Benthic community structure on the U.S. South Atlantic slope off the Carolinas: Spatial heterogeneity in a current-dominated system

Abstract

Faunal communities of the continental slope and rise seaward of North and South Carolina (U.S.A.) are strongly influenced by the Gulf Stream, the Western Boundary Undercurrent, and an increasingly steep declination of the slope toward Cape Hatteras. Sixteen stations in depths of 600–3500 m were sampled to characterized the sediment and benthic macrofauna. Box cores were taken along four transects: Cape Hatteras, Cape Lookout, Cape Fear and Charleston. On the Cape Hatteras transect infaunal densities at the 600-m station were as high as those typical of shallower waters, and the dominant organisms were species that are more characteristic of continental shelf depths. There is high, nearly continuous sedimentation of terrigenous fine sandy sediments that are funneled over the Cape Hatteras slope by southerly-flowing, long-shelf currents. We postulate that organic material is transported rapidly over the site and the high depositional rates are enhanced by scavenging activities of filter-feeding organisms. Large, deep-burrowing deposit feeders serve to carry organic material deep into the sediments. The shallow stations on the Charleston transect were dominated by sand waves generated by the Gulf Stream, while deeper stations were enriched by macroalgae transported downslope. Transects off Cape Lookout and Cape Fear were more typical of those found elsewhere in the western North Atlantic. Macrofaunal analysis yielded 1202 species, 520 of which were new to science. Annelids were the dominant taxa in terms of density and numbers of species. A partial zoogeographic barrier was identified between Cape Lookout and Cape Fear. Species diversity was high at most stations, with the highest at an 800-m site off Charleston where 436 species were taken from nine ☐ cores (0.81 m2). Diversity data from off the Carolinas support previously stated views that the deep sea is a resource of high species richness and biodiversity. Densities of macrofauna from all depths were generally higher than in previously published accounts. The faunal assemblage at each station was consistent over 2 years in that samples from one station were always more similar to one another than to any other station. Stations grouped into upper slope, middle slope, lower slope and continental rise assemblages; the upper slope stations were the most variable, both in density and community composition. Transect differences in types of sediment and macrofaunal communities along and across isobaths, indicated considerable regional heterogeneity.