Benthic faunal assemblages and carbon supply along the continental shelf/shelf break-slope off Cape Hatteras, North Carolina

Abstract

Abstract Patterns of benthic faunal abundances, biomass, and productivity were examined in the continental shelf-break/upper-slope and mid-slope region of the Ocean Margins Program study area off Cape Hatteras, NC in July 1994, and July and August 1996. Macrofaunal abundances were comparable to or slightly higher than other shelf-slope locales in the North Atlantic. Similar to previous studies in the region, there were no clear depth (75–900 m) or latitudinal (36°20′N–35°25′) trends. Sta. S300 in 300 m had greatest abundances (539,000±38,400 m−2) for individuals >0.3 mm, more than 3 times higher than the average for all stations. Annelids of all sizes dominated numerically, equaling >80% of all macrofauna regardless of size. The majority of infauna were found in the upper 5 cm, but direct visual observations and geochemical evidence from other studies imply a deep-burrowing benthos. Meiofauna (excluding benthic foraminifera) were twice as abundant at shelf-break/upper-slope stations than mid-slope stations, while foraminifera were more abundant at deeper stations. Meiofaunal-sized polychaetes and nematodes were found to at least 7–8 cm below the sediment surface. Bacterial inventories at shelf-break/upper-slope depths were high relative to other shelf regions, but declined precipitously deeper than 500 m. Relative biomass patterns were similar for all stations, highest for macrobenthos and lowest for bacteria. Although densities were high, the contribution of nematodes to benthic biomass was 0.3 mm) at station S300, while metazoan meiofauna contributed from 0.6 g C m−2 at station N-274 to 11 g C m−2 at M76, averaging 2.2±2.4 g C m−2. Bacterial biomass over the upper 10 cm was ∼4 times higher at shelf-break/upper-slope stations than mid-slope stations, averaging 1.05±1.14 g C m−2 and ranging from 5 g C m−2 at M76 to 0.12 g C m−2 at MLB-679Rb. Benthic production estimates track biomass patterns and are estimated at 188 g C m−2 yr−1 for shelf-break/upper-slope and 87 g C m−2 yr−1 for mid-slope stations, respectively. Although bacterial biomass was considerably less than macrobenthos, production estimates for bacteria were almost 4 times higher on average at shelf-break/upper-slope stations than for macrofauna (147 compared with 40 g C m−2 yr−1). Sustained inputs of labile planktonic debris likely contribute to the abundant, deep-burrowing, and extremely active benthic community off Cape Hatteras, and are probably higher than presently estimated.