Indicator organisms, sometimes called bioindicators, are plant or animal species known to be either particularly tolerant or particularly sensitive to pollution. The health of an organism can often be associated with a specific type or intensity of pollution, and its presence can then be used to indicate polluted conditions relative to unimpacted conditions.

Tubificid worms are an example of organisms that can indicate pollution. Tubificid worms live in the bottom sediments of streams and lakes, and they are highly tolerant of sewage. In a river polluted by wastewater discharge from a sewage treatment plant, it is common to see a large increase in the numbers of tubificid worms in stream sediments immediately downstream. Upstream of the discharge, the numbers of tubificid worms are often much lower or almost absent, reflecting cleaner conditions. The number of tubificid worms also decreases downstream, as the discharge is diluted.

Pollution-intolerant...