Abstract

The influence of seamount-intensified flows on the structure of infaunal assemblages was examined at two sand-covered sites located 2.3 km apart atop the summit plain of Fieberling Guyot (32°27.6′N 127° 48.0′W). Both sites experience strong, tidal bottom currents with flows exceeding 20 cm/s on a daily basis (4 mab). Estimates of shear velocity (u*) did not differ significantly between the two sites. However, differences in sediment composition and density produced different sediment transport regimes at the two sites. At Sea Pen Rim (SPR), located on the NW perimeter (635 m), sedimentary particles were composed primarily of basaltic sands that experienced negligible transport during the study period. At White Sand Swale (WSS, 580 m), a narrow valley enclosed on three sides by basalt outcrops, sediments were composed almost entirely of foraminiferal sands that moved daily. Sediment organic content and microbial abundances were similar at the two sites. Infauna (>300 μm) had higher densities at WSS (1870/m2) than SPR (1489/m2), but lower expected species richness. Although the 2 sites shared nearly 50% of identified species, peracarid crustaceans, echinoderms, sponges, and bryozoans were proportionally more important in the stable substrates of SPR, while turbellarians, bivalves, and aplacophorans were better represented in the shifting sands of WSS. The infauna of WSS lived deeper in the sediment column (>50% below 2 cm) than that of SPR (> 50% in the upper 1 cm), at least partly because the majority (83%) at WSS were subsurface burrowers with motile lifestyles. Tube-building and epifaunal lifestyles were more common at SPR than WSS, as were surface-deposit and filter-feeding modes. Fences and weirs were deployed at the study sites for 6.5-wk and 6-mo periods to manipulate bottom stress. Changes in faunal patterns within weirs at WSS reinforced our conjecture that contrasting sediment transport regimes explain between-site differences in community structure. Fence effects varied with deployment period and site. Topographic features on Fieberling Guyot produce heterogeneous sedimentary settings characterized by different transport regimes. Our results suggest that substrate mobility exerts primary control over infaunal community structure at the two high-energy sites.

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