In soft bottoms, which constitute the largest ecosystem in terms of spatial coverage, granulometry is thought to be the major structuring factor. As the sediments become coarser, polychaetes diversity tends to increase, until larger clusters prevail (i.e., boulders, pebbles, and gravels) that will unavoidably cause a decline to the interstitial fauna. In soft sediments, few species of amphipods occur, mainly of the genus Ampelisca. The performance of P/A index has been comprehensively tested in soft-substratum communities of the English Channel and the NW Spain coasts after some severe oil spill events. At the most affected communities P/A index rapidly increased in values for about 3 years after the episode; afterwards, P/A gradually decreased. These results were attributed to the very low abundance of amphipods due to their high mortality after the oil spill, and to their subsequent recovery, as they started to re-colonize the sediment when the environmental conditions improved. An exception to the above pattern was observed at two more sites, where the ratio showed constantly high values during the study period, due to a very strong dominance of opportunistic polychaetes, such as Chaetozone setosa, Spiochaetopteros costarum, Melinna palmata, and Diplocirrus glaucus. So, the spill seemed to have little effect on these two sites, as the community structure is probably driven by some other ecological factors.
The performance of the same methodology in several communities of the eastern Mediterranean (Thermaikos Gulf), gave confounded results (Figure 1). Thermaikos Gulf has been characterized as moderately polluted by several methods (i.e., diversity indices, biotic indices, abundance/biomass comparison, multivariate analyses), with a northward gradient toward organic enrichment. Yet, P/A values were higher to the less-disturbed sites (four stations), as opposed to the organically polluted ones (two stations). This was rather unexpected, since all polychaetes collected from the last sites were typical opportunistic species. However, these two sites were characterized by significantly reduced species richness and abundance, linked to their proximity to a small outfall acting as a point source of various pollutants. Considering amphipods, a total of 14 species have been collected, among which Elasmopus rapax and Ampelisca diadema - a well-documented resistant to pollution and a typical sensitive species, respectively -dominated. In that case we have two facts that violate the theoretical assumptions of P/A index: (1) some samples were very impoverished, creating large problems to the function of most ecological indicators and
S1 W1 S2 W2 S3 W3 S1 W1 S2 W2 S3 W3 Station 1 Station 2
S1 W1 S2 W2 S3 W3 S1 W1 S2 W2 S3 W3 Station 3 Station 4
S1 W1 S2 W2 S3 W3 S1 W1 S2 W2 S3 W3 Station 5 Station 6
Figure 1 The performance of P/A index in assorted soft sediments (Thermaikos Gulf), where S = summer, W = winter, and 1-3 the successive years of the study. Note that station 5 and 6 were the most affected sites.
(2) amphipod fauna is proportionally distributed among sensitive and resistant species. Thus, the basic thought behind P/A (i.e., polycheta = resistant to pollution; amphipoda = sensitive to pollution) seems inappropriate to these results. A probable explanation would consider the differences in the sediment type between these two examples; the sorted fine sands or muddy fine sands in Atlantic, as opposed to the unsorted sediments of Thermaikos with shell fragments of various sizes creating a type of hard substratum which attracts several amphipode species. Therefore, the wide-scale applicability of P/A seems to be restricted, as it is affected by the geography and the habitat heterogeneity that influence the biodiversity of an area.
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