Use of RP Ratio in Applied Ecology

Since red algae, and more conspicuously brown algae, have been shown to be extremely sensitive to changes of environmental factors, since the mid-1980s, the R/P index has also been used by a number of Mediterranean algol-ogists as a synecological index to assess environmental conditions, if expressed as mean of values calculated for each sample from various phytobenthic communities present in a given area (community mean R/P value). Such index tends to increase in connection with ecological degradation or environmental instability (in extreme cases it cannot be calculated for lack of Phaeophyceae), whereas it tends to decrease under conditions of highly structured or stable environments. As an example, community mean R/P values of some communities with Cystoseira spp., that are among the most important ones on rocky substrata in the Mediterranean Sea, calculated in areas characterized by good environmental conditions over the spring-summer period, are reported in Table 1. Such values are generally considered as reference values on which the estimate of environmental conditions is based.

However, in studies aiming environmental assessment and monitoring, they should be employed with some caution. Indeed, in vegetation surveys carried out in communities from the same areas, through monthly sampling over one year, the mean R/P value was found to change from one month to the next. As a consequence, it should be noted that comparisons with reference values are more significant when samples are collected in the same season as the reference one, and they belong to communities lying in the same geographical areas as the reference one.

More recently, in order to evaluate the ecological environmental conditions of small coastal areas, the use of a different index has been proposed, resulting from the ratio between the community mean R/P value for each community present in the surveyed area and the reference community mean R/P value of the corresponding communities. The values obtained from this ratio have been grouped into five classes, as reported in Table 2. Each class refers to a specific scale of assessment defining the ecological conditions of a given environment.

Table 1 Spring-summer mean R/P values of some phytobenthic communities with Cystoseira occurring along the coasts of Sicily

Phytobenthic communities

Mean R/P value

Community with Cystoseira amentacea

4.5

Community with Cystoseira crinita

3.4

Community with Cystoseira sauvageauana

6.2

Community with Cystoseira spinosa

3.8

Community with Cystoseira dubia

5.5

Community with Cystoseira zosteroides

6.2

Table 2 Range of values of index 'community mean R/P value/ reference community mean R/P value' and corresponding classes and ecological conditions

Ecological

Range of values

Class

conditions

>4 or not computable due to lack of

1

Bad

Phaeophyceae

3.9-2.1

2

Low

2.0-1.4

3

Moderate

1.5-1.1

4

Good

<1

5

High

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