Bioassessment with Benthic Macroinvertebrates

Bioassessment with Benthic Macroinvertebrates

Bioassessment using benthic macroinvertebrates (BMIs) is a cost-effective tool for assessing aquatic condition because invertebrates (mostly insects) have a diverse community structure with individual species residing within the stream for a period of months to several years.  BMIs are sensitive, in varying degrees, to temperature, dissolved oxygen, scouring, sedimentation, nutrient enrichment and chemical and organic pollution.  Bioassessment measures integrate the effects of water quality over time and provide citizens with an understanding of ecological health of the river system.

SYRCL’s River Monitoring program began bioassessment in 2004 following training from the California Aquatic Bioassessment Laboratory. Sampling was conducted at nine sites according to a protocol following standards of the California Department of Fish and Game authorities, and repeated during the same early fall period in 2005 and 2006.  In 2007, the State Water Resources Control Board released more comprehensive and authoritative protocols as part of the Surface Water Ambient Monitoring Program. SYRCL adopted and implemented the new protocol (SWAMP BMI Protocol 2007) after confirming the comparability of resulting data. Both procedures include evaluation of physical habitat as well as collection.

Composite samples from each site are sent to professional taxonomists for identification. SYRCL has used two taxonomists certified by the Southwest Association of Freshwater Invertebrate Taxonomists and following the standard level of taxonomic effort.  The Bug Lab at the Utah State University provided reports from their work on Yuba BMIs in 2007 and Yuba BMIs in 2008.   The Bug Lab and the taxonomist used for prior samples provided many metrics of community composition which are useful as indices of diversity and water quality. These metrics can be used to indicate levels of impairment or disruptions in community structure.

BMI data in Yuba Shed includes eight metrics which were available for all years of sampling, and also determined to be sensitive to the effects of hydropower projects or other disturbances present in the Yuba River watershed.  These metrics are explained in the table below. Some of these metrics function as part of a composite index of hydropower effects (Rehn et al. 2008).  Others are more generally understood to be useful indicators of stream condition in Northern California, as used, for example, in the Russian River Index of Biological Integrity (Harrington 1999). Charts show reference values according to evaluative categories determined in these studies.  While the terms “optimal” and “suboptimal” are not prescribed by these authors, they correspond with breaks for separating values into three categories of condition determined in these studies.





Taxa richness is the number of distinct taxa in the sample, and normally decreases with decreasing water quality. Taxa richness was calculated for the number of unique genera, and families. Comparisons taxonomic richness among samples is appropriate when the same level of taxonomic effort was used.


Total number of taxa in the orders Ephemeroptera, Plecoptera and Trichoptera which are relatively sensitive to pollution.  EPT decreases with impairment


Percent of individuals in sample of the single most common taxa. This metric is inversely related to diversity, and typically increases with pollution or other forms of impairment.


"This metric is based on the Hilsenhoff Biotic Index that uses a set of taxon-specific tolerance values to calculate an overall, or community level tolerance. Based on a scale of 0 (highly intolerant) to 10 (highly tolerant), this metric was originally designed to serve as a measure of community tolerance to organic pollution in Wisconsin streams, but is commonly used as a general index of pollution tolerance. Application of this metric to all types of disturbance is complicated by the fact that the original values are regionally specific, and by the fact that different organisms can respond uniquely to different pollution or disturbance types. For example, a genus that is highly tolerant to sediment or organic pollution might be very intolerant to disturbance from heavy metals or pesticides.  However, this index has been found to be useful as a general measure of community tolerance to human disturbance, although its use should be treated with caution. In future iterations, it is proposed that there will be disturbance-specific tolerance values" (Ode et al 2003).  Tolerance Value Index is the weighted average for all the individuals in the sample: (n1*TV+n2*TV...)/N.  


Shannon Diversity Index is a measure of community structure defined by the relationship between the number of distinct taxa and their relative abundances.  The Shannon diversity index was calculated for each sampling location for which there were a sufficient number of individuals and taxa collected to perform the calculations.  The calculations were made following Ludwig and Reynolds (1988, equation 8.9, page 92).


Evenness is a measure of the distribution of taxa within a community.  The evenness index used in this report was calculated following Ludwig and Reynolds (1988, equation 8.15, page 94: H'/lnH').  Value ranges from 0‑1 and approach zero as a single taxa becomes more dominant.   "....a measure of biodiversity which quantifies how equal the community is numerically" (


Percent of individuals in sample classified as predators. Predators are one of five common functional feeding groups of BMIs, and one that is of less relative abundance in impaired waters.


Percent of individuals in sample classified as scrapers. Scrapers may be of less relative abundance in impaired waters.