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Taxonomic Resolution
An aquatic assemblage is defined as a group of phylogenetically related taxa within the aquatic community (Hutchinson 1967). 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); and corals are key in certain coastal areas (Fisher 2007). 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). The assessment of status and diagnosis of associated causes and sources can be strengthened by the addition of a second assemblage. However, it should be recognized that the sampling, processing, and calibration of endpoints are better developed in some assemblages rather than others.
Taxonomic resolution should be determined based on the needs and objectives of the biological assessment program, a balance between information (gain or loss) and available resources (Bouchard et al. 2005). 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 and usually provides rapid results. However, decisions on the taxonomic level to use for biological assessment (species or genus, versus others) should depend on some understanding of the taxonomic or phylogenetic diversity of the group in the geographic region of focus or interest. The higher the ratio of genera to families, it is less likely that a family-level identification approach will yield the specificity of information needed (Hawkins and Norris 2000). 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 low-gradient 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 typically used (size and age class). Studies looking at specific types of perturbation are likely to require genus-level or preferably species-level identifications (Bouchard et al. 2005). However, in certain cases, a tiered approach in which only those families with wide intrafamilial variation in tolerance are identified to finer levels is likely to be more cost-effective than identifying all taxa with fine-level/resolution (Chessman et al. 2007).
Whatever assemblage(s) are used, agencies 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 and Tribes 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 possible, send them out to recognized independent 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.
Some Frequently Asked Questions
Question: What is the effect of using one vs. two assemblages on bioassessment outcomes?
Answer: Two assemblages provide a more comprehensive assessment of aquatic life use attainment status and biological condition classes than do single assemblages, as determined from analyses in Rockdale, County, Georgia and statewide databases from Maryland and Ohio (see Figure below).
- 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 (not presented here), offering multiple lines of evidence.
Comparison of Aquatic Life Use attainment status for Ohio Rivers and Streams – fish and macroinvertebrate assemblage assessment singly and in combination.
Question: How does taxonomic resolution, i.e., lowest taxonomic unit (LTU - generally genus) versus family level (FTU) affect bioassessment outcomes?
Answer: Suitability of FTU for bioassessment may depend on the characteristics of the benthic assemblages in a particular study region (e.g., more or less taxonomic diversity within families). More diversity within families (high LTU:FTU ratio) will reduce the suitability of family-level IBIs for accurate assessments. As an example, the Figure on the left illustrates the average weighted value for FTU (e.g., Chironomidae) versus the range of response values for the respective genus and species. Generally, FTU can detect three levels of condition: good, poor, and “somewhat in between”; this supports general use pass/fail assessments. LTU is necessary to support assessments that can discriminate multiple condition classes along the BCG; this supports the application of multiple levels of aquatic life uses. LTU provides more detailed information about tolerance, which leads to the development of improved biological indices.
Weighted values for selected families and lower taxonomic levels within these families in response to un-ionized ammonia concentrations.
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.
Bouchard, R.W., D. Huggins, and J. Kriz. 2005. A review of the issues related to taxonomic resolution in biological monitoring of aquatic ecosystems with an emphasis on macroinvertebrates. 2005. Prepared by Central Plains Center for BioAssessment, Kansas Biological Survey. Prepared for United States Environmenal Protection Agency. Pages 38.
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.
Chessman, B., S. Williams, and C. Besley. 2007. Bioassessment of streams with macroinvertebrates: effect of sampled habitat an taxonomic resolution. Journal of the North American Benthological Society 26:546-565.
Fisher, W.S. 2007. Stony Coral Rapid Bioassessment Protocol. EPA/600/R-06/167. Office of Water and and Office of Environmental Information, US Environmental Protection Agency, Washington, D.C. 72 pgs.
Hawkins, C.P. and R.H. Norris. 2000. Performance of different landscape classifications for aquatic bioassessments: introduction to the series. Journal of the North American Benthological Society 19(3):367-369.
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.
Hutchinson, G.E. 1967. A treatise on limnology. Volume 2: Introduction to lake biology and the limnoplankton. John Wiley and Sons, Inc., New York, New York.
USEPA. 1996. Biological criteria: Technical guidance for streams and rivers. Authors: G.A. Gibson, M.T. Barbour, J.B. Stribling, J. Gerritsen, and J.R. Karr. EPA 822-B-96-001. Office of Science and Technology, USEPA, Washington, D.C.
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]