There are some organisms which occur under certain specific conditions of environment. These organisms have developed resistance to such conditions under which the growth of other organisms is usually suppressed. A large number of species of Chlamydomonas, Pandorina, Oscillatoria, Euglena, Melosira, and Navicula etc. frequently occur in organically enriched waters.
Their appearance in water bodies indicates the onset of pollution caused by dead, decaying and decomposing organic matter and excreta. On the other hand, form like Cladophora, Ankistrodesmus, Bulbochaete, Chaetophora, Draparnaldia Cyclotella, Cymbella, Chromulina, Dinobryon, Phacus; Phacotus etc. usually occur in cleaner waters.
These organisms are intolerant to polluted conditions. Their growth is suppressed and is usually replaced by pollution tolerant forms. Forms like Cladophora glamerata and Volvox aureus prefer well aeriation waters rich in dissolved oxygen. These organisms cannot tolerate organic matter enrichment as decomposition of organic matter creats oxygen deficient conditions.
A large number of organisms show a remarkable degree of resistance to heavy metal pollution. These onanisms may or may not occur in localities uncontaminated with heavy metals. However, with the onset of heavy metal pollution within the system it is only these plants, which can tolerate greater concentration of heavy metals, survive. Others gradually disappear as the pollution rises.
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Therefore, it is simply not the presence but the exclusive presence of forms tolerant to higher concentration of one or more heavy metals which is usually considered to be indicative of heavy metals pollution within the system. Species of Anthoxanthium are tolerant to higher concentration of zinc. Agrostis species can tolerate a high concentration of copper. Festuca species and species of Impatiens can tolerate higher concentrations of lead and cadmium respectively. These plants can serve as useful indicator of heavy metals for which they have developed resistance.