[The use of biological indices on environmental quality assessment]

The aquatic ecosystems quality can be evaluated through the analysis of physico-chemical and biological parameters. However, the biological parameters differ from the physico-chemical ones, having the advantage of providing information on the water quality concerning longer periods of time--specially if one considers sessil organisms--thus reflecting, in a better way, the general ecological conditions of the water body. It is necessary, though, that the taxonomic information be translated into simple numerical values (indices), in order to simplify its interpretation as well as providing subsidies for the elaboration of water quality criteria. Several numerical biological indices can be applied, some of them to specific communities, and others to the hole biological community of an aquatic environment. Three kinds of indices are more generally applied to evaluate the pollutant impacts on aquatic communities: biotic, diversity and community comparison indices. The biotic indices will measure the variation in terms of tolerance and relative sensitivity of the occurring organisms to a certain pollution condition; the diversity indices evaluate the effect of pollution in terms of the community structure; the community comparison indices (also named similarity or dissimilarity indices) establish the effects of pollutants on the community composition. It is important, for the numerical indices users, to know the limitations of each method, so that the interpretation of the values obtained may be made in the light of such limitations.