COSM

The Continental Shelf

Continental Shelf Habitat
sbezThe Georgia Bight is the reentrant formed by the southeastern coastline from North Carolina to the tip of Florida and overlies the continental shelf above the Blake Plateau. This coastal morphology focuses tidal energy and causes the large tidal range on the Georgia Coast. This continental shelf is home to numerous invertebrates and vertebrates. (From USGS CMG InfoBank Atlas: South Atlantic Bight EEZ regions).

The inner edge of the continental shelf abuts against the Georgia barrier islands and carries an abundance of infaunal, epifaunal, and pelagic organisms (Prezant, Toll, Rollins, and Chapman, (2002). Even in the absence of a significant megaflora, many species are recorded from the inner shelf edge. This fauna is comprised of many mollusks (clams, snails, and cephalopods), arthropods (shrimp, crabs, stomatopods, and chelicerates), sponges, bryozoans, echinoderms (sand dollars, star fish, basket stars, and echinoids), worms and many other groups of invertebrates, as well as vertebrates, including sharks and rays, bony fish, turtles, and marine mammals (dolphins and whales). Many of these organisms live buried in the sandy substrate or skim the boundary between the sand bottom and overlying water. Others are highly mobile and burrow through the sand or swim in the overlying water mass.

Many of these invertebrates are protected by calcium carbonate shells that are washed ashore after the inhabitants die, that are fully capable of being preserved in the fossil record as body fossils. The movements of many of them make traces in or on the sea bottom, that are fully capable of being preserved in the fossil record as trace fossils. The host of burrowing invertebrates living in the sea bottom continually overturns the sediment destroying internal layering and homogenizes it in a process known as bioturbation.

The Charleston Bump

The Blake Plateau lies off the outer edge of the continental shelf from Cape Hatteras to the south tip of Florida. The Florida-Hatteras Slope

Bounds the edge of the shelf where it drops in elevation onto the Blake Plateau at about -500 m of elevation. The base of the escarpment forming the slope directs the path of the Gulf Stream as it flows out of the Florida Straights. The Charleston Bump is a rugged topography lying off Charleston on the Blake Plateau and is swept by strong currents flowing across the hard bottom. The faunas of the Blake Plateau are being investigated by the Estuary to the Abyss Project along a transect from -10 m to the deep waters off the Blake Escarpment.

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The Charleston Bump is a hardground on the inner part of the Blake Plateau that is being actively explored by SERTC and NOAA to characterize its fauna. (Thanks to Elizabeth Wenner at SERTC).

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A bottom profile along the Latitude 31o-30′ Transect begins on St. Catherines Island and heads east, crossing several hard-bottom habitats, including inner- and mid-shelf live-bottom reefs (such as Gray’s Reef), shelf edge reefs, and rugged hard-bottom reefs of the Blake Plateau (such as the Charleston Bump).

Gray’s Reef

Gray’s Reef National Marine Sanctuary was established on January 1981. It is one of the largest near shore live-bottom reefs in the southeastern United States. Located 17.5 nautical miles off St. Catherines Island, GA, it is one of 14 marine protected areas that make up the National Marine Sanctuary System and one of three marine sanctuaries that make up the Southeast Region.

Within the 17-square-nautical-mile Gray’s Reef sanctuary, there are both rocky ledges and sandy flats. The rock ledges, submerged beneath 60 to 70 feet of water, are as tall as 6-8 feet and are highly complex, having nooks and crannies and exposures for invertebrates to attach to and in which fish may hide. These animals form a living carpet of animals that in places completely obscures the substrate, giving the habitat of Gray’s Reef its common name, a “live bottom.”

Sub-Fossil mammal bones and human artifacts of worked bone have been collected from Gray’s Reef and documented. They indicate that about 15,000 years ago, at the end of the last glacial epoch, sea level was lowered about (-22m MSL) and the area at Gray’s Reef was home to terrestrial animals and to humans. The mainland of Georgia extended 96.5 km (60 miles) further east onto the continental shelf, about 60 km (39.4 mi) beyond Gray’s Reef.

Gray’s Reef is home to a moored weather buoy, Station 41008 – GRAYS REEF of the National Data Buoy Center (31o24’00″N, 80o52’30″W). This station is a principal source of real-time weather data for the St. Catherines Sea Turtle program.

location

Location of Gray’s reef approximately 18 nautical miles off Sapelo Island and Geologic Cross-Section from Sapelo Island to Gray’s Reef. (modified after NOAA Gray’s Reef Web Site).

Last updated: 11/30/2015

Sea Turtle Program at St. Catherines Island • (912) 478-1744 • jgaskin@georgiasouthern.edu