Why Monitor Macroinvertebrates?

Macroinvertebrates are organisms that are large (macro) enough to be seen with the naked eye and lack a backbone (invertebrate). They inhabit all types of running waters, from fast flowing mountain streams to slow moving muddy rivers. Examples of aquatic macroinvertebrates include insects that live in water during their larval or nymph form, crayfish, clams, snails, and worms. Most aquatic macroinvertebrates live part or most of their life cycle attached to  submerged rocks, logs, and vegetation.  

Aquatic macroinvertebrates are good indicators of stream quality because:

  • They are affected by the physical, chemical, and biological conditions of the stream.
  • They can’t escape pollution and show the effects of short- and long-term pollution events.
  • They may show the cumulative impacts of pollution.
  • They may show the impacts from habitat loss not detected by traditional water quality assessments.
  • They are a critical part of the stream’s food web.
  • Some are very intolerant of pollution.
  • They are relatively easy to sample and identify.

The basic principle behind the study of macroinvertebrates is that some are more sensitive to pollution than others. Therefore, if a stream site is inhabited by organisms that can tolerate pollution and the more pollution sensitive organisms are missing there is likely a water quality problem.

For example, stonefly nymphs, aquatic insects that are very sensitive to most pollutants, cannot survive if a stream’s dissolved oxygen falls below a certain level. If a biosurvey shows that no stoneflies are present in a stream that used to support them, a hypothesis might be that dissolved oxygen has fallen to a point that keeps stoneflies from reproducing or has killed them outright.  

This brings up both the advantage and disadvantage of the biosurvey. The advantage of the biosurvey is that it tells us very clearly when the stream ecosystem is impaired, or “sick,” due to pollution or habitat loss. It is not difficult to realize that a stream full of many kinds of crawling and swimming “critters” is healthier than one without much life. The disadvantage of the biosurvey, on the other hand, is that it cannot definitively tell us why certain types of creatures are present or absent.  In this case, the absence of stoneflies might indeed be due to low dissolved oxygen. But is the stream under oxygenated because it is inherently sluggish or because pollutants in the stream are damaging water quality by using up the oxygen? The absence of stoneflies might also be due to other pollutants discharged by factories or running off farmland, water temperatures that are too high, habitat degradation such as excess sand or silt on the stream bottom that has ruined stonefly sheltering areas, or other conditions.

Thus a biosurvey should be accompanied by an assessment of habitat and water quality conditions in order to help explain biosurvey results.  LJEA and other organizations have performed chemical analyses of water in the Lake James watershed for many years. When a biosurvey indicates a potential problem, the chemical monitoring data may help identify the source of the problem.

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