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Biotic index

Biotic Index (also called "Family Biotic Index") is a scale for showing the quality of an environment by indicating the types of organisms present in it. It is often used to assess the quality of water in rivers. It is measured from 1 to 10 and corresponds to the four basic water quality (Excellent, Good, Fair or Poor). The concept of the Biotic Index was developed by William M. Beck in an effort to provide a simple measurement of stream pollution and its effects on the biology of the stream

To assign a biotic index value to a specific water site, the tester first collects macroinvertebrates from portions of the sample area of the stream, river or lake and separate them into groups of similar-looking organisms. More extensive testing can be done by looking for certain microscopic organisms.

Then an identification key is used to help determine which category or group the organism belongs in and allows a numerical value be assigned to that organism. A worksheet is then used to calculate the final value or score of all the organisms found. Depending upon the worksheet's equations, the score determines the condition of the water quality.

Usefulness of macroinvertebrates

Aquatic macroinvertebrates have some general characteristics that make them very useful to assess stream health.

1. They are abundant and found in water bodies throughout the world.

2. They are not extremely mobile.

3. They carry out part or all of their life cycle within the stream or river.

Macroinvertebrates limited mobility and extended presence in the water means that they are exposed on a continuous basis to water quality in that stream or river. In particular, many of these organisms breath dissolved oxygen that is in the water. They are also easier to see at the time of sampling.

Not all the macroinvertebrates found in samples are listed on the biotic index scoring sheets. This is because some do not rely on oxygen within the water for survival. Many are able to collect air from the atmosphere and hold a bubble alongside their body to use like a scuba diver uses a tank of oxygen.

For those macroinvertebrates that do rely on dissolved oxygen, some can only live in water that has a lot of oxygen. Others can live in water that doesn't have much oxygen dissolved in it at all. Generally, it is assumed that the more pollution there is in the water, the less oxygen.


The biotic index works by assigning different levels of tolerance to pollution to the different types of organisms. The types of macroinvertebrates and other organisms found during sampling are broken into 4 groups:

1: Pollution intolerant: These organisms are highly sensitive to pollution. (IE: Stonefly or Alderfly Larva)

2: Semi-Pollution intolerant: These organisms are sensitive to pollution. (IE: Dragonfly Larva or Crawfish)

3: Semi-Pollution tolerant: These organisms will be found in clean and slightly polluted waterways. (IE: Snails or Black Fly Larva)

4: Pollution tolerant: These organisms will be found in polluted, as well as clean aquatic ecosystems (IE: Leechs,Bloodworms)

Some index worksheets combine groups 2 and 3 together giving only 3 groups. Each group has a number assigned to it and is multiplied by the number of organisms found in that group. This is why identifying the type of organism is important.

See also

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  • William E. Sharpe, William G. Kimmel, and Anthony R. Buda (2002)."Biotic Index Guide." Pennsylvania State University.
  • Water Action Volunteers - "Stream Monitoring." University of Wisconsin Extension office.