The Great South Channel (GSC) is a bathymetric feature between the Nantucket Shoals and Georges Bank that connects the southern boundary of the Gulf of Maine with the northwest Atlantic Ocean. The mid-channel is characterized by the presence of sporadic, large, and isolated bedforms on a generally sediment-starved seafloor. Exhibiting distinctive east–west oriented straight crests, these immobile bedforms are poorly understood. In an area of active sediment transport, we have found that these bedforms have not migrated since 1998. As a result of their immobility they have ecological significance at the microhabitat scale. They increase seafloor stability on their lee side, by shielding the seafloor from erosive bottom current, thus providing a refuge for species that otherwise would not occur. Moreover, for an large straight, isolated (LSI) bedform of approximatively 16 m in height, the area shielded from erosive currents extends up to 90 m downcurrent from the bedforms.

By combining a quantitative geomorphometric characterization of high-resolution bathymetry and the concurrent analysis of coregistered seafloor images, we describe these features in detail. The fauna in the area surrounding the bedforms is found to be organized in a consistent, microscale patchiness and it is illustrated in a bedform schematic. On the whole, the immobile LSI bedforms serve as collectors of sand, which builds up on the north-facing (stoss) side forming ripples on the lower parts of the slope. Here is where the highest concentration of winter flounder can be found. On the south-facing (lee) side, the seabed is sheltered from flowing sand and the exposed, coarse gravel lag deposit is richly colonized by sessile fauna. Furthermore, the sediment in this area is very heterogeneous. Toward the center, the sediment is characterized by the presence of cobble and pebble in a matrix of gravel, as well as absence of sand. Moving away from this core area toward either side, bigger fragments can be found up to boulder size, which are exhumed by the current. In this part some sand can overcome the barrier created by the bedform and a thin layer of finer sediment is found atop and between the boulders. Besides the sessile fauna, monkfish and other mobile fauna can be found.