The Florida Reef (also known as the Great Florida Reef, Florida reefs, Florida Reef Tract and Florida Keys Reef Tract) is the only living coral barrier reef in the continental United States. It lies a few miles seaward of the Florida Keys, is about 4 miles (6 to 7 km) wide and extends (along the 20 meter depth contour) from Fowey Rocks just east of Soldier Key to just south of the Marquesas Keys. The barrier reef tract forms a great arc, concentric with the Florida Keys, with the northern end, in Biscayne National Park, oriented north-south and the western end, south of the Marquesas Keys, oriented east-west. The rest of the reef outside Biscayne National Park lies within John Pennekamp Coral Reef State Park and the Florida Keys National Marine Sanctuary. Isolated coral patch reefs occur northward from Biscayne National Park as far north as Stuart, in Martin County. Coral reefs are also found in Dry Tortugas National Park west of the Marquesas Keys. There are more than 6,000 individual reefs in the system. The reefs are 5,000 to 7,000 years old, having developed since sea levels rose following the Wisconsinan glaciation.
The densest and most spectacular reefs, along with the highest water clarity, are found to the seaward of Key Largo (in and beyond John Pennekamp Coral Reef State Park) and Elliott Key (the northernmost 'true' Florida Key) where the two long keys help protect the reefs from the effects of water exchange with Florida Bay, Biscayne Bay, Card Sound and Barnes Sound. The bays and sounds (all between the Florida Keys and the mainland) tend to have lower salinity, higher turbidity and wider temperature variations than the water in the open ocean. Channels between the Keys allow brackish water from the bays to flow onto the reefs (especially in the middle Keys), limiting their growth.
Reef structure and communities
The Florida Reef consists of two ridges separated from the Florida Keys by the Hawk Channel. Closest to the Keys is a sand ridge called White Bank, covered by large beds of sea grass, with patch reefs scattered across it. Further out to sea on the edge of the Florida Straits is the second ridge forming the outer reefs, covered by reefs and hard banks composed of coral rubble and sand.
Almost 1,400 species of marine plants and animals, including more than 40 species of stony corals and 500 species of fish, live on the Florida Reef. The Florida Reef lies close to the northern limit for tropical corals, but the species diversity on the reef is comparable to that of reef systems in the Caribbean Sea.
The Florida Museum of Natural History defines three communities on the Florida reefs. The hardbottom community lies closest to the Florida Keys and consists primarily of algae, sea fans (gorgonians) and stony corals growing on limestone rock that has a thin covering of sand. The stony corals in hardbottom communities include smooth starlet coral (Siderastrea radians), mustard hill coral (Porites astreoides), golfball coral (Favia fragum), elliptical star coral (Dichocoenia stokesii) and common brain coral (Diploria strigosa). Hardbottom provides habitat for anemones, mollusks, crabs, spiny lobsters, seastars, sea cucumbers, tunicates and various fish, including grunts (Haemulon spp.), snappers (Lutjanus spp.), groupers (Epinephelus spp.), Atlantic blue tang (Acanthurus coeruleus), Ocean surgeon (Acanthurus bahianus) and Great barracuda (Spyraena barracuda).