled and were also higher than the original MBSS set of reference sites (see dashed line in Fig. 5). Subsequently, the reference criteria used in the original MBSS IBIs were reviewed to identify changes that would result in greater confidence that the new reference sites could be defined as “minimally disturbed.” Based on analysis of urban effects on stream condition (Vølstad et al. 2003), the presence of original reference sites with relatively high levels of urban land (i.e., 5% to 20%) indicated that not all reference sites were minimally disturbed; instead, many were impaired.

Cumulative distribution of stream miles with benthic macroinvertebrate IBI scores for (1) MBSS sites sampled in 2000-2004 (solid line), (2) subset of 2000-2004 sites meeting original reference criteria (dashed line), and (3) subset of 2000-2004 sites meeting new reference criteria (dotted line). (Southerland et al. 2005)Therefore, the MBSS changed the minimum allowable forested land use from >25% to >35% of the catchment area, maximum allowable urban land use from <20% to <5%, and minimum allowable riparian buffer from 15m to 30m. These changes in land use and riparian width thresholds resulted in a smaller proportion of stream sites meeting the reference site criteria. Using the original reference site criteria, 152 of the 1098 Round One sites (14%) were designated as reference sites. Using the new criteria, 196 of the total 2508 sites (8%) were designated as reference. These new reference sites were of higher quality (dotted line in Figure below) than sites meeting the original reference criteria. This result is consistent with greater confidence that the sites are minimally disturbed.

Figure to left - Cumulative distribution of stream miles with benthic macroinvertebrate IBI scores for (1) MBSS sites sampled in 2000-2004 (solid line), (2) subset of 2000-2004 sites meeting original reference criteria (dashed line), and (3) subset of 2000-2004 sites meeting new reference criteria (dotted line). (Souterland et al. 2005)

As the criteria for selecting reference sites are tightened to ensure minimally disturbed sites are chosen, it is important that these sites still represent the range of natural factors likely to control the biotic composition in the region of interest. For example, stream size, gradient, and elevation can be major determinants of potential biotic composition. Therefore the set of reference sites should cover and be limited to the range of natural conditions important in the region of interest. If the streams represent an elevation gradient, reference sites should also represent that elevation gradient; if streams considered represent a range of sizes, the set of reference sites should also represent these sizes.


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

Learn more about reference condition by reading:

Citations:

Bryce, S.A., D.P. Larsen, R.M. Hughes, and P.R. Kaufmann. 1999. Assessing relative risks to aquatic ecosystems: A Mid-Appalachian case study. Journal of American Water Resources Association. 35:23-36.

Davis, W.S., and J. Scott. 2000. Mid-Atlantic Highlands Streams Assessment: Technical support document. EPA-903-B-00-004. U.S. Environmental Protection Agency, Region 3, Office of Research and Development, Ft. Meade, MD.

Southerland, M.T., G.M. Rogers, M.J. Kline, R.P. Morgan, D.M. Boward, P.F. Kazyak, R.J. Klauda, and S.A. Stranko. 2005. New biological indicators to better assess the condition of Maryland streams. CBWP-MANTA-EA-05-13. Report for the Maryland Department of Natural Resources, Annapolis, MD.

Vølstad, J.H., N.E. Roth, M.T. Southerland, and G. Mercurio. 2003. Pilot study for Montgomery County and Maryland DNR data integration: Comparison of benthic macroinvertebrate sampling protocols for freshwater streams. EPA-903-R-03-005. U.S. Environmental Protection Agency, Office of Environmental Information and Mid-Atlantic Integrated Assessment, Fort Meade, MD.

 

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