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Taxonomic Resolution
An assemblage is defined as an association of interacting populations of organisms in a given waterbody. Four principal assemblages are used in bioassessment and attainment decisions: benthic macroinvertebrates, fish, algae, and aquatic macrophytes. However, zooplankton are key indicators in lakes while vegetation and birds are key indicators in wetlands and riparian zones (Baker et al. 1997; Bryce et al. 1999; Hughes et al. 2000). Each assemblage serves a different function in the aquatic community, has different habitat ranges and preferences, and may be susceptible to stress in varying manners and degrees. Although a single assemblage may be sufficient to make an attainment determination, USEPA recommends the use of at least two to enhance confidence in the assessment findings (USEPA 1996). Data comparisons have shown that the assessment of status and diagnosis of associated causes and sources can be influenced by the addition of one assemblage .
Taxonomic identification of each assemblage to genus or species level provides reliable information about sensitivity, tolerance, and ecological/environmental relationships. Genus/species identifications improve assessments using richness values or metrics as key endpoints. Identification to family level requires less expertise to perform and usually speeds up the assessment process. However, the scientific determination of level of taxonomy should include knowledge of adaptive radiation within the fauna (i.e., the determination of the number of genera and/or species per family). The higher the ratio of genera to families, the less likely that a family-level identification approach will yield the specificity of information needed. For example, the benthic fauna of lakes are generally highly diverse taxa within few families and require a taxonomic resolution higher than family level. Naturally depauperate systems, such as lowgradient streams or oligotrophic ecosystems, may warrant family-level indices of macroinvertebrates. However, family-level indices are inappropriate for vertebrates (e.g., fish and birds) and macrophytes in oligotrophic systems. In these cases, population-level data are needed (size and age class). [For more information, please see A Review of the Issues Related to Taxonomic Resolution in Biological Monitoring of Aquatic Ecosystems with an Emphasis on Macroinvertebrates by the Central Plains Center for Bioassessment (2005, 38pp, 770 KB,About PDF]
Whatever assemblage(s) are used, States must document the rationale for choosing them and include in their methodologies the value and purpose of the assemblage(s) in assessment and attainment decisions. Similarly, States must clearly document the chosen level of taxonomic rigor for each assemblage. A "voucher collection" of each assemblage for each major basin, ecoregion, site class, or other appropriate study unit is highly recommended. Such a collection contains a representative of each taxon and serves as a basin record and reference for checking identifications. A senior aquatic taxonomist should check the specimens entered into the collection for accurate identification and, if necessary, send them out to recognized experts for verification. Ideally, the voucher collection should be housed in a museum or university. The State's protocols for establishing and maintaining such a collection should also be described (or referenced) in its methodology.
What
is the effect of using one vs. two assemblages on bioassessment outcomes?
Two assemblages provide a more comprehensive assessment of Aquatic Life Use
attainment status as well as biological condition classes than do single assemblages,
as determined from analyses on statewide databases from Maryland and Ohio,
and data from Aquatic Life Use.
- Attainment status agreement was 44-66% based on 2 assemblages vs. single assemblages alone
- Single assemblages alone under-estimated impairment compared to 2 assemblages by 10-25%.
- Condition class (e.g., exceptional, good, fair, etc.) agreement using two assemblages was 34-45% in the Maryland and Rockdale Co. datasets, respectively.
- Multiple biological condition classes are important in applications of the Biological Condition Gradient (BCG), regardless of whether attainment of aquatic life use is evaluated.
References
Baker, J.R., D.V. Peck, and D.W. Sutton (eds.). 1997. Field operations manual for lakes. USEPA. EPA/600/R-97/003. Corvallis, OR.
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.
Hughes, R.M., S.G. Paulsen, and J.L. Stoddard. 2000. EMAP-surface waters: a national, multiassemblage, probability survey of ecological integrity. Hydrobiologia 423:429-443.
USEPA. 1999. Rapid Bioassessment Protocols for Use in Streams and Wadeable Rivers: Periphyton, Benthic Macroinvertebrates and Fish. Second Edition. Office of Water, Washington, D.C. Authors: M.T. Barbour, J. Gerritsen, B.D. Snyder, and J.B. Stribling. EPA 841-B-99-002. Office of Water, Washington, D.C.