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 statlons profiles of median grain slze, % organic carbon and ' K total n~trogen were measured to a sediment depth of 15 cm. Macrofauna densltles decreased exponentially from -8000 m' at 208 m to -550 m ' at the deepest stations (3673 and 4460 m). Most of the macrofauna was concentrated In the upper 1 cm of the sediment. The hlghest proportion (70 to 8OUi1) was in the upper 1 cm found at lntermedlate 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-fccders at the deep statlons and a high proportion of f~lter-feeders plus surface deposit-feeders at ~ntermediate 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 depositfeeders. Median gram size decreased with increasing water depth from -90 pm at 208 m to -8 pm below 2000 m. The vert~cal profiles showed very homogeneous sedlments to a depth of 15 cm at all stations, probably due to bioturbation by the relat~vely abundant depos~t-feeders. In general, the % of total N and organlc C in the upper 1 cm of the sediment Increased with Increasing water depth, but showed a peak at -1000 m The C% C and especially N were Ioxver In the deeper sediment layers at the deeper stations, resulting In C -N ratlos 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) d ~d not decrease with depth in the sediment, ~n d icating a rapid burial of organic matter by the hlgh numbers of surface deposit-feeders at these stations.