The Polychaeta/Amphipoda (P/A) index simply refers to the ratio between the abundance of polychaete species to the one of amphipods. The basic thought behind this approach is the different sensitivity of these two taxo-nomic groups. Polychaetes, as a taxon, are in general resistant to pollutants, since many species are positively correlated with organic enrichment. As a result, after disturbances, several opportunistic polychaete species are favored and increase their abundance. Amphipods, as a taxon, are more sensitive and experience high mortality after severe pollution events. Still there are some species, such as Elasmopus rapax, Jassa falcata, Podocerus variegatus, and Corophium sp., which can proliferate by increased organic load. Several forms of this index have been tested (e.g., the opportunistic species instead of the total poly-chaete fauna, or some genera of polychaetes), since this taxon is the most important in disturbed areas because it contains a large number of both tolerant and sensitive species, whose gradient can reflect the degree of organic pollution. The use of log10 has been suggested for the calibration of the P/A index, with the addition of one monad to the denominator (i.e., log10 (P/A + 1)). So, the index has low values, below 1 (<1), when the benthic communities are not disturbed and exceeds 1 (>1) when they are affected from pollution events. Important attributes of such an index should be its reliability in ecological quality assessment, as well as the easy use and broad applicability to various ecosystems.
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