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A Region Under Stress-- Home
A Region Under Stress-- Introduction

Environmental Setting-- The Natural System
Physiography
Climate
Geology
Hydrology
Watersheds and Coastal Waters
Kissimmee-Okeechobee-Everglades Watershed
Big Cypress Watershed
Charlotte Harbor Watershed
Estuaries and Bays
Florida Reef Tract
Coral Reefs and Sea Level

Environmental Setting-- The Altered System
Drainage and Development
Public Lands
Agriculture
Urbanization
Water Use
Water Budget

Water and Environmental Stress
Loss of Wetlands and Wetland Functions
Soil Subsidence
Degradation of Water Quality
Mercury Contamination
Effects on Estuaries, Bays, and Coral Reefs

Summary and Research Needs
References

Related Links

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U.S. Department of the Interior
U.S. Geological Survey
Circular 1134

The South Florida Environment - A Region Under Stress

Environmental Setting--
The Natural System


Photo of Western Florida Bay at Cape Sable
Figure 21. Western Florida Bay at Cape Sable. Click on image to open larger picture (27.5k).

Watersheds and Coastal Waters

Estuaries and Bays

A series of interconnected bays--Biscayne and Florida Bays, Card and Barnes Sounds-lie between the mainland and the Florida Keys (fig. 17). The bays are semitropical environments that support a variety of biological communities that are dependent on the distribution of sediment, salinity, temperature, and tidal flow. The bays and estuaries are protected in a shallow depression formed between the Keys and the mainland. On the Keys side, bedrock consists of coralline Key Largo Limestone (Hoffmeister and Multer, 1968). This generally low ridge of limestone that forms the upper Florida Keys is as much as 18 ft above sea level at Windley Key and extends from Soldier Key in the north to Big Pine Key in the southwest. On the mainland side of the bays, the bedrock is the Miami Limestone which is dominated by two lithologies-an oolitic facies forming the Atlantic Coastal Ridge and a nonoolitic bryozoan facies beneath the Everglades to the west (Hoffmeister and Multer, 1968).

Florida Bay is a triangular area of about 850 mi2 (fig. 1) whose western side opens directly to the Gulf of Mexico (fig. 21). Except for tidal channels between the Keys, it is almost completely enclosed to the south and east. The rock under Florida Bay is porous limestone, which is similar to the bryozoan facies of the Miami Limestone in the eastern Everglades to the north. As much as 16 ft of marine sediments, mud, peat, and sand has accumulated on the limestone and formed a latticework of mud banks in Florida Bay. The banks enclose more than 40 shallow depressions, locally termed "lakes" that are between 4 and 6 ft deep (Enos and Perkins, 1978). Mangroves grow where the banks come near the surface and create small islands (Enos, 1989). The irregular lattice pattern was created by a complex process of erosion and deposition during the slow inundation of an Everglades-like marsh by a rising sea (Davies and Cohen, 1989). Mangroves that grow inland along storm berms, rills, and sloughs joined with mangroves on elevated shorelines to form a perpendicular meshwork that trapped marine sediments, thus forming the lattice pattern of banks and depressions (Wanless and Tagett, 1989). Thin layers of freshwater mud and peat that fill depressions in the limestone indicate that Florida Bay was similar to the Everglades when sea level was lower (Davies and Cohen, 1989).

Numerous interconnected bays and estuaries also lie along the western coast between Florida Bay and Charlotte Harbor, including the Ten Thousand Islands. In the Ten Thousand Islands area, oyster bars and mangrove islands create an intricate pattern of protected backwaters. Longshore currents from the north have deposited silica sand to form offshore bars that are parallel to the coastline. On top of these sandbars, dense mats of oysters grow perpendicular to the tidal flow and thus gain a feeding advantage. Mangroves grow on these intertidal bars and with time deposit tough, fibrous layers of peat. Eventually, further growth of the oyster bars may so restrict tidal flow that oyster growth declines. The mangroves, however, will continue to cover the bars and to connect adjacent islands. Later, sediments will fill the lagoons between the bars. Even so, mangrove land building appears to be balanced by the gradual drowning of offshore islands by a rising sea (Scholl, 1964; Scholl and others, 1969; Parkinson, 1989).


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