DENSE INFAUNAL ASSEMBLAGES ON THE CONTINENTAL-SLOPE OFF CAPE-HATTERAS, NORTH-CAROLINA

Unusually dense assemblages of benthic infaunal invertebrates have bee n discovered in continental slope sediments off Cape Hatteras, North C arolina. Densities were highest on the upper slope, ranging from 24,05 5 to 61,244 ((X) over bar = 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) over bar = 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 stat ion at 2000 m sampled six times in 1984-1985 and again in 1992, had de nsities consistently higher than 8500 individuals m(-2). Species richn ess and diversity are consistently lower on the Cape Hatteras slope th an at other locations off North Carolina and elsewhere in the western North Atlantic. The 1992 studies indicated that the upper slope infaun al assemblages (similar to 600 m) were dominated by oligochaetes, whil e the middle slope assemblages (800-1400 m) were dominated by the poly chaete Scalibregma inflatum. This latter depth range could be defined into two assemblages based upon suites of less abundant species. At de pths of 1500-2000 m, a lower slope assemblage dominated by various dep osit feeding polychaetes and oligochaetes was found. Results from the 1984-1985 studies suggest seasonal ol year-to-year patterns in the dom inance of S. inflatum and Cossura longocirrata. Unusually high sedimen tation 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 Hat teras sediments have revealed a mixture of both marine and terrestrial ly 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.