Patterns in polychaete abundance and diversity from the Madeira Abyssal Plain, northeast Atlantic

Polychaete abundance and diversity patterns from the Madeira Abyssal Plain (MAP) were studied together with data from three other sites on the northea stern Atlantic abyssal plains. Polychaete abundance at MAP was significantl y lower than at any of the other sites, including those lying under compara ble productivity regimes, Analysis of diversity, using rarefaction and spec ies counts per unit area, suggests that MAP is extremely species poor and s hows dominance by a few common species. The MAP site is characterised by a superficial layer of turbidite sediment, and the hypothesis is put forward that the unusual sediment characteristics at MAP have affected macrofaunal abundance over a vast area ( > 2000 km(2)). Analysis of species composition indicates that the MAP site is not faunistically unique; rather it contain s a high proportion of widespread, abundant, cosmopolitan species. We sugge st that these are the opportunists of the abyssal benthic habitat. Differen ces in abundance between the other abyssal sites are the result of both pro ductivity and local environmental conditions. Equitability at the other nor th Atlantic sites is not affected by productivity, although the actual numb er of species per unit area is affected, showing a south-north gradient, (C ) 2000 Elsevier Science Ltd. All rights reserved.