Rhodophyceae Phaeophyceae Ratio RP Ratio

In the latter half of the 1930s, in his study of the benthic marine vegetation of Cote des Alberes (Banyuls, France), Feldmann compared the rich flora by himself found in that area with some others known from other Mediterranean areas. As it appeared immediately, the French flora turned out to be the richest among the ones considered; particularly, its overall richness consisting of 496 species was found out to be similar to that of the Mediterranean as a whole, then estimated as ranging around 494 species. From that early result Feldmann concluded that the floristic richness is not suitable to characterize geographically an algal flora and even less to carry out significant comparisons between floras, since it is to be regarded as relative information depending mostly on the accuracy by which the floristic survey was carried out and on the kind ofmethodology adopted. In order for it to provide reliable information, the floristic study should be made by samplings carried out over various periods during the year as well as at various depths.

On the other hand, by comparing floras from the same area (Gulf of Naples), carried out and published by four different authors at different times, Feldmann observed that, despite noticeable differences in floristic richness (from 171 to 423 species), the percent incidence of each algal group (Chlorophyceae (green algae), Phaeophyceae (brown algae), and Rhodophyceae (red algae)) appeared to be quite similar in the four compared floras.

Following that result, Feldmann set out to compare 28 floras from corresponding geographical areas in the Atlantic, from the Arctic zone to the equatorial zone, including floras from five different areas of the Mediterranean Sea. From this comparison he found out that green algae showed percent values rather variable: they were generally lower in floras of temperate regions (including the Mediterranean) and higher in floras ofboth arctic and equatorial regions; conversely, as for the other two algae groups, Feldmann observed that from north (Arctic zone) to south (equatorial zone), a progressive increasing of percent values of red algae could be appreciated, as opposed to a regular and progressive decreasing of the percent values of brown algae (Figure 1).

From these results, Feldmann realized that red and brown algae were the taxonomic groups more sensitive to environmental changes along the north-south geographical gradient. Furthermore, he observed that the ratio between the number of red algae and that of brown algae occurring in each flora varied gradually according to the latitude of the 28 studied floras, ranging from about 1 in Arctic and sub-Arctic floras, to about 5 in tropical and sub-equatorial floras (Figure 2). Irregular geographical borders between zones may easily account for a number of uncharacteristic values with respect to latitude. It is a well-known fact that geographical borders in some areas very close to continents cannot perfectly fit the parallels,

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Floras considered by Feldmann

Figure 1 Percentage values of red, brown, and green algae in each of the 28 floras compared by Feldmann and ordered according to latitude (from north toward south). Trend line and corresponding R2 value ^coefficient of determination) are reported for each taxonomic group. 1 - Arctic regions; 2 - Iceland; 3 - Trondhejm; 4 - The Faeroes; 5 - Western Sweden; 6 - Clare Island; 7 - Isle of Man; 8-Holland; 9-Cherbourg; 10-Saint-Malo; 11 - Gulf of Guascony; 12-Rovinj (Adriatic Sea); 13-Gijon; 14-Banyuls (Mediterranean Sea); 15-Massachussett; 16-New England; 17-Gulf of Naples (Mediterranean Sea); 18-Minorca (Mediterranean Sea); 19-Azores; 20 - Algeria (Mediterranean Sea); 21 - Tangier; 22 - South Carolina; 23 - Bermudas; 24 - Canary Islands; 25 - Florida; 26 - Bahamas; 27 - Virgin Islands; 28 - Brazil.

o % Chlorophyceae a o % Chlorophyceae a

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Floras considered by Feldmann

Figure 2 R/P and (R + C)/P values in the 28 floras compared by Feldmann and ordered according to latitude (from north to south). Trend line and corresponding R2 value ^coefficient of determination) are reported in the graph. See legend of Figure 1 for place names of floras.

but show some deviations (northward and/or southward) locally resulting in one geographical area being engulfed within another. Taking all this into consideration, Feldmann thought the ratio R/P to be a sufficiently reliable, as well as extremely discriminating, index, for easy and quick description of the overall biogeographical characteristics of a flora.

The relevance of Feldmann index lies also in the fact that it enables the appreciation of similar environmental conditions in faraway geographical areas, as long as their respective floras are found to possess the same R/P values. This arises from the assumption that the floristic composition of a certain area depends on the influence of environmental characteristics on each taxonomic algal group.

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