VIABLE DIATOMS AND CHLOROPHYLL-A IN CONTINENTAL-SLOPE SEDIMENTS OFF CAPE-HATTERAS, NORTH-CAROLINA

Continental slope sediments off Cape Hatteras, North Carolina, were sampled by box coring in late summer, 1992. The chlorophyll a concentrations measured in sediments from 16 sites at depths ranging from 530 to 2003 m averaged 19.9 mg chi a m(-2), a concentration much higher than observed elsewhere on the eastern U.S. continental slope, indicating high depositional rates for microalgal material. The variability in the chlorophyll a values suggests strong environmental heterogeneity at both small and large spatial scales in this slope habitat, probably a consequence of both topography and bioturbation. Viable diatoms were found in sediment samples across the range of depths sampled, and up to 14 cm deep in sediments, indicating high rates of deposition and bioturbation. Bulk sediment samples contained planktonic, tychopelagic and benthic diatoms, indicating that both phytoplankton and benthic microalgae from the continental shelf may be sources of organic matter for these slope sediments.