Site History and Resources
Overview
Gray's Reef National Marine Sanctuary is one of the largest nearshore live-bottom reefs in the southeastern United States, and it is the only marine protected area in federal waters (U.S. Exclusive Economic Zone) in the South Atlantic Bight, an area of continental shelf stretching from Cape Hatteras, N.C., to Cape Canaveral, Fla. Located 17.5 nautical miles offshore of Sapelo Island, Ga., the 16.68-square-nautical-mile sanctuary contains both rocky ledges and sandy flats (Figure 2). Unlike reefs built by corals, Gray's Reef comprises scattered limestone rock outcroppings that stand above the sandy substrate of the nearly flat continental shelf. The reef also supports soft corals, non-reef building hard corals, bivalves and sponges, as well as associated fishes and sea turtles.
Geology
The Gray's Reef sanctuary is a consolidation of marine and terrestrial sediments (sand, shell and mud) that was laid down as loose aggregate between 6 and 2 million years ago. Some of these sediments were likely brought down by coastal rivers draining into the Atlantic and others were delivered by currents from other areas.
These sediments continued to accumulate until a dramatic change began to take place on Earth during the Pleistocene Epoch, between 2 million and 10,000 years ago. During this time, the area that is now Gray's Reef was periodically exposed land and the shoreline was at times as much as 70 nautical miles east of its present location, as sea levels rose and fell at least seven times. As the glacial ice melted for the last time starting 18,000 years ago, the water flowed back into the sea, filling the ocean basins back to their original levels (Figure 3).
Designation
In the 1960s, extensive biological surveys of the ocean floor off the Georgia coast were conducted by Milton "Sam" Gray, a biological collector and curator at the University of Georgia Marine Institute on Sapelo Island, Ga. (Figure 4). In 1961, Gray first recognized this unique, near-shore hard-bottom reef off Sapelo Island. In 1974, the name "Gray's Reef" was proposed for this live-bottom habitat to commemorate Gray's valuable contribution to the understanding of offshore habitats and marine organisms, especially those of the near-shore continental shelf of Georgia. Collections made during the surveys still remain under the protective supervision of the University of Georgia Natural History Museum and maintained as the "Gray's Reef Collection."
In June 1978, the Coastal Resources Division of the Georgia Department of Natural Resources nominated Gray's Reef for consideration as a national marine sanctuary. The designation was approved and signed by President Jimmy Carter on Jan. 16, 1981 and was publicly announced in the Federal Register (46 FR 7942).
Water and Climate
Gray's Reef National Marine Sanctuary is a small but very important part of the broad continental shelf off the southeastern coast of the United States, sometimes known as the South Atlantic Bight (Figure 5). The South Atlantic Bight extends from Cape Hatteras, N.C., to Cape Canaveral, Fla. The outer reaches are dominated by the Gulf Stream flowing northeastward. The inner area is defined by the cuspate curves of the coastline between the two capes and is dominated by tidal currents, river runoff, local winds, seasonal storms, hurricanes and seasonal atmospheric changes. The Gray's Reef sanctuary lies at the break between the inner- and mid-shelf zone of the South Atlantic Bight and is subject to seasonal variations in temperature, salinity and water clarity. It is also influenced by the Gulf Stream, which draws deep, nutrient-rich water to the region, and carries and supports many of the tropical fish species and other animals found seasonally in the sanctuary. Ocean currents and eddies also transport fish and invertebrate eggs and larvae from other areas, linking this special place to reefs north and south (NMSP 2006, Blanton et al. 2003).
Primary productivity at the Gray's Reef sanctuary is likely supported by input of nutrients from freshwater land runoff, as well as deep, nutrient-rich water from upwelling along the western edge of the Gulf Stream. Due to agitation from periodic high seas, re-suspension of organic material in the sediment adds to the productivity of sanctuary waters. Water column and benthic primary production are both important contributors to the overall productivity of the sanctuary, though benthic primary productivity is thought to be an order of magnitude higher than that of the water column. In addition, the Gulf Stream likely supplies planktonic larvae of invertebrates and fishes originating in the Caribbean and Gulf of Mexico (NMSP 2006).
Habitat
Gray's Reef National Marine Sanctuary is underlain by aragonitic limestone. These rocky features vary from flat, smooth surfaces to exposed vertical scarps and ledges with numerous overhangs, crevices and slopes (Riggs et al. 1996). The irregularities of the bathymetry can be attributed to the easily erodable limestone that has dissolved and pitted, creating the appearance of isolated ledges and patches of hard bottom. Exposed surfaces are colonized to varying extents by algae and sessile and burrowing invertebrates, which in turn provide shelter, foraging habitat and nursery areas for a large diversity of fish. Interestingly, percent cover of benthic species, with the exception of gorgonians, is significantly greater on ledges in comparison to the sparse live bottom. In addition, total percent cover and cover of macroalgae, sponges and other organisms is significantly lower on short ledges (<58.5 cm height) in comparison to medium (58.5-89.2 cm) and tall ledges (>89.2 cm) (Figure 6) (Kendall et al. 2007). The series of rock ledges and sand expanses has produced a complex habitat of caves, burrows, troughs and overhangs that provide a solid base upon which temperate and tropical marine flora and fauna attach and grow. This rocky platform, with its rich carpet of attached invertebrate and plant organisms, is known locally as a "live-bottom" habitat (NMSP 2006).
Live-bottom habitats are structurally complex and provide a number of microhabitats. Although the Gray's Reef sanctuary is the most intensely surveyed live-bottom feature in the region, diver-focused survey methods have provided only basic information on the extent and distribution of the live-bottom areas within the sanctuary. Video transects, coupled with side-scan and multi-beam sonar mapping suggest that sand habitats (rippled sand and flat sand) dominate, accounting for 75% of the sanctuary area. Approximately 24% of the sanctuary is sparsely or moderately colonized live bottom, and less than 1% of the sanctuary is considered densely colonized live bottom (Kendall et al. 2005).
Sediments covering the vast areas of sand in the sanctuary are probably re-suspended and redistributed during times of high wave action that accompanies winter and tropical storms. These shifting sands can uncover barely emergent limestone rock areas or, conversely, cover areas that were previously exposed. The effect of storm-suspended sediments has even been observed to scour entire low-relief ledges, removing all but the hardiest of attached marine organisms (Figure 7) (McFall pers. comm.).
Living Resources
The live-bottom habitat of the Gray's Reef sanctuary is of particular biological importance, given the extensive sands that cover most of the broad continental shelf. The sanctuary contains biological assemblages consisting of sessile invertebrates such as sea fans, sea whips, hydroids, anemones, ascidians, sponges, bryozoans and corals living upon and attached to naturally occurring hard or rocky formations with rough, broken or smooth topography, and whose structural complexity favors the aggregation of turtles, fishes and other fauna (Figure 8) (McFall 1998).
Gray's Reef National Marine Sanctuary attracts reef-associated fishes including bottom-dwelling and mid-water fish species such as sea bass, snapper, grouper and mackerel, as well as their prey. An estimated 180 species of fish, encompassing a wide variety of sizes, forms and ecological roles, have been recorded at Gray's Reef. Some fish species are dependent upon the reef for food and shelter, and rarely venture away from it during their life. Many of these fishes are nocturnal seeking refuge within the structure of the reef during the day and emerging at night to feed. Some species of reef-dwelling fish disperse to sandy habitats or to other reef areas north and south or offshore for feeding and spawning. Other reef residents, such as gag and black sea bass, rely on the inshore areas and estuaries in early life stages.
In addition to reef-associated fishes, Gray's Reef serves as habitat for a number of other fish species. King mackerel, Spanish mackerel, great barracuda, Atlantic spadefish and cobia make up the majority of pelagic species that are targeted for recreational angling. The high abundance of schooling baitfishes, such as Spanish sardine and round scad, likely attracts these pelagic predators to sanctuary waters. Approximately 30 species spawn in the vicinity of the sanctuary and only a third of these are reef-associated (Walsh et al. 2006, Sedberry et al. 2006). The large areas of sandy habitat in the sanctuary form another habitat that is not as rich in fish species, and is not targeted by recreational fishermen. These sandy areas support a number of species including flounders, tonguefishes, cusk eels, stargazers, and lizardfishes (Walsh et al. 2006, Gilligan 1989).
Sea turtles known to occur in the South Atlantic Bight include the Kemp's ridley, hawksbill, leatherback, green and loggerhead. Except for the loggerhead, all these species are federally listed as endangered. The loggerhead sea turtle is the most abundant sea turtle in the South Atlantic Bight and is federally listed as threatened (Figure 9). Gray's Reef is an important area for loggerheads to rest and forage throughout the year, especially during the summer nesting season, when females may nest two to four times on area beaches, laying approximately 120 eggs per nest.
Marine mammals on the southeastern United States continental shelf include cetaceans, occasional pinnipeds (harbor seals and sea lions) and sirenians (West Indian manatees). Atlantic spotted dolphins (Figure 10) and bottlenose dolphins (most likely from the Western North Atlantic coastal stock, see Torres et al., 2003), are the most common marine mammals at the Gray's Reef sanctuary. Both species have been designated as depleted under the Marine Mammal Protection Act. There are four species of federally listed endangered whales in the region: northern right, humpback, sperm and fin. Of these, only the highly endangered northern right whale - whose only known calving grounds are off coastal Georgia and northern Florida - has been observed in the vicinity of the sanctuary during the winter.
Pelagic birds, many of which are seasonal migratory species, occur on the middle and outer shelf regions of the South Atlantic Bight, particularly along the western edge of the Gulf Stream. More than 30 species of marine birds occur off the southeastern coast of the United States. Seabirds observed in the sanctuary area include gulls, petrels, shearwaters, Northern Gannet, phalaropes, jaegers and terns (NMSP 2006).
Maritime Archaeological Resources
To date, no downed aircraft or shipwrecks have been documented within Gray's Reef National Marine Sanctuary. However, Gray's Reef is an area of great interest for submerged archaeological and historical resources. Fossil oysters, scallops and snails embedded in the sandstone at the sanctuary indicate that the reef was once a shallow coastal environment (Figure 11). Fragments of mammal bones and a projectile point located in the sanctuary may indicate that the current reef area could have been inhabited by Paleoamericans - ancient peoples of the Americas who were present at the end of the last ice age - when it was above sea level (NMSP 2006).