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Natural Classification
In developing a bioassessment program, USEPA recommends classifying waterbodies more specifically than simply by waterbody type (e.g., river, lake, etc.), because it is highly unlikely that the biological condition of any given waterbody type is uniform throughout any anthropogenically-defined boundary. The classification of waterbodies is useful in partitioning natural variability and distinguishing it from variability resulting from human-induced changes. Classification of waterbodies can be based on a combination of characteristics, i.e., watershed drainage size, ecological regions, elevation, temperature, and other physical features of the landscape and/or waterbody for each waterbody type (e.g., large rivers, wadeable streams, headwater streams). The number of sites sampled and the availability of candidate reference sites within each class may limit the number of classifications.
Ideally, jurisdictional boundaries are superseded by ecological drivers. Data that represent a continuous scale and are based on variables such as site size, ion chemistry, substrate, elevation, and latitude/longitude have been found very useful for classification (Moss et al. 1987; Reynoldson et al. 1997; Bailey et al. 1998; Marchant et al. 1999; Joy and Death 2000; Oberdorff et al. 2002). Ecoregions, combined with other factors, have been used successfully in some parts of the country as a primary classification scheme. Ecoregions are a system of classification that are based on similarities of natural geographic features and land use patterns. Ecoregions are not the only classifications of freshwater ecosystems; other landscape factors such as elevation and rainfall can be used to classify waterbodies. Site-specific information may also prove useful in delineating patterns not recognized at larger scales. The extent to which the aquatic resource types within a State or Tribal jurisdiction are recognized and described influences how effective and reliable the resultant bioassessment framework will be. Common classes for streams and rivers include coldwater and warmwater, but can also include subcomponents such as different grades of each resource type. Other specific waterbody characteristics that are addressed here would be blackwater (bog-oriented or tannic source), spring-fed (alkaline, Clearwater), and glacial originated streams (turbid coldwater). For other waterbody types, analogous stratification schemes exist or are in development.
Figure. Example of Bioregions as established by Mississippi
Partitioning the waterbodies of an agency by natural variability in the biota results in a classification that greatly improves assessment of ecological condition. As an example, natural classification in Mississippi resulted in five bioregions (not counting the delta in gray) as a basis for bioassessment. Bioregions are geographically distinct regions of waterbodies that roughly correspond to ecoregions or aggregations of ecoregions.
For more information, visit EPA's
- Properly Classifying Sites is Key in Lessons Learned for Bioassessment
- Ecoregion Maps and GIS Resources
- Ecoregions and Watersheds
References
Bailey, R.C., M.G. Kennedy, M.Z. Dervish, and R.M. Taylor. 1998. Biological assessment of freshwater ecosystems using a reference condition approach: comparing predicted and actual benthic invertebrate communities in Yukon streams. Freshwater Biology 39:765-774.
Joy, M.K., and R.G. Death. 2000. Predictive modeling of freshwater fish as a biomonitoring tool in New Zealand. Freshwater Biology 47:2261-2275.
Marchant, R., A. Hirst, R.H. Norris, and L. Metzeling. 1999. Classification of macroinvertebrate communities across drainage basins in Victoria, Australia: consequences of sampling on a broad spatial scale for predictive modeling. Freshwater Biology 41:253-268.
Moss, D., M.T. Furse, J.F. Wright, and P.D. Armitage. 1987. The prediction of the macroinvertebrate fauna of unpolluted running-water sites in Great Britain using environmental data. Freshwater Biology 17:41-52.
Oberdorff, T., D. Pont, B. Hugueny, and J.P. Porcher. 2002. Development and validation of a fish-based index for the assessment of ‘river health’ in France. Freshwater Biology 47:1720- 1734.
Reynoldson, T.B., R.H. Norris, V.H. Resh, K.E. Day, and D.M. Rosenberg. 1997. The reference condition: a comparison of multimetric and multivariate approaches to assess water-quality impairment using benthic macroinvertebrates. Journal of the North American Benthological Society 16:833-852.