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florida reef tract
Watersheds and Coastal WatersFlorida Reef Tract
Corals and coral reefs (fig. 21) are most abundant and best developed offshore of the Florida Keys seaward of Hawk Channel. These reefs which started growing about 6,000 years ago (Shinn and others, 1989), form a tract that is almost 150 mi long and about 4 mi wide extending to the edge of the Florida Straits (fig. 1). The reef tract is not uniform, but consists of a series of ridges and channels parallel to the Keys. Two zones of discontinuous, but parallel, ridges are evident--the inner ridge, White Bank, composed of skeletal sand and scattered patch reefs; and the outer ridge, a discontinuous shelf margin of reefs and hard banks, composed of coral rubble and skeletal sand that form the seaward edge of the reef tract (Ginsburg and James, 1974). The corals that formed the Key Largo Limestone and built the upper Florida Keys are living today on the reefs. The best examples of living reefs exist off Key Largo where the islands retard exchange with Florida Bay and the offshore water is dominated by the waters of the Florida Current (Ginsburg and Shinn, 1964; Hoffmeister, 1974; Lidz and Shinn, 1991).
Small patch reefs and large grass beds, as well as large areas of bare sand, are scattered on White Bank. Most of the common life forms of the outer reef occur on the patch reefs with the interesting exception of elkhorn coral, which is absent in patch reefs and in the Key Largo Limestone, but is a prominent reef-building coral of the outer reefs (Shinn, 1963). Also, the dominance of marine species differs between the outer reefs and the patch reefs, as does the growing shape of some corals. Sea fans and whips seem more common on the patch reefs than on the outer reef. The percentage of grass- feeding fishes is higher on patch reefs than on the outer reefs. These fishes utilize the patch reefs as a daytime resting place and then move onto nearby grass beds to feed at night. Patch reefs often have a halo of white sand around their perimeter; this is usually caused by the browsing of the black- spined sea urchins on the adjacent seagrasses (Ogden and others, 1973).
The greatest variety of corals and coral-reef animals live on the outer reefs (Jaap, 1984). Dustin (1985) listed more than 40 species of stony corals from the Florida Reef Tract. The outer reefs have the most stable temperature and salinity and the clearest water because of the proximity of the Florida Current. Most reef-building corals require clear water for photosynthetic algae living in their soft tissues. The corals, in turn, benefit from the oxygen and nutrients produced by the algae (Muscatine, 1990). The Florida Current, which moves northward parallel to the reef tract, provides a rich source of plankton, an important food source for many fishes and invertebrates of the outer reefs. The fish population on the outer reefs, which is one of the most varied in the world, contains more than 500 recorded species (Starck, 1968). The varied morphologies of reef corals provide a haven for fish, crustaceans, mollusks, worms, and sea urchins. Also, the dead coral limestone offer attachment surfaces to a multitude of marine algae and invertebrates. Nearly 1,400 species of marine plants and animals were recorded for a small area of the Florida Reef Tract (Voss and others, 1969).
These coral reefs are, perhaps, the most diverse and colorful marine habitats within the continental United States. Although they have developed near the northern limit for coral reefs (Mayor, 1914) and are subjected to winter water temperatures that can be fatal to corals (Vaughan, 1918), the reefs of Florida rival those of many other areas of the Caribbean in diversity and beauty. Like coral reefs elsewhere, they are among the most highly productive marine ecosystems (Erez, 1990). They thrive in regions typically low in nutrients because they have evolved mechanisms for conserving and efficiently recycling food. This ability of reefs to thrive in a nutrient-poor setting led Odum (1971) to describe a coral reef as "an oasis in a desert ocean."
U.S. Department of the Interior, U.S. Geological Survey
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