Water: Monitoring & Assessment
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Selecting Metrics to Determine Stream Health
Metrics are used to analyze and interpret biological data by condensing lists of organisms into relevant biological information. In order to be useful, metrics must be proven to respond in predictable ways to various types and intensities of stream impacts. This manual recommends using a multimetric approach that combines several metrics into a total Biosurvey Score. The four primary and two optional metrics discussed in this chapter have been tested extensively in the mid-Atlantic region and have been shown to respond in predictable ways to stream impacts. In other parts of the country, other metrics and scoring systems may be more appropriate. For example, the Benthic Index of Biotic Integrity (B-IBI), developed by Dr. James Karr, is another multimetric approach, using different metrics, that has been tested in the Tennessee Valley, the Midwest, and the northwest. The River Watch Network suggests that, while you should always use multiple metrics to summarize your data, you shouldn't rely solely on an overall score to interpret your data; individual metrics can also provide a wealth of information. In any case you will need to select metrics that have been proven to respond predictably to various impacts. As always, consult with your program's biological advisor for help in selecting appropriate metrics for your region and for determining whether an overall biosurvey score is recommended. Below are metrics that are commonly used in rocky bottom streams. This is only a partial list of the dozens of metrics used by monitoring programs throughout the country. These metrics fall under four general categories: 1) taxa richness and composition, 2) pollution tolerance and intolerance, 3) feeding ecology, and 4) population attributes. Metrics marked with a (*) are included in the recommended suite of metrics in this manual. The River Watch Network's Benthic Macroinvertebrate Monitoring Manual contains detailed guidance on selecting, calculating, aggregating, and interpreting the metrics discussed below. (See Dates, G. and J. Byrne in References and Further Reading) Taxa Richness and Composition Metrics
- Total Number of Taxa *: the total number of taxa found in the sample.
- Number of EPT Taxa *: the combined number of mayfly (E), stonefly (P) and caddisfly (T) taxa found in the sample. The number of taxa in each of these macroinvertebrate orders can also be reported separately since each order may respond differently to various impacts.
- Number of Long-Lived Taxa: the number of organism families found in the sample (such as giant stoneflies and dobson flies) that live more than one season.
- Percent Abundance of the Major Groups *: the percent of the sample that is comprised of individuals in each of the selected major groups (mostly orders).
- Percent Model Affinity (Bode, 1991): used in conjunction with Percent Composition of the Major Groups, this metric measures the similarity of the sample to a model "nonimpacted" community of organisms (adjusted for ecoregional conditions) based on the percent composition of the major groups.
- Quantitative Similarity Index (from Shackleford, 1988): used in conjunction with Percent Composition of the Major Groups, this metric shows the percent similarity between two sites based on the percent of the sample in each of the major groups.
- Dominants in Common (from Shackleford, 1988): the number of dominant (5 most abundant families) families common to two sites.
- Number of Intolerant Taxa: the number of taxa in the sample that are in the 10-15% of the least tolerant taxa in a region or that have a pollution tolerance value of 1 (based on the Hilsenhoff scale of 0-10).
- Percent of Individuals in Tolerant Taxa: the number of taxa in the sample that are in the 10-15% of the most tolerant taxa in a region or that have a pollution tolerance value of 10 (based on the Hilsenhoff scale of 0-10).
- Number of Clinger Taxa: the number of families in the sample that live by clinging to the bottom of the stream.
- Sensitive Taxa Index *: the pollution tolerance values (based on the Hilsenhoff scale of 0-10) assigned to each family aggregated into an overall pollution tolerance value for the sample.
- Percent Composition of Functional Feeding Groups: the percentage of the total number of individuals in the sample that belong to each of the five functional feeding groups (scrapers, shredders, filtering collectors, gathering collectors, and predators).
- Percent Abundance of Scrapers *: the percent of the total number of individuals in the sample that use bottom-growing algae as their primary food source.
- Percent Abundance of Shredders *: the percent of the total number of individuals in the sample that use leaves and other plant debris as their primary food source.
- Percent Abundance of Predators: the percent of the total number of individuals in the sample that eat other animals as their primary food source.
- Percent Dominance (of the most abundant family) *: the percentage of the total number of individuals in the sample that are in the sample's most abundant family.
- Percent Dominance (of the three most abundant families): the percentage of the total number of individuals in the sample that are in the sample's three most abundant families.
- Organism Density Per Sample (total abundance): the total number of individuals in the sample (calculated if a subsample is used).