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A series of interconnected bays - Biscayne Bay, Card Sound, Barnes Sound, and Florida Bay - lie between the mainland and the Florida Keys. They are semitropical environments supporting a variety of biological communities dependent upon the distribution of sediment, salinity, and tidal flow. The bays and estuaries are calm because they are protected by the reefs. They are open to the sea by inlets between the keys and are interconnected by narrow channels through shallow mudbanks. The bedrock consists of coralline and oolitic limestones. Thin layers of lime mud, quartz sand, freshwater calcite mud, or peat cover the limestone. The freshwater mud and peat, plus sediment-filled depressions in the limestone, indicate that these bays once were freshwater lagoons.

FIGURE 16. Florida Bay and the Florida Keys. [larger image]

Florida Bay is a triangular area of about 2,200 km² (850 mi²) (fig. 16) whose western side opens directly to the Gulf of Mexico. Except for numerous tidal channels between the Keys, it is completely enclosed to the south. The rock under Florida Bay is the oolitic limestone which in most areas is covered with a thin layer of fine calcareous sediment. Green algae may have formed most of this sediment. The depth of the bay is commonly 1.2 to 1.8 m (4 to 6 ft), but several long winding bars join to create a lacelike pattern of banks enclosing shallow depressions. Mangroves grow where the banks come near the surface and create small islands. The irregular lattice pattern was caused by the slow inundation of an Everglades-like marsh by a rising sea. Mangroves growing inland along rills and sloughs joined with mangroves on elevated shorelines to form a perpendicular meshwork which trapped marine sediments, thus forming the lattice pattern of banks and depressions.

The Ten Thousand Islands area along the southwest coast might also be considered a shallow bay. There, numerous 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 parallel to the coastline. Dense mats of oysters grow on top of these sandbars 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 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.