Summary #
A biotic index is a numerical score that summarizes the biological health of a stream based on the types of organisms found there. This page explains how biotic indices work, how they are calculated, and what the scores mean.
Macro invertebrates can be very descriptive of the overall water quality of a waterway, though they cannot pinpoint specific chemical parameters. Benthic (bottom dwelling) organisms: reflect the long-term condition of the stream from a variety of sources, have a high species diversity and are more sensitive to changes in the ecosystem than fish, and live at the bottom of the waterway, where pollution associated problems are often magnified.
Biotic surveys can be conducted in a number of ways. It is best to use a variety of capture methods as the insects have a wide variety of habitats. Many of the macro invertebrates are actually insect larva that bear little resemblance to the adult that many are familiar with. Some prefer to burrow into the sandy bottom of a stream, like midges. Others cling to the bottoms and sides of rocks. Some organisms prefer to live in quiet pools while others live in riffles. Riffles are the areas of water that are relatively fast-moving and flow over and around many rocks.
Macro invertebrate populations will respond readily to pollution of many kinds. It is important to remember this when doing a biotic index. If there are multiple pollution types in a stream, each has its own effect. The macros also respond differently to different types of pollution. Agricultural runoff with result in one kind of population change, acid rain another and AMD still another.
Sources/Links (as provided in the source text):
Related Pages #
- Developing a Monitoring Plan
- Water Quality — What biological monitoring tells us
- Collection Methods — How to collect biological samples
- Macroinvertebrate Guide — Identifying stream organisms
Source and Last Reviewed #
Source: AMR Clearinghouse (amrclearinghouse.org). Migrated to AML-Connect. Last Reviewed: 2026-03-13.
[Admin note: Some external links in this article may be outdated. Verify before relying on them. Flag dead links for removal or replacement.]
Tags: monitoring, sampling, assessment
