Biological monitoring can inexpensively screen for problems that are not specifically tested for using common chemical kits. The presence of heavy metals, pesticides, herbicides, and other toxins in a stream may be indicated by a lack of biodiversity in a stream. Since certain organisms can tolerate more pollution than others, the greater the variety of organisms - the better the water quality.
Biological monitoring can also indicate the presence of problems that extend over time.
The organisms that live in a stream are affected by the quality of the water in the stream. The organisms that cannot leave can be viewed as indicator organisms. Some pollutants are only in the water for a short period of time before they dilute out or wash downstream. These short term events might not show up in testing but they can have a dramatic impact on the organisms living in the stream. The presence of indicator organisms tells us a history of water quality in the stream.