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III.B.2. Benthos

The benthic habitat the mid-Atlantic region is characterized by a medium-grained sand inshore grading to finer sediments at the shelf break. The shelf demonstrates a ridge-and-swale (hill-and-valley) topography on the inner and part of the outer shelf, with the ridges having coarser surficial sediments than the swales. At the shelf break, the topography is modified by the major submarine canyons which incise the shelf to varying distances shoreward and form complex benthic habitats. The continental slope sediments are made up of clays on the upper slope and change to fine silts on the continental rise. The slope area between the major canyons can show diversified types of topography from smooth slope to rugged configuration in areas of minor canyons caused by mass wasting (see section III.A.1 for more specific information).

The fauna in the Middle Atlantic Bight region is dominated by four groups: Arthropoda; Annelida; Mollusca; and Echinodermata. These four groups are dominant in terms of numerical abundance as well as biomass, however the relative ranking changes depending on which measure is used (Wigley and Theroux, 1981). Table III.B.2-1 shows the dominance of these four taxa.

Table III.B.2-1. Number and Biomass of Four Common Taxa in the Middle
Atlantic Bight

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Boesch (1979) reported on macroscale and mesoscale patterns of benthos distribution in the mid-Atlantic region. The author concluded that no latitudinal (north-south) variation of across-shelf macrobenthos distribution was evident within the sampled area. A total of 697 species and an additional 84 unidentified taxa were collected during the 2-year study. This is in direct contrast to Wigley and Theroux (1981) who reported only 435 species collected during 3 years of sampling over a much greater area--the entire Middle Atlantic Bight, including bays and sounds and deeper water--and including many more samples--667 versus 159 in Boesch's (1979) study. Although comparison of these two studies is tentative, it appears that the differences can be attributed, at least in part, to different sieve mesh sizes--Boesch (1979) used 0.5 mm and Wigley and Theroux (1981) used 1.0 mm--and the fact that Wigley and Theroux (1981) grouped many congeners. Further contrast

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between the two studies reveals differences in numerical abundances and relative ranking of dominant groups. Boesch (1979) reports that annelids (primarily polychaetes) are the numeric dominants over the shelf whereas Wigley and Theroux (1981) rank annelids third, behind arthropods and molluscs in the Middle Atlantic Bight. Boesch's (1979) density values generally range from 2,000 to 10,000 individuals per m2 and are typically greater than two times higher than Wigley and Theroux's (1981) values which had a mean of 1,400 individuals per m2. This is predominantly because of the finer sieve mesh size used by Boesch. Biomass estimates were equivalent between the two studies, another indication that Boesch's (1979) numerical data were composed of smaller individuals. Boesch (1979) also stated that the hypothesis of Wigley and Theroux (1976) that density of macrofaunal organisms decreased with increasing water depth was not correct and proposed a five-zone faunal pattern: inner shelf, 0-30 m; middle-shelf, 30-50 m; outer shelf, 50-100 m; shelf break, 100-200 m; and slope, >200 m--with mesoscale density trends correlating with shelf topography--ridges and swales. The author reported that the inner and mid-shelf zones supported lower densities of organisms than the outer shelf and shelf break. The swale habitats also tended to have greater densities than the ridges; swale habitats had densities about 2 to 3 times higher than adjacent habitats. Densities on the slope quickly decreased with depth. The upper slope had a median density of 2,000 individuals per m2 and the middle slope decreased to a median density of 390 individuals per m2. Figure III.B.2-1 shows the

faunal zones and their reported median densities (from Boesch, 1979).

The mean densities of higher taxonomic groups, as reported by Wigley and Theroux (1981), are listed in Table III.B.2-2 for the entire Middle Atlantic Bight region and bathymetric division.

The middle Atlantic and north Atlantic continental slope physiography is dominated by numerous submarine canyons which cut into the slope in varying degrees. It is hypothesized that submarine canyons were formed in at least two ways: 1) they were the result of riverine erosional processes during glacial periods when sea level was much lower, and 2) they were formed by mass wasting (slumping) of shelf edge sediment that built up over time until the gravitational forces caused it to collapse, much in the manner of an avalanche. Recent studies have proposed that the canyons are still eroding into the shelf. One method of erosion proposed is that internal waves are being concentrated by funneling them up the submarine canyons to the canyonhead, where they are concentrated and result in an area of increased energy which slowly erodes the shelf. The second method proposed is bioturbation, or the disturbing of the environment by organisms. In this case,

the organisms at the head of the canyons have been shown to create extensive burrow systems into the more stable clay substrate. It is postulated that the burrow systems eventually collapse and the sediments form turbidity currents which flow to the main axis of the canyon. The major canyons along the Middle Atlantic Bight incise deeply into the shelf (up to 150 km shoreward from the shelf edge) while the minor canyons are located on the upper and middle slope, or barely reach the shelf edge (see section IV.A.2 for a complete discussion of submarine canyons).

Although, in general, the biomass and numbers of organisms decrease in a seaward direction from the outer shelf to the slope and beyond, submarine canyons have been shown to have increased biomass and numbers of organisms. The submarine canyons offer a unique environment for the concentration of fauna. The primary

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FIGURE III.B.2-1

Distribution of total density of macrobenthos by major habitat. Horizontal lines represent ranges, bars represent the mid-ranges, and vertical lines represent medians.

SOURCE: Boesch, 1979.

TABLE III.B.2-2.

Mean number of individuals listed by major taxonomic
groups for each bathymetric class, representing the
entire Middle Atlantic Bight region.

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