Vertical distribution of macrozoobenthos within the sediment on the continental slope of the Goban Spur area (NE Atlantic)

Abstract

Macrofauna density and biomass were estimated in 4 sediment layers (0-1, 1-5, 5-10 and 10-15 cm) from 12 stations ranging from 185 to 4460 m water depth in the Goban Spur area (NE Atlantic). At the same stations profiles of median grain size, % organic carbon and % total nitrogen were measured to a sediment depth of 15 cm. Macrofauna densities decreased exponentially from ~8000 m-2 at 208 m to ~550 m-2 at the deepest stations (3673 and 4460 m). Most of the macrofauna was concentrated in the upper 1 cm of the sediment. The highest proportion (70 to 80%) was in the upper 1 cm found at intermediate depths (670 to 1425 m), whereas at the deeper stations the fauna was more evenly distributed. Polychaeta were the most abundant taxon, with a high proportion of subsurface deposit-feeders at the deep stations and a high proportion of filter-feeders plus surface deposit-feeders at intermediate depth. Biomass values showed a very variable pattern, mainly due to a few extremely large individuals at some of the stations. Excluding these large individuals, a maximum biomass value was observed around 1000 m water depth. At most stations the biomass was concentrated below the upper 1 cm, which was also reflected in the vertical distribution pattern of the mean individual weight, showing small individuals in the upper 1 cm and larger individuals in the deeper sediment layers. Both density and biomass fell within the range of values usually observed at these depths. Only the biomass value at 1034 m was extremely high and concentrated in the upper 1 cm of the sediment. This may be due to pulsed inputs of fresh material at this station, whereas at the deepest stations (3673 and 4460 m) a constant low input of refractory material supports the fauna of mainly deep-living subsurface deposit-feeders. Median grain size decreased with increasing water depth from ~90 μm at 208 m to ~8 μm below 2000 m. The vertical profiles showed very homogeneous sediments to a depth of 15 cm at all stations, probably due to bioturbation by the relatively abundant deposit-feeders. In general, the % of total N and organic C in the upper 1 cm of the sediment increased with increasing water depth, but showed a peak at ~1000 m. The % C and especially N were lower in the deeper sediment layers at the deeper stations, resulting in C:N ratios of ~12 to 15 in contrast to ~7 to 8 in the upper 1 cm of the sediment. The C:N ratio at the midslope stations (~1000 to 1500 m) did not decrease with depth in the sediment, indicating a rapid burial of organic matter by the high numbers of surface deposit-feeders at these stations.