Benthic metazoan biomass, community structure and bioturbation at three contrasting deep-water sites on the northwest European continental margin

Size structure, taxonomic composition and bioturbation potential of benthic metazoan communities were examined at three contrasting sites on the northwest European continental margin as part of a wider study of biogeochemical cycling in the deep-sea benthic boundary layer. Sampling was conducted in the Hatton–Rockall Basin (1100 m depth), on the North Feni Ridge (1920 m) and in the South Rockall Trough (3580 m) on two cruises bracketing the expected 1998 spring phytodetrital flux. Density and biomass of epibenthic megafauna declined with depth and were consistent with previous data from the deep northeast Atlantic. Aggregations of the hexactinellid sponge Pheronema carpenteri occurred at 1100 m. The percentage contribution of suspension-feeders to megafaunal biomass at the lower slope sites was much higher than expected for these depths. Macrofauna (sensu stricto) were consistently undersampled by a box corer relative to an hydraulically damped multiple corer. Estimated macrofaunal densities declined with depth. Polychaete trophic group structure was very different at the three sites, with more small carnivores at 1100 m, surface deposit and interface-feeders at 1920 m, and a higher representation of subsurface deposit-feeders at 3580 m. Meiofaunal densities were low in comparison with published data from comparable depths in the northeast Atlantic. Meiofaunal numbers increased at all three sites from May to July 1998. The increase at 1920 m was statistically significant and was associated with the greatest phytodetrital accumulation observed during the study period. The relative contribution of the mega-, macro- and meiofauna to benthic metazoan biomass did not vary consistently with depth, and no trend was apparent in a review of data from other studies in the North Atlantic. Depth-related patterns in community size structure recorded from previous North Atlantic studies were not all replicated, suggesting the importance of local hydrodynamics and other environmental factors. There was no obvious link between faunal parameters and biodiffusion coefficients or mixing modes estimated from radiotracer profiles. Diffusion-analogue 210Pbexcess profiles recorded at 3580 and 1100 m resulted from a combination of macrofaunal and megafaunal sediment mixing, some of it advective. At 1920 m, a combination of deep megafaunal subduction and near-surface concentration of macrofaunal feeding activity produced subsurface radiotracer peaks. The poorly known burrowing megafauna probably play a significant role in bioturbation at all three sites.