| Summary: | Live (rose Bengal stained) benthic foraminiferal assemblages were examined from four locations in the Northeast Atlantic. Three sites were sampled seasonally as part of the NERC “Biogeochemistry in the Deep Ocean Benthic Boundary” (BENBO) Thematic Programme: Sites A (3600 m water depth) and C (1900 m) were located in the Rockall Trough, and Site B (1100m ) in the Hatton-Rockall Basin. These sites receive decreasing organic carbon inputs with increasing water depth, and are subject to different bottom-current velocities. Wet-sorting sediment residues (>125 μm; 0.0-1.0 cm sediment depth) revealed abundant (264 to 1952 indivduals/27.3 cm 2 ) and diverse (54 to 106 species/27.3 cm 2 ) assemblages. Abundance decreased with water depth, while species diversity increased. All three sites were dominated by soft-bodied and agglutinated species, with the proportion of calcareous individuals decreasing with depth (46.3% at Site B, 44.8% at Site C, and 16.8% at Site A). Site A yielded a lower continental slope assemblage transitional between shallower, bithyal faunas and deeper, abyssal faunas. Sites B and C were dominated by typical bathyal species. Faunas fluctuated seasonally at all three sites. Small individuals of Hoeglundina elegans were more abundant at Site A, and the phytodetritus species Eponides pusillus was more abundant at Sites B and C, in samples collected after phytodetritus inputs compared with those collected before. At a fourth site, the Darwin Mounds (950 m; northern Rockall Trough), diverse (Fisher α: 18.9 to 37.2) foraminiferal assemblages were associated with dead tests of the giant xenophyophore Syringammina fragilissima. These large structures provided distinct microhabitats inside the test branches, attached to the outer surface of the test, and in mud trapped between the test branches. Foraminifera were more abundant within the xenophyophore microhabitats (160 to 664 individuals 10 cm -3 ) than in the surrounding sediments (55 to 110 individuals 10 cm -3 ). The factors controlling the ...
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