Introduction
Gray's Reef National Marine Sanctuary off the
coast of Georgia is one of the largest near-shore
"live-bottom" reefs of the southeastern United
States. It is just one of 15 marine protected areas that make up the
National Marine Sanctuary System and is governed by the
National Marine Sanctuaries Act.
Gray's Reef National Marine Sanctuary is
currently the only protected natural reef area on the continental
shelf off the Georgia coast and one of only a few natural marine
protected areas in the ocean between Cape Hatteras, NC and
Cape Canaveral, FL. The approximately 22 square mile sanctuary
(about 14,000 acres) of Gray's Reef is just a
small part of the U.S. territorial Atlantic Ocean, yet its value as
a natural marine habitat is recognized
nationally and internationally.
"Live bottom" is a term used to refer to hard or rocky seafloor
that typically supports high numbers of large invertebrates such as sponges, corals
and sea squirts. These spineless creatures thrive in rocky areas, as many are able
to attach themselves more firmly to the hard substrate, as compared to sandy or
muddy "soft" bottom habitats. Within the Gray's Reef National Marine Sanctuary there
are rocky ledges with sponge and coral live bottom communities, as well as sandy
bottom areas that are more typical of the seafloor off the southeastern U.S. coast.
A Different Kind of Reef
Gray's Reef is not considered a coral reef such as those found
in the tropics, as its foundation was not built by living hard corals. Instead, it
was formed by the cementing and consolidation of marine and terrestrial sediments
(shell fragments, sand and mud) which were originally deposited as a blanket of
loose grains between six and two million years ago. Some of these sediments were
brought to the coast by rivers and others were probably transported to the region by
ocean currents. The "cement" that glued the grains together more than 2 million
years ago was briny calcium-carbonate seawater. The resulting rock that is the foundation of Gray's Reef is
a carbonate-cemented sandstone.
During the Pleistocene Epoch (~2 million to 10,000 years ago,
and after Gray's Reef had formed) the climate oscillated between glacial ice ages
and interglacial warmer periods. As a result, sea level fluctuated dramatically as
glacial ice expanded (i.e., sea level fell) and melted (i.e., sea level rose). At
one time, Gray's Reef was dry land! The sanctuary was periodically exposed above sea
level when the shoreline was as much as 80 miles east of today's location. The
fossils of large Pleistocene terrestrial animals such as mammoths have been found at
the reef, indicating that either these beasts lived in the area when sea level was
low, or their remains were washed down old river channels and deposited at what is
now the sanctuary. To learn more about gearcharological and paleontological research
at Gray's and J-Y Reefs conducted by Dr. Ervan G. Garrison, a professor in the
Geology and Anthropology Departments of the University of Georgia (UGA) click, here.
An ever growing number of scientists have been working on the
36,000 year old gray whale mandible (jawbone) found by the University of Georgia's
Dr. Scott Noakes at JY Reef near Gray's Reef. Prior to this discovery, no known
evidence of gray whales had ever been documented offshore Georgia. After two years
and numerous dives, the mandible was recovered from the seafloor in sections
totaling approximately 1.5 m in length. The mandible was sent to Emory University's
Carlos Museum in Atlanta, Georgia for reassembly and conservation. During this time,
several Emory and UGA student interns gained valuable hands on experience in
conservation practices. Due to the importance of the find and the desire to display
and generate interest in the Atlantic Ocean's whales, the Smithsonian National Museum of
Natural History (SNMNH) offered to make multiple quality casts of the mandible.
The casts will be displayed on the UGA campus; UGA Marine Extension on Skidaway
Island, Savannah, GA; and the Georgia Aquarium in Atlanta. The SNMNH will retain one
cast for their research efforts on the now extinct Atlantic gray whales. The
original mandible is currently being used as a model for students in UGA's
Scientific Illustration department in the Lamar Dodd School of Art to prepare the
casts for display. To learn more about the discovery, excavation and conservation of
the Atlantic gray whale mandible, click here.
Habitat of the Reef
Today, the rocky ledges can be as tall as six feet but lie under
60 to 70 feet of ocean water. The rocky ledges are complex - they have nooks and
crannies, and caves and bumps that provide plenty of places for invertebrates (animals without backbones) to latch
on to or hide in. Those invertebrates provide food for many fishes that also shelter
in the cracks and crevices, or that hover above the reef. The rocky places provide a
firm base for a variety of invertebrates that live their lives permanently attached
to the rock. These animals include bryozoans (moss animals), ascidians or tunicates
(sea squirts), sponges, barnacles, and hard-tubed worms. These animals form a dense
carpet of living creatures that in places completely hides the rock. That gives the
habitat of Gray's Reef its name - "live bottom".
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