Testing for chemical pollution in our nation's streams has traditionally meant using analytical chemistry. In recent years, environmental agencies have endorsed biological monitoring to enhance or replace chemical monitoring. This concept was applied to air quality and used by miners who took canaries into deep mines with them. If the canary died, the miners knew the air was bad, and that they had to leave the mine. Biomonitoring an aquatic system uses the same theoretical approach.
Aquatic organisms are subject to pollutants in the stream as it flows by, day and night.
Consequently, the health of the organisms reflects the quality of the water they live in. If the pollution levels reach a critical concentration, certain organisms will migrate away, fail to reproduce, or die, eventually leading to the disappearance of those species at the polluted site. Normally, these organisms will return if conditions improve in the system. Each component influences the health of the biological community individually and together. Toxic chemicals are only a single factor within the water-chemistry component.