Ambient Benthic Macroinvertebrate Community Monitoring

Benthic macroinvertebrates are animals without backbones, who inhabit the bottom of rivers and streams, as well as many other waterbody types.  These organisms are very well studied and have a long history of use as indicators of water quality. Certain types, including mayflies, stoneflies, and caddisflies, can survive only in the cleanest water quality conditions.  Other major groups of macroinvertebrates are true flies, beetles, worms, crustaceans, and dragonflies.

The DEEP Water Monitoring Group has used benthic macroinvertebrate communities to help characterize stream and river water quality since the mid-1970s. 

Indicators of Water Quality

Natural, undisturbed, macroinvertebrate communities are typically comprised of a balance of many species from the major orders of aquatic insects, which include the mayflies, stoneflies, caddisflies, true flies, dragonflies, and beetles. In addition, in a healthy community, the species present will represent a balance of different feeding strategies including shredders, filterers, and predators. When a diverse and robust benthic macroinvertebrate community is present in a particular section of stream or river, it is a sign that the waterbody is ‘healthy’ and the water quality conditions are good. On the other hand, a waterbody that has poor water quality will typically support a macroinvertebrate community that is dominated by only a few species and feeding strategies.  

Data Collection

Benthic macroinvertebrate sampling occurs between September 15th and November 30th annually. This fall ‘index period’ is designed to represent worst-case water quality conditions, as the fall macroinvertebrate community is exposed to the greatest level of stress from any potential water quality issue through the low-flow summer months.

For high gradient streams, samples are collected from 2 square meters of riffle habitat using a rectangular ‘kick’ net.  Field staff use standardized protocols to insure any macroinvertebrates living in the stream bottom or attached to substrate in the sampling area are collected in the net. 

A single sample can contain thousands of macroinvertebrates.  Therefore, the sample is preserved and brought back to the laboratory for ‘subsampling.’ This process entails randomly selecting 200 organisms for more detailed analysis.  This 200 organism subsample is then sent to a professional taxonomist for identification down to the species level. 

Data Analysis

Macroinvertebrates have been studied for decades and the life history traits and pollution sensitivity of each species is well known.  Using the species list, DEEP calculates two numbers for the macroinvertebrate structure of each site sampled – a macroinvertebrate multimetric index (MMI) score and a Biological Condition Gradient (BCG) tier.  This  information is used to determine whether a section of stream supports or does not support the designated use goal for aquatic life as established by the State’s Water Quality Standards.  This information is reported every two years in the biennial Integrated Water Quality Report.

Macroinvertebrate Multi-Metric Index (MMI) Score

The MMI score is a composite score generated from several macroinvertebrate based water quality indices; the score of a site can range from 0-100.  In general, a MMI value greater than 48 points indicates good water quality; the higher the score, the better the water quality.  MMI scores less than 43, however, are indicative of poor water quality conditions.  These types of communities tend not to meet the water quality standard. 

Biological Condition Gradient (BCG) Tier

Complementary to the MMI is the macroinvertebrate Biological Condition Gradient (BCG). Based on published scientific literature, the Connecticut BCG model illustrates the relationship between the amount of stress on the environment and its effect on biological communities. Stressors are diverse and may be biological, chemical or physical in nature. In accordance with the BCG Model, as the level of stress gets progressively greater, the biological communities, which start out in a natural condition, begin to change as they respond to the stress.

Each site is assigned to an integer tier value between 1 and 6.  The BCG model attempts to mimic how trained environmental professionals would rank macroinvertebrate community data on a scale from completely natural (Tier 1) to completely dysfunctional due to human disturbance (Tier 6). This universal gradient (1 to 6) provides resource managers with a common language for comparison of different types of data, (e.g. other biological data such as fish or diatom communities), different methods of collection, and even different ecological systems. In other words, a Tier 2 site is considered to be 'good' no matter where, how, when or what is being evaluated.

Related Documents

  • Ambient Biological Monitoring-Benthic Macroinvertebrates Quality Assurance Project Plan (QAPP)

Related Pages

For More Information Contact:

Mary Becker
DEEP Water Monitoring Unit Supervisor
79 Elm Street, Hartford, CT 06106
(860) 424-3062

Content last updated May 24, 2022.