VERTICAL-DISTRIBUTION OF MACROZOOBENTHOS WITHIN THE SEDIMENT ON THE CONTINENTAL-SLOPE OF THE GOBAN SPUR AREA (NE ATLANTIC)

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 station s profiles of median grain size, % organic carbon and % total nitrogen were measured to a sediment depth of 15 cm. Macrofauna densities decr eased exponentially from similar to 8000 m(-2) at 208 m to similar to 550 m(-2) at the deepest stations (3673 and 4460 m). Most of the macro fauna 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 dep ths (670 to 1425 m), whereas at the deeper stations the fauna was more evenly distributed. Polychaeta were the most abundant taxon, with a h igh proportion of subsurface deposit-feeders at the deep stations and a high proportion of filter-feeders plus surface deposit-feeders at in termediate depth. Biomass values showed a very variable pattern, mainl y due to a few extremely large individuals at some of the stations. Ex cluding these large individuals, a maximum biomass value was observed around 1000 m water depth. At most stations the biomass was concentrat ed below the upper 1 cm, which was also reflected in the vertical dist ribution pattern of the mean individual weight, showing small individu als in the upper 1 cm and larger individuals in the deeper sediment la yers. Both density and biomass fell within the range of values usually observed at these depths. Only the biomass value at 1034 m was extrem ely 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 refract ory material supports the fauna of mainly deep-living subsurface depos it-feeders. Median grain size decreased with increasing water depth fr om similar to 90 pm at 208 m to similar to 8 mu m below 2000 m. The ve rtical profiles showed very homogeneous sediments to a depth of 15 cm at all stations, probably due to bioturbation by the relatively abunda nt 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 similar to 1000 m. The % C and especially N were lowe r in the deeper sediment layers at the deeper stations, resulting in C :N ratios of similar to 12 to 15 in contrast to similar to 7 to 8 in t he upper 1 cm of the sediment. The C:N ratio at the midslope stations (similar to 1000 to 1500 m) did not decrease with depth in the sedimen t, indicating a rapid burial of organic matter by the high numbers of surface deposit-feeders at these stations.