From Coastal Wiki
Definition of bioindicator:
species whose disappearance or disturbance gives early warning of the degradation of an ecosystem. 
This is the common definition for bioindicator, other definitions can be discussed in the article
Bioindicators are necessary for certain hydrophobic pollutants, because these compounds tend to be adsorbed by biota soon after entering the ecosystem. This makes that the concentrations in the water might not reflect the contamination in the biota.
See alsoCommon biomarkers for the assessment of marine pollution
- ↑ Lawrence E (ed.), 2000. Henderson’s Dictionary of Biological Terms. 12th edition. Prentice Hall, Pearson Education Limited. Harlow, Great Britain.