Until recently, very few indices have been proposed for the biomonitoring of hard-substratum communities, despite the attractiveness of the subject, mostly due to the difficulties involved in the research of rocky bottoms. Recent efforts (biomonitoring of the photophilic, the sciaphilic, and port communities) conducted in the Mediterranean gave the opportunity for the establishment of such indices.
On rocky bottoms, zoobenthos is commonly related to algal structure and zonation. This does not necessarily mean dependence to the presence of particular macroal-gal species, but rather to the morphological complexity and architecture of the algal cover and epiphytes. P/A index was applied on both a disturbed and undisturbed coastal area of the Aegean (Thermaikos Gulf and Chalkidiki peninsula, respectively). The results clearly show that the application of this index is inappropriate in hard bottom (Figures 2 and 3). P/A gains negative values in all the disturbed sites, and clearly failed to detect any pollution effect. The reason for these results is the different response of the fauna in hard substratum with respect to pollution that contradicts P/A index assumptions. In hard bottoms, as opposed to soft sediments, amphipods constitute a major group with many species capable of proliferating by the increased organic load, building very dense populations. So, in the case of the photophilic algae community in Thermaikos, the species Elasmopus rapax, Corophium sextonae, Caprella acanthifera, and Stenothoe monoculoides increased their density, as did the species Elasmopus rapax, Corophium acutum, C. sextonae, and Erichthonius brasiliensis in ports. In contrast, in the undisturbed sciaphilic algae community, no amphipod species showed increased dominance. Consequently, the index performs unlikely to its predictions (P/A < 1 = unpolluted; P/A> 1 = polluted), since it is the denominator that increases with the pollution effect, as if the polychaetes were the sensitive group and amphipods the resistant.
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