Since from the cold temperate to the tropical regions, the diversity of brown algae (Phaeophyceae) is drastically reduced and red algae (Rhodophyceae) begin to dominate, Feldmann introduced the R/P (Rhodophyceae/ Phaeophyceae) ratio, as a phytogeographic index useful to characterize the flora of a given region. It varies from 1.08 in the cold temperate regions to a maximum of 4.59 in the tropical regions. Since it does not take into account the number of green algae (Chlorophyceae), particularly abundant in tropical floras, Cheney suggested to replace the R value with the R + C value. In Cheney's view, (R + C)/P ratio is more sensitive and accurate showing a wider range (1.7:7) than R/P ratio (1.08:4.59).

Feldmann index, or that modified by Cheney, cannot be used in characterizing floras of geographical areas where conspicuous upwelling occurs, or where brackish waters prevail. In those areas, the floristic richness decreases considerably and, because such a decrease affects mainly brown algae, the Feldmann index is misleading.

The R/P index can also be used as an ecological indicator if expressed as mean of values calculated for each sample from various phytobenthic communities present in a given area. In fact, it grows with the increase of pollution or instability (in extreme cases it cannot be calculated due to the absence of brown algae), and decreases in conditions of environmental stability.

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