Variation in megafaunal assemblages on the continental margin south of New England

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Abstract

Megafaunal assemblages at four locations on the slope south of New England were surveyed using a towed camera sled, with photographs taken at automatic 15 s intervals. A total of 284,692 organisms were seen in 94,380 m2 of sea floor viewed. Classification and ordination analysis defined four megafaunal zones that corresponded to different regions of the slope: an upper slope zone, an upper-middle slope zone, a transitional zone on the lower-middle slope, and a lower slope zone. Each of these zones could be further subdivided into shallow and deep portions of higher species similarity. Faunal densities were high in the upper and lower slope zones and low in the two middle slope zones. The boundaries between zones were frequently marked by abrupt shifts in faunal density and trophic structure. High species overlap between adjacent zones suggested a pattern of continual species turnover along the depth gradient, with zones representing regions of lesser faunal changes separated by regions of greater faunal change. Species replacement with depth was very abrupt in steep areas and gradual in flat areas. The pattern of species turnover and the depths of faunal boundaries varied among geographic locations, reflecting differences in local topography and geology. Patterns of density, and species composition, with depth were very similar at three of the locations but differed significantly at the eastern edge of Georges Bank, a site characterized by very steep topography and numerous glacial erratics and outcrops. Faunal assemblages in zones were frequently dominated by one or two very abundant species, and the boundaries between zones usually reflected the depth limits of these species. At each location, depths of major faunal boundaries coincided with relative changes in slope declivity rather than absolute angles of inclination.

Environmental heterogeneity provided by different substrata may further enhance species turnover along the depth gradient. Variable relief also may allow extension of depth ranges by providing refuge from predation and/or altering food resources. Factors controlling the distribution of megafaunal assemblages on sediment-covered slopes may be related to the effect of local topography on current intensities and related differences in food availability.

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