Dense infaunal assemblages on the continental slope off Cape Hatteras, North Carolina

Unusually dense assemblages of benthic infaunal invertebrates have been discovered in continental slope sediments off Cape Hatteras, North Carolina. Densities were highest on the upper slope, ranging from 24,055 to 61,244 (X¯=46,255) individuals m⁻² in nine samples taken at a 600-m site in 1984 and 1985, and from 15,522 to 89,566 (X¯=37,282) individuals m⁻² in single samples at 15 stations over a wider depth range of 530 to 1535 m in 1992. A lower slope station at 2000 m sampled six times in 1984–1985 and again in 1992, had densities consistently higher than 8500 individuals m⁻². Species richness and diversity are consistently lower on the Cape Hatteras slope than at other locations off North Carolina and elsewhere in the western North Atlantic. The 1992 studies indicated that the upper slope infaunal assemblages (600m) were dominated by oligochaetes, while the middle slope assemblages (800–1400 m) were dominated by the polychaeteScalibregma inflatum. This latter depth range could be defined into two assemblages based upon suites of less abundant species. At depths of 1500–2000 m, a lower slope assemblage dominated by various deposit feeding polychaetes and oligochaetes was found. Results from the 1984–1985 studies suggest seasonal or year-to-year patterns in the dominance ofS. inflatum andCossura longocirrata. Unusually high sedimentation rates and organic carbon flux have been recorded from the slope off Cape Hatteras and may account for the high infaunal productivity in the area. Most of the dominant infaunal organisms are species more typical of shallow, coastal habitats rather than deep-sea species that dominate other areas of the U.S. Atlantic continental slope. Parallel investigations regarding the nature of organic matter in the Cape Hatteras sediments have revealed a mixture of both marine and terrestrially derived carbon, only a small percentage of which is composed of the smaller molecular weight polyunsaturated fatty acids more typical of continental slope sediments. It is likely that the high percentage of refractory organic matter would favor the survival of preadapted shelf species over those from adjacent slope environments.