Extending from the northern end of Biscayne National Park to the Tortugas, the Sanctuary encompasses 2,800 square nautical miles of water surrounding the Florida Keys. It is the second largest site in the National Marine Sanctuary System. Comprised of colonies of many tiny animals called polyps, corals make up the framework of the reef ecosystem. They provide food and shelter to many fish, invertebrates and plants that inhabit the reef. Off the coast of the Florida Keys is the third largest barrier reef in the world.
Corals exist only in a very delicate balance of warm, clear water, sunshine and proper salt levels.
Stress from repeated physical contact by divers, snorkelers, boats and anchors, as well as increasing pollution and disease, threaten the very survival of the fragile coral. The sunny, shallow water seagrass beds provide food and shelter for many of the fish and invertebrates that make the reef their home. Damage from boat propellers is one of the major threats to the survival of the seagrass communities of mainland Florida and the Florida Keys. Lining the islands of the Florida Keys, the very dense roots of the mangrove trees serve as an important nursery habitat for the young of many of the animals found on the reef. They also play an important role by trapping eroding sediment, thereby giving shape and structure to the islands themselves.