United States
Environmental Protection
Agency
EPA/820/F-11/006 | March 2011 | water.epa.gov
BIOLOGICAL ASSESSMENTS:
Key Terms and Concepts
While the world constantly changes, our basic uses for water remain the same: We drink it, we bathe in it, we grow
food with it. Our demand on water resources has a direct impact on the health and wellbeing of waterbodies and
those species they support. To maintain healthy waterbodies, the Environmental Protection Agency (EPA) and many
states and tribes have established regulations and programs to help restore and maintain the health of the nation's
waters.
The first step in restoration and maintenance is to evaluate the condition of waterbodies and to understand
the relationship between stressors and their impact on aquatic communities. Biological assessments provide
information on the condition of the biological community and help document the response of this community to
stressors.
What is a Biological Assessment?
A biological assessment is an evaluation of the condition of a
waterbody by sampling species that spend all or part of their lives
in that waterbody. A biological survey is conducted to collect a
representative sample of the biological community found in the
waterbody. For each site sampled specific attributes, known as
biological indicators, are compared to the conditions expected for
that indicator based on reference sites.
• Biological survey. A systematic method for collecting a
consistent, reproducible and reliable sample of the aquatic biological community in a waterbody.
• Biological indicators. The groups of organisms used to assess the condition of an environment. Depending on
the type of waterbody being sampled, biological indicators used in biological surveys may include the following:
- Fish (trout, sunfish, perch, salmon)
- Benthic macroinvertebrates (insects, snails,
crayfish, worms)
- Periphyton (algae)
- Amphibians (frogs, salamanders)
- Macrophytes (aquatic plants)
- Birds (residential or migratory)
• Reference Sites. Data collected at reference sites provide a benchmark for assessing the biological condition of
surveyed sites. Ideally, reference sites are sites that have not been disturbed by anthropogenic stress. However,
the impact of human activities on aquatic systems in the United States is widespread and pervasive. Reference
sites, therefore, typically have been influenced to some degree by human activities and may represent our best
approximation of natural conditions. Data from reference sites can be used to develop management targets for
protection and restoration of aquatic resources.
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Metrics: Measuring Biological Conditions
Metrics are quantitative measures of biological indicators, and can provide information on both the present and
past effects of anthropogenic stress on aquatic systems. Each metric is given a score, which can be interpreted as a
measure of the condition of the sampled site relative to reference sites. Definitions and examples of typical metrics
are described in the table below.
METRIC
Total Taxon Richness
Relative Richness
Percent Occurrence
of Anomalies
DEFINITION
The number of different species (or genera or
families) counted in a sample collection.
The percentage of the number of species in a sample
that are either pollution sensitive or pollution
tolerant species compared to the total taxon richness.
The percent of sampled fish that have visible, external
anomalies (e.g. tumors, lesions, abnormalities).
EXAMPLE
There are 41 different species of benthic macroinvertebrates from
34 different genera, 22 families and 8 different orders observed
in one sample. The expected richness in reference sites is 40 or
more species. The metric value of the sample (41) is comparable to
reference site values.
19.5 percent in a sample of benthic macroinvertebrates are pollution
sensitive species. The reference site percentage is 36 percent or
higher.
12 percent of a fish sample show external anomalies. Samples from
the reference sites show less than 1 percent.
Biological Index. To gain a more comprehensive view of an aquatic community, multiple types of metrics are
combined into a biological index. The typical biological index may include information from up to 7 to 12 different
metrics.1 The index of biological integrity (IBI) for fish is an example of a biological index created from metrics.
Alternatively, models such as RIVPACS, can be used to compare the observed species (0) at a site to the expected
species (E) based on reference sites.2 The 0/E ratio is then used as a measurement of "taxonomic completeness".
Biological Assessments in the Field: An Example
The following photographs3 give an example of a biological assessment in Maine. A biological survey has been
conducted on each stream to determine their communities' structures. In this case, the benthic macroinvertebrate
assemblage is the biological indicator, and metrics such as relative richness have been selected. The condition
represented by undisturbed/minimally disturbed streams, or reference sites, is used to assess changes in different
metric values from other communities with increasing stress.
As the level of stress increases in these streams, the aquatic communities' compositions were measurably changed.
Species that are sensitive to stress disappeared and more pollution tolerant species were found.
Those species are identified in the following images to indicate their general level of sensitivity to stress and ability
to tolerate pollution: Intermediate species: S. Moderately tolerant species: M. Tolerant species: T.
Undisturbed/Minimally Disturbed Nutrient Enriched
Stream (Reference Site) Stream
Drainage from a Shopping
Mall Parking Lot
1 Karr, J.R., K.D. Fausch, P.L. Angermeier, P.R. Yant and I.J. Schlosser. 1986. Assessment of biological integrity in running water: A method and its rationale. Special
Publication 5, Illinois Natural History Survey, Champaign, IL.
2 Wright, J.F. 2000. An introduction to RIVPACS. In Assessing the Biological Quality of Fresh Waters: RIVPACS and Other Techniques. J.F. Wright, D.W. Sutcliffeand M.T.
Furse (eds.), pp. 1-24. Freshwater Biological Association, Ambleside, UK.
3 Photographs used with permission from the Maine Department of Environmental Protection.
For more information, please refer to the Primer on Use of Biological Assessments to Support Water Quality Management.
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