A bioindicator is an organism or biological response that reveals the presence of the pollutants by the occurrence of typical symptoms or measurable responses, and is therefore more qualitative. These organisms (or communities of organisms) deliver information on alterations in the environment or the quantity of environmental pollutants by changing in one of the following ways: physiologically, chemically or behaviourally. The information can be deduced through the study of:

1. their content of certain elements or compounds

2. their morphological or cellular structure

3. metabolic-biochemical processes

4. behaviour, or

5. population structure(s).

The importance and relevance of biomonitors, rather than man-made equipment, is justified by the statement: "There is no better indicator of the status of a species or a system than a species or system itself. Sourece: http://en.wikipedia.org/wiki/Bioindicator