| Summary: | As part of an European Union Marine Science and Technology (MAST) initiative, polychaete assemblages were studied from the Madeiran Abyssal Plain (MAP). Three other sites from the northeast Atlantic were used for comparative purposes. The sites lie under a gradient of surface productivity from a seasonal high productivity site on the Porcupine Abyssal Plain (PAP) to an oligotrophic non-seasonal site on the Cap Verde Abyssal Plain (EUMELI). Two sites of intermediate productivity were on the Tagus Abyssal Plain (TAP) and MAP. Data was analyzed to determine abundance, distribution within sediment, diversity and faunal similarity of MAP with the other three sites. MAP was show to have reduced abundance (approx. 250 ind m -2 )and diversity compared to the other three sites. It is not apparent why MAP should show low polychaete abundance as it lies under a higher surface productivity regime than EUMELI. One possible explanation comes from a biogeochemical analysis which suggests that a turbidity current has passed through in the last 1000 yrs, re-working the sediment. Within sediment distribution of polychaetes at MAP revealed penetration of the 0–5 cm layer, similar to PAP and unlike the superficial distribution recorded form the oligotrophic EUMELI site. Analysis of beta diversity at MAP does not lend support to the idea of a latitudinal diversity gradient, being lower than both northerly and southerly sites. Surface and sub-surface deposit-feeding cirratulids, spionids and paraonids dominate the fauna. A dominant predator, Sigambra sp., was abundant in three of the sites. At the species level, faunal similarity was low, in the region of 15–20% between-site and 30–50% within-site. It appears that although similar in terms of functional groups, the sites are quite separate in terms of species composition, supporting the hypothesis that the abyss is not populated by one uniform species pool. Detailed taxonomic work on the more abundant “cosmopolitan” species is required to verify biogeographical distributions.
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