The deep-sea macrobenthos on the continental margin of the northwest Atlantic Ocean

Abstract DSRV Alvin was used to take 62 small box core samples between 203 and 3659 m deep on the continental margin off the northeastern United States. Abundance and wet-weight biomass of total macrobenthic communities decreased sharply with depth. Standing stocks were higher in the Hudson canyon than in adjacent areas of similar depths, but only in the canyon head at upper continental slope depths, where the canyon is geologically ‘inactive’ and is filling with organic-rich, fine-grained sediments. At lower continental slope and upper continental rise depths no such differences could be discerned. Diversity increased directly and gradually with depth and varied inversely with sample size (= animal abundance), but it was not related to the canyon. Assemblages defined using a percent similarity index were ordered across the depth gradient, with a peculiar canyon fauna only in the organic-rich canyon sediments on the upper continental slope. The greatest faunal discontinuity (= least similarity between samples) was encountered between the upper and the lower continental slope and not at the continental shelf break (about 300 m). Percent sand was high in the sediments deeper in the canyon on the upper continental rise, but the fauna had a species composition, community structure, and biomass that could not be considered different from the continental rise outside of the canyon, where the sediments were silt and clay. The canyon on the upper continental slope traps organic-rich sediments. Although the low concentrations of organic matter on the continental rise in the canyon would imply that the organic matter from shallow water does not reach this depth, pore water concentrations of ammonia and nitrate were consistently higher in canyon sediments than outside the canyon on the rise, indicating metabolism (biologic activity) in these sediments is greater in the canyon, presumably a result of labile organic matter being trapped and funneled by the canyon across the continental rise.