Abstract
Abstract In three sedimentary regions in the deep (>3400 m) Venezuela Basin, nematode abundance and dry weight biomass (ean ±1 S.E.) were higher in hemipelagic sediments (94 ± 1.5 individuals, 88 ± 2.5 μg per 10 cm−2) than in pelagic (67 ± 3 individuals, 32 ±33 μg per 10 cm−2) or turbidite (36 ± 3 individuals, 30 ± 3 μg per 10 cm−2) sediments. Abundance of nematodes appears to be at least partially related to geographic position within the basin; hemipelagic sediments lie closer to an area of elevated surface production near the Lesser Antilles than do the pelagic or turbidite sediments. Abundance of nematodes is also directly correlated with macrofauna abundance and presence of sedimentary lipids. Normal hierarchical classification indicates the presence of two faunal groups in the sediments: a ‘sand’ fauna in coarser pelagic sediments (median grain size, 65 μm) and a ‘silt-clay’ fauna in the finer hemipelagic and turbidite sediments (median grain size in both, 0.65 μm). Animals among the two faunal groups differ mostly in their feeding morphology: selective deposit feeding species, unable to utilize large, pelagically derived particles (mainly foraminiferan tests) are more abundant in finer hemipelagic and turbidite sediments, whereas species capable of rasping food particles off large sedimentary particles (epistrate feeders) are more abundant in pelagic sands than in finer sediments. Species diversity is higher in the hemipelagic than turbidite and pelagic sediments. Two reasons postulated for this are (1) the possible input of relatively fresh, surface-derived organic matter that might permit a large number of species to exist in hemipelagic sediments than in the other two, and (2) an optimally heterogeneous grain size distribution that might allow the species to coexist equitably. All three sediments support more diverse nematode assemblages than those in the North Carolina slope region, probably reflecting the greater physical stability of the abyssal than bathyal sea floor.
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