LEAFLET 14 5
ENVIRONMENT
The climate of the Florida Keys is subtropical to tropical, with rare, brief, below-freezing temperatures. The plants, animals, and ecosystems are a blend of temperate and tropical species.
Because the Keys receive some of the lowest amounts of rainfall in
Florida, because they are surrounded by salt water, and because the rocks of the Keys are permeable, obtaining adequate supplies of fresh water has always been a problem. There are no reliable natural sources of potable groundwater, although some small, unpredictable and fluctuating lenses of fresh-to-brackish water occur at shallow depths. Fresh water must be obtained by the pioneers' technique of capturing rain runoff in cisterns, by importation via the pipeline along US 1, or by desalinization.
Elevations over most of the Keys are less than 10 feet above mean sea level, although Key Largo and Key West have small areas that rise slightly over 15 feet. The islands slope very gradually up from the sea to flattened, gently rounded tops. Relief is slight on the bedrock surfaces, seldom exceeding one or two feet. Irregularities of the rock surfaces are a result of the heterogeneous topography of the coral reefs that created the islands, and also the result of erosion and solution of the limestone rocks after exposure above the sea. Solution features, such as pitted and pinnacled surfaces, occur everywhere on the Keys. Sinkholes, up to several feet in diameter and several feet deep, are abundant but many are filled with peat or carbonate sediments, which masks them from casual detection. Vegetation preferentially takes root in them, providing clues to their location.
Compared to the rest of Florida, there is very little quartz sand on the Keys. Most of the sand is of carbonate origin, not quartz sand. Carbonate sand is derived from the erosion of limestone, from particles precipitated in water, or as by-products in the life processes of some marine plants and animals. A few islands, notably Long Key and Bahia Honda, have beaches of loose carbonate sand that veneers the bedrock; most other beaches are exposed, pitted and pinnacled limestone. Extensive commercial development and construction has resulted in large quantities of crushed limestone "fill" covering many areas of the Keys.
The subtropical Florida Keys present somewhat of a paradox with respect to vegetation. In contrast to the usual picture of tropical, verdant rainforests and luxuriant plant cover, large areas of the islands present