Florida Keys National Marine Sanctuary

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Chapter 3. SITE DESCRIPTION

a - General features of the site

Terrestrial surface under sovereignty, excluding wetlands:

266 sq. km

Wetland surface:

0 ha

Marine surface:

9947 sq. km

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Wetland surface (ha): Extensive (adjacent Everglades NP, and mangroves surround the islands of the Keys.

b - Physical features

Brief description of the main physical characteristics in the area:

See below.

Geology:

The Florida Keys are located at the southern edge of the Floridan Plateau, a large carbonate platform composed of 7,000 m of marine sediments. The plateau incorporates all of Florida and the adjacent continental shelves of the Gulf of Mexico and Atlantic Ocean. Sediments have been accumulating in the region for 150 million years and have been structurally modified by subsidence and sea level fluctuation. The crystalline and sedimentary basement rocks of the South Florida Basin underlie the plateau. The basin is a block-faulted feature associated with the breakup of North America and Africa during the Mesozoic era. Further block-faulting during this era created the Straits of Florida, the water body separating the plateau from the Bahamas and Cuba. Subsequent sea level transgressions flooded the area, initiating episodic reef building and marine deposition. Between 100,000 and 125,000 years ago, sea level was approximately 6 m higher than it is today. Sediments were deposited in a series of bays and lagoons in South Florida, while a large reef complex flourished to the east. To the south, tidal exchange between the Atlantic Ocean and the Gulf of Mexico formed a large series of cross-bedded, carbonate (oolitic) sand bars. Sea level fluctuations attributed to glaciation are largely responsible for the region's current morphology. During the Wisconsin Glaciation, sea level dropped between 15 and 30 m, exposing the entire platform to marine and subaerial erosion. Sea level rose again approximately 6,000 years ago, flooding the area and forming the current physiographic regions. Lithified remnants of the ancient reef complex formed the Upper Keys, while the Lower Keys were formed from the oolitic sand bars. Florida Bay occupies the southern portion of the old lagoonal structure.

Topography:

Length of sandy beaches: 50 km

Hydrodynamics:

Estuarine areas:

Florida Bay lies between the extensive freshwater ecosystem of the Everglades and the marine environments of the Florida Keys National Marine Sanctuary. Florida Bay is approximately 1,550 km2 and an average depth of 1.5 to 2 m. Its most distinct feature is a patchwork of interconnected mud banks composed of shelly calcareous silt, which forms a series of oval-shaped basins 4.8 to 6.4 km long, 5.1 to 7.7 km wide, and 1.5 to 1.8 m deep. To the west, these banks gradually mix with the more clastic sediments of the southwest continental shelf. The bay has been termed an active lime-mud factory, with silts and muds composed of 90 percent calcium carbonate, with aragonite the primary constituent mineral. Biogenic sediments derived from a variety of marine organisms (primarily the green algae Penicillus) continually accumulate. Because of the bay's shallow depth, large seasonal variations in temperature and salinity are common, and abundant sediment contributes to turbidity levels. As winter storms pass through the area, large amounts of sediment-rich cool water are transported through the channels between the Keys to the Florida Reef Tract. During periods of warm, stable weather, tidal currents can transport high-temperature water in the same direction. This influx directly affects reef production by changing water temperature, salinity, and turbidity levels

Underwater formations:

The porous limestone foundation of the Florida Keys provides for localized discharge of groundwater around the islands of the Keys.

Others:

The Florida Reef Tract: linear zones of living coral reefs and associated habitats paralleling the Keys for 130 km.

Mean annual precipitation (in mm) 12,450 mm.

Freshwater springs: Existence and brief description, including marine offsprings