Biological monitoring involves identifying and counting macroinvertebrates. The purpose of biological monitoring is to quickly assess both water quality and habitat. The abundance and diversity of macroinvertebrates found is an indication of overall stream quality. Macroinvertebrates include aquatic insects, crustaceans, worms, and mollusks that live in various stream habitats and derive their oxygen from water. They are used as indicators of stream quality. These insects and crustaceans are impacted by all the stresses that occur in a stream environment, both man-made and naturally occurring.

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 effects of short and long-term pollution events.
• They are relatively long lived the life cycles of some sensitive macroinvertebrates range from one to several years.
• They are an important part of the food web, representing a broad range of trophic levels.
• They are abundant in most streams. Some 1st and 2nd order streams may lack fish, but they generally have macroinvertebrates.
• They are a food source for many recreationally and commercially important fish.
• They are relatively easy to collect and identify with inexpensive materials.

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

For example, stonefly nymphs, which 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 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. Different macros occupy different ecological niches within the aquatic environment, so diversity of species generally means a healthy, balanced ecosystem. The disadvantage of the biosurvey, on the other hand, is 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 flows too sluggishly, 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 run off from farmland, water temperatures that are too high, habitat degradation such as excess sand or silt on the stream bottom 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.