For a number of decades since the end of the nineteenth century, algologists have based environmental descriptions of different geographical areas on the floristic richness and, most notably, on the qualitative composition of algal flora (different genera present, number of species belonging to different genera). This approach has resulted in diverging, often subjective, interpretations relying in part on the importance ascribed to the presence or absence of certain genera or of specific groups of species in each flora.

J. Feldmann first proposed the adoption of a biogeo-graphical index (Feldmann index or R/P ratio: number of Rhodophyceae species to number of Phaeophyceae species in a given marine flora); since this ratio has been found to increase gradually from the floras of the Arctic and sub-Arctic zones to those of tropical and subequator-ial zones, it allows us to deduce to which geographical zone any flora could belong.

Given the close relation of these two groups of algae with all environmental factors, they interact with and to whose changes they are sensitive; today the ratio is also employed, with slight modifications, to assess and describe local environmental conditions.

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