Polychaete diversity at tropical Atlantic deep-sea sites: environmental effects

This study assesses how differences in nutrient flux and bottom currents affect the diversity and trophic structure of deep-sea polychaete communities from the tropical northeast Atlantic at 20 degrees to 21 degrees N latitude. Faunal assemblages were studied from 3 sets of USNEL box cores (0.25 m(2)) taken at depths of 1700, 3100 and 4600 m. In terms of primary productivity, the cores were taken beneath water which is eutrophic, mesotrophic and oligotrophic, respectively. Total polychaete abundance, abundance of dominant species, and faunal similarity and diversity were compared among the 3 sites. Polychaete abundance decreased with increasing depth. Community structure reflected environmental characteristics as well as the influence of coastal upwelling. Polychaete diversity showed a parabolic distribution, with depth peaking at about 2000 m. Species richness observed at the eutrophic site was generally higher than observed at other deep-sea regions of comparable depth at temperate latitudes. This points to regional variation in processes contributing to diversity patterns of deep fauna. We suggest that the diversity pattern observed results from non-equilibrial interactions between production and disturbance in the form of current energy and bioturbation. Diversity patterns observed in the EUMELI sites seem best explained by the intermediate productivity theory. The rate of production encountered at the eutrophic site may overcome much of the impact of physical and biological disturbances, favouring a diverse species assemblage.