Spatial Heterogeneity of benthos on the Carolina continental slope: large (100 km)-scale variation

Abstract

Large-scale spatial heterogeneity of macrofaunal and microbial commun~ties was examined on the continental slope off North and South Carolina, USA, by comparing 3 sites, separated by 130 to 150 km and all at 850 m water depth. Significant variation was found among macrofaunal assen~blages at all 3 sites, and between microbial counts at 2 sites. We investigated the hypothesis that 100 km scale heterogeneity was driven by variation in organic C flux to the sea floor. The northernmost site (Site 111, off Cape Hatteras, NC) was found to have macrofaunal abundances (> 55 000 m-2) higher than any previously recorded from this depth, and significantly hlgher than those at Site I1 (off Cape Lookout, NC) (21 319 m-2) or Site I (off Charleston, SC) (9438 m-2). Trends in macrofaunal abundance did not follow those of sediment TOC (total organic carbon), but agreed well with estimates of total carbon flux for the 3 sites. Mixing coefficients determined from profiles of naturally occurring 234Th (half life 24 d ) indicate that the sediments at Site I11 are mixed 2 to 6 times faster than at the other 2 sites, which is consistent with the trends in macrofaunal abundance and biomass. Using ',C-based sedimentation rates and sediment carbon content, we estimated carbon flux to be 0.6. 20 and >?0 g C m-2 yr-l at Sites I, I1 and 111, respectively. Inventories of 2 3 4 ~h and downcore concentration profiles of dissolved SO,,' , ZC02 and CH, within the sediment provided evidence that the flux of metabolizable carbon was greater at Site I11 than at the other sites. Polychaetes, which comprised 43, 74 and 65 % of the fauna at Sites I, I1 and Ill, respectively, exhibited lower diversity, higher dominance, and a completely different species composition at Site I11 than at the other 2 sites. Scalibregma inflatum and Aricidea quadrilobata comprised 33 % of total macrofauna at Site 111, but were absent at Sites I and 11. The species composition, high dominance, and prevalence of juveniles among polychaetes at Site I11 is suggestive of a response to organic enrichment. Enrichment of the Site I11 benthos is attributed to physical oceanographic and geophysical causes, including Gulf Stream-induced upwelling, a confluence of currents focused by bottom topography, and lateral inputs resulting from mass wasting processes. Despite significant differences in macrofaunal abundance, Sites I and I1 exhibited considerable overlap in microbial counts, polychaete species composition, dominance and diversity patterns.