Diversity patterns in macrobenthos across a continental slope in the NE Atlantic

Different estimates were used to assess the diversity of the total macrofau na and its major taxonomic groups separately from a broad bathymetric range at a site in the NE Atlantic. In the Goban Spur region, a transect was sam pled from the shelf to the abyssal plain over a depth range from similar to 200 to similar to 4500 m and in the Porcupine Sea Eight two stations were sampled (at 3670 In and 4115 In). Species diversity (the number of species per number of individuals) increased with increasing water depth. both when expressed as Hurlbzrt's E(S-n) and as Shannon's H' log e. The expected num ber of species in a 100-individual sample E(S-100) of total macrofauna incr eased from 30 on the shelf to 68 on the abyssal plain. Evenness (the propor tional abundance of species), estimated with Shannon's J', also increased w ith water depth from 0.66 to 0.91, whereas dominance (Simpson's D) decrease d from 0.09 to 0.01. Species richness (the number of species per unit of ar ea). however, showed a parabolic pattern with a peak at the upper slope. Th e largest number of species was found at the slope station at 1425 m (232 s pecies within 0.66 m(2)). It is argued that species richness is not a synon ym of species diversity, but that species richness depends both on species density (which decreases with increasing water depth) and on species divers ity. Across the whole bathymetric range (similar to 200 to similar to 4500 m) a total of 696 species within 8327 specimens in a total sampled area of 4.12 m(2) were counted, yielding mean values of similar to 12 individuals p er species and similar to 169 species pet m(2). Different communities were found to exist on the shelf, slope and abyss. It is suggested that this cou ld have been caused by different selection processes. Differences in life-h istory strategies and organic-matter supply could (at least partly) explain the different community structures and diversity patterns found along the depth gradient. (C) 1999 Elsevier Science B.V. All lights reserved.