Stream Benthic Macroinvertebrates - Water Quality Assessment

One method to monitor the water quality in streams is to survey the benthic macroinvertebrates living in the stream. Benthic macroinvertebrates are creatures without backbones that live at the bottom. Common creatures are crayfish, snails, clams, and a large variety of insect (and non-insect) larvae. This means that if you find sensitive creatures living in the water, you can conclude that the water quality must be good. On the other hand if you only find creatures that are very tolerant of poor water quality, you can conclude that the water quality is poor.

One of the advantages of using benthic macroinvertebrates for stream water quality monitoring is that the creatures are relatively non-mobile and generally live in the water a fairly long period of time. This means that if you find a sensitive creature alive in the stream, the water quality most have been good throughout its life. As a result, a single sample of macroinvertebrates provides far more longer term information than a single water quality sample used for chemical analysis. However, the lack of sensitive creatures just says that the water quality is not good but it does not provide any indication of what is the cause.

Additional Resources

There are a large number of organizations that use various methods for benthic macroinvertebrate sampling for water quality monitoring. Many are happy to involve volunteers in this type of monitoring.

Benthic Macroinvertebrate Identification

Introductory - high taxonomic level

Advanced - Family taxonomic level

Educational Materials

Benthic Macroinvertebrate Monitoring Programs

  • Maryland Stream Waders is a Maryland Department of Natural Resources program for annual monitoring of water quality across Maryland. The program has a strong scientific design basis and has been shown to collect data equal in quality to their professional Maryland Biologic Stream Survey. Volunteers collect the sample and turn it in to DNR for professional identification.
  • Web searching for macro invertebrates or water quality monitoring will provide an abundance of additional resources.

Benthic Macroinvertebrate Reference Books

  • Voshell, J. Reese Jr, A Guide to Common Freshwater Invertebrates of North America, The McDonald & Woodward Publishing Company, Blacksburg, Virginia, (2002). This is my favorite. In addition to detailed illustrations, there are extensive descriptions.

Photograph of a stream macroinvertebrate

Caddisfly larvae

A pollution-sensitive organism typically found in healthy streams