EVIDENCE THAT SEDIMENT TYPE INFLUENCES THE HORIZONTAL AND VERTICAL-DISTRIBUTION OF NEMATODES AT A DEEP-SEA SITE

The relationship between sediment type and the nematode fauna was studied at the HEBBLE site (40.degree.27''N, 627%20''W), which is at 4820 m depth on the Scotian Rise in the northest Atlantic. Because of the hydrodynamic regime, the fine-grained, surficial sediments (brown mud) are eroded and redeposited several times per year, so the properties of brown mud can be expected to be relatively homogeneous, regardless of depth of burial. Brown mud accumulates in topographic lows, so although 0-1 cm layer in our samples is brown mud, the 1-2 and 2-3 cm layers can be either brown mud or foraminiferan mud. Where both sediment types occur in a single horizontal plane (i.e. the 1-2 cm layer and the 2-3 cm layer), nematodes are more abundant in the brown mud than in the foraminiferan mud, and this pattern occurs across feeding and tail-morphology functional groups. In those samples in which brown mud occurs from 0 to 3 cm, nematodes do not differ significantly in abundance among the three depth layers, suggesting that the usual decrease in abundance with depth in the sediment is not simply a function of proximity to the sediment surface, but relates to a change in the quality of the sediment with depth. Further, in brown mud, nematodes tend to be more abundant in the 1-2 cm layer than in the 0-1 cm layer. At the HEBBLE site, there is reason to suspect that small organisms are at risk on the sediment surface. The unexpected nematode-abundance profile may occur for the same reason.