The choice of spatial frameworks for organizing environmental information influences the effectiveness of the research, assessment, and management of many aquatic-resource problems, particularly nonpoint-source pollution (Omernik and Griffith, 1991). It also can lead to the generation of large amounts of information and to large expenditures of money to produce statements about the biological integrity or use attainability of watersheds or large hydrologic units. Unless properly structured, the information collected within a framework may not be useful when compared with information from units in other regions of the country. The use of differences in land and water interactions, regional variations in attainable water quality, distinct biogeographical patterns (Wallace, 1869; MacArthur, 1972), and similarities and differences in ecosystems to delineate ecoregions makes the application of ecoregions in environmental analyses a powerful tool with which to organize environmental information.
Ecoregions can be distinguished by landscape-level characteristics that cause ecosystem components to reflect different patterns in different regions (Omernik, 1987). The delineation of ecoregions is based on patterns in geology, soils, geomorphology, dominant land uses, and natural vegetation. Omernik (1987) originally identified 76 ecoregions in the conterminous United States by using information from small-scale maps.
One of the values of the ecoregion concept in lake restoration and management is that it provides a rational basis for setting regional rather than national lake water-quality standards. The approach can take into account regional factors related to attainable water quality and thus can be used to designate lakes for protection and to establish lake-restoration goals that are appropriate for each ecoregion (National Research Council, 1992). The National Research Council of the National Academy of Science has similarly endorsed the use of the concept in the restoration and management of streams, rivers, and wetlands (National Research Council, 1992). Although the ecoregion concept has been applied and tested rather extensively in streams, rivers, and lakes, its application to wetlands, ground water, and estuaries has not been refined. Additional variables may be needed to determine the spatial distribution of ground-water, estuarine, and wetland ecoregions. For ground water, the additional variables may include redox potential, depth, and the geochemical environment. For wetlands, variables may include ground- and surface-water interactions. For estuaries, tidal influence, salinity profiles, depth, and substrate type may help define boundaries.
The U.S. Forest Service (USFS) developed a hierarchial framework on the basis of earlier work to provide a scientific basis for ecosystem management in the National Forests and Grasslands, as well as in other USFS programs (U.S. Forest Service, 1993). Ecological units are defined on the basis of potential natural communities, soils, hydrologic function, landform and topography, lithology, climate, air quality, and natural processes for cycling plant biomass and nutrients.
At the ecoregion scale, units are recognized by differences in global continental and regional climatic regimes and physiography. The hierarchy defines three levels of ecoregions--domains, divisions, and provinces. Domains are based on broad climatic patterns; for example, polar, dry, humid, temperate, and so forth. Divisions are defined by isolating levels of vegetative associations that are defined by broad climatic regions. Provinces reflect broad vegetative regions that correspond to climatic subzones, which are based on continental climatic patterns; for example, length of dry season. Similar soil orders also characterize provinces.
Omernik and Gallant (1990) defined ecoregion aggregations in terms of 8 broad-level named regions generalized from the 76 ecoregions mentioned above. They are as follows:
The use of ecoregions as the spatial framework for collecting and analyzing environmental information has the following advantages:
Because environmental characteristics are interrelated (for example, climate and surficial geology affect soil formation; soil formation and climate affect vegetation type, which further affects soil formation; and all these factors affect land use, which affects vegetative succession and soil formation), spatial distributions of many of the features coincide, reinforcing patterns that would not be entirely identifiable from any single variable (Gallant and others, 1989). Other factors that can be used to define ecoregions include bedrock geology, physiography, hydrologic drainage areas, lake phosphorous concentrations, and sensitivity of surface waters to acidic deposition.
An example of an ongoing effort to subdivide ecoregions is the U.S. Geological Survey/USEPA Region 3 project in the Central Appalachian Ridge and Valley ecoregion of West Virginia, Virginia, Maryland, and Pennsylvania. This ecoregion consists of sharply folded sedimentary strata that have been eroded, which has resulted in a washboardlike relief of resistant ridges that alternate with valleys of less-resistant rock (Gerritsen and others, 1994). The region has been divided into the following subregions that correspond to ridges and valleys of different parent material (fig. 2; Omernik and others, 1992):
Table 1 shows how selected criteria traditionally used to classify streams within the Ridge and Valley ecoregion (elevation, conductivity, temperature) relate to subecoregional classification, which incorporates these characteristics into their structure. Because these parameters (streamwater type, dominant fishery) are used to delineate subregion streams, they also can be used as criteria for reference conditions. Differences between subregions are accurately determined only if the best possible conditions are used for references and if accurate measurements of the same parameters are taken in all subregions.
Table 1. Preliminary stream classification and subregions of the Central Appalachians Ridge and Valley ecoregion
[From: Gerritsen and others, 1994]
Area Steamwater type Dominant fishery Corresponding subregion.
Highland Low conductivity Cold water Shale ridge, sandstone ridge.
High conductivity (owing to calcareous do. Sandstone ridge.
cement in rock formations or
minor limestone strata.
Valley Limestone spring (high conductivity) do. Limestone valley.
Limestone influenced (high conductivity) Cold or warm water Do.
Low conductivity do. Shale valley.
More recently, the development of an ecoregional framework (Omernik, 1987; Ohio Environmental Protection Agency, 1987; Whittier and others, 1987) has provided the basis for subregionalization (Gerritsen and others, 1994) in several parts of the country. Subecoregions provide a framework for establishing ecological expectations (reference conditions), which are based on the sampling of many minimally impaired reference sites within the subregion. These physical, chemical, and biological data are stratified (within a subregion) by the type and character of the water-body class to form a reference-condition data base for the subregion.
The establishment of ecological criteria is the central purpose of many water-quality-management activities. The concept of biocriteria implies a comparison of a test-site observation to the highest level of attainable ecological condition in a subregion. The USEPA is using " reference conditions" as the basis for making comparisons and detecting attainment of aquatic life use (U.S. Environmental Protection Agency, 1990, 1994). Such conditions should be applicable to an individual water body, such as a stream segment, and to water bodies generally on a regional scale. The reference condition is a critical element in the development of a biocriteria program.
In areas where least-impaired or best-available sites have been significantly altered, the search for suitable sites must be extended over a wider area; multiState cooperation in the form of data- and reference-site sharing is the basis for such searches. If no suitable sites are found, then historical data, expert opinion, and (or)empirical models can be used to determine reference expectations for the region (Gibson and others, 1994). Historical data alone may not suffice owing to potentially questionable methods, lack of QA information, surveys made at impaired sites, and insufficiently documented methods and (or) objectives. Empirical water-quality models must be carefully evaluated before being used solely in the development of reference conditions. Because they generally are deficient in community-level evaluations, a consensus of expert opinion, as well as modeling and historical data, should be used in determining the reference condition. In any event, the goal of establishing reference conditions is to define the natural potential of the reference sites as being equivalent to that for natural lakes--best of ambient conditions or prediction regardless of the extent of human degradation that currently exists in the area. The development of reference-site selection criteria for reservoirs [J. Gerritsen, and others, Tetra Tech, Inc., written commun. (draft report), 1994] showed that although natural conditions for reservoirs are nonexistent, operational criteria for establishment of reference models can be established.
Reference sites must be carefully selected because they will be used as a benchmark against which test sites will be compared. The ideal reference site will have extensive natural riparian vegetation, a diversity of substrate materials, natural physical structures, a natural hydrograph, and a minimum of known human-induced disturbances or discharges. There also should be a representative and diverse abundance of naturally occurring biological assemblages (Hughes and others, 1986).
However, it also is recognized that pristine conditions no longer exist, and, in practice, the level of the acceptable conditions for reference sites may be based on socioeconomic demands. Consider a county in which all the streams have been converted into canals or ditches; consequently, the habitat has been completely altered. Some of the canals receive point-source discharges, as well as nonpoint-source input, while others have clean water. On the basis of the framework described above, there would be two drastically different approaches for establishing reference conditions. The decision on the approach to be taken rests with the acceptance that the substantial habitat impairment of canals will not support naturally occurring biological assemblages, as defined by Hughes and others (1986). In the first approach, a composite of the best biological condition of the canals within the region is determined to represent the reference condition and, thus, the biological expectations. The alternative would be to take the best of the nonchannelized streams in an adjacent county within the same ecoregion or subregion and establish expectations more similar to a natural condition. The latter approach provides a more stringent basis for judgment of impairment. A decision to use this approach implies that there is acceptance of degraded physical habitat and may remove incentives toward efforts at improving overall ecological conditions. However, Gibson and others (1994) cautioned against the wholesale acceptance of significantly altered systems and stipulated that resultant criteria are interim goals subject to improvement. The result of the approach for establishing reference conditions in significantly altered systems, however, is nontechnical in nature and falls within the charge of policymakers.
Different approaches for ecological assessments focus on different indicators, use of different sampling methods, sampling during different index periods, use of specialized data-evaluation procedures, and measurement of data at various scales. Regardless of the specific measurements or samples being taken, pilot studies or small-scale research may be needed to define, evaluate, and calibrate individual indicators. Past efforts that have been made to evaluate the use of metrics illustrate procedural approaches to this task (Angermeier and Karr, 1986; Karr and others, 1986; Davis and Lubin, 1989; Boyle and others, 1990; Barbour and others, 1992; Karr and Kerans, 1992; Kerans and others, 1992; Lyons, 1992; Resh and Jackson, 1993). Indicator metrics can be calibrated by evaluating the response to varying levels of stressors (Jongman and others, 1987). Sites must be carefully selected for controlled prospective studies to cover a wide range of suspected stressors. In general, impaired sites are selected to provide knowledge on the directional changes of indicators by using either single or aggregated metrics subjected to known stressors singly and in combination. The combination of selected impaired and reference sites is the basis for developing an empirical model of indicator response to stressors.
Comparison of the medians and ranges across environmental strata (ecoregions, subecoregions, stream or lake size, seasons) can help determine if it is necessary to segregate the data by the strata. For example, seasonal influences on an indicator can be examined by comparing the median and range between two samples obtained at the same site in different seasons and by using the same methods. If differences exist between the two seasons, then data from these two seasons should not be combined; that is, for this particular ecoregion, only spring data should be compared with spring data, and fall data with fall data. Separate criteria (or reference expectations) should be developed for spring samples and fall samples. Data that do not show such distinct seasonal differences may be combined. A similar approach can be applied to data that originate in different ecoregions, subregions, or sizes of water bodies.
For classification of reference conditions based on the best available (selected) sites, it is assumed that most of, if not all, the sites are minimally impaired. Therefore, the upper 50th percentile (values above the median) can be used as the delineation between what is considered to be impaired versus nonimpaired for each indicator or metric. When scoring each metric, the values in the upper range would receive the maximum score, and quartiles below the median would receive progressively lower scores. This approach is conducive to metrics that may have a modal response, rather than a monotonic one, because upper bounds on the expected condition can be established. Hypothetically, taxa richness may be best in a region when the number of taxa is between 25 and 35. However, in a water body with nutrient enrichment, the number of taxa may be 37. In this approach, the condition would be noted as indicating some degree of impairment owing to probable nutrient enrichment.
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