AMERICA'S SUBTROPICAL WONDERLAND

Mangrove Swamp

The southward-creeping waters of the glades eventually meet and mingle with the salty waters of the tidal estuaries. In this transition zone and along the gulf and Florida Bay coasts a group of trees that are tolerant of salty conditions, called "mangroves," form a vast, watery wilderness. Impenetrable except by boat or alligator-back, it occupies hundreds of square miles, embracing both the shifting zone of brackish water and the saltier coastal waters.

Three kinds of trees are loosely called "mangroves." The water-tolerant red mangrove grows well out into the mudflats, and is easily recognized by its arching, stiltlike roots. Black-mangrove typically grows at levels covered by high tides but exposed at low tide, and is characterized by the root projections called pneumatophores that stick up out of the mud like so many stalks of asparagus growing in the shade of the tree. White-mangrove has no peculiar root structure and grows, generally, farther from the water, behind the other trees. Sometimes all three are found in mixed stands.

This mangrove wilderness, laced by thousands of miles of estuarine channels (called "rivers" and "creeks") and broken by numerous bays and sounds, is extremely productive biologically. The brackish zone is particularly valuable as a nursery ground for shrimp. The larvae and young of these marine crustaceans and of other marine animals remain in this relatively protected environment until they are large enough to venture into the open waters beyond the mangroves.

MANGROVE SWAMP. (elevation: sea level to 1 foot above sea level) (click on image for an enlargement in a new window)

The shrimp represent a $30 million industry, and the sports-fishing business of the area is said to exceed that figure by far. Both would suffer if any damage occurred to this ecosystem. The greatest danger is the alteration in the flow of fresh waters from the glades and cypress swamps that occurs when new canals are built and land is drained for cultivation or development. The flow carries with it into the estuaries organic materials from the rich glades ecosystem; these supplement the vast quantities of organic matter derived from the decay of red mangrove leaves. Thus, a reduction in the amount of nutrient-laden fresh water flowing into the mangrove region will affect the welfare of the ecosystem, and indirectly the livelihood or recreation of many persons.

The productive zone of brackish water varies in breadth according to the flow of fresh water. In the wet summer it moves seaward as the flow of fresh water from the glades pushes the tides back. In the drier winter the bay and gulf waters move inland and the brackish zone is quite narrow. With an extended low-rainfall cycle, the brackish zone moves so far inland that mangroves invade the sawgrass glades. A series of wet years enables the sawgrass to march toward the bay, and a dry cycle causes it to retreat. The drainage and canal-building operations of man in south Florida can be extremely disruptive here, since too little (or too much) fresh water flowing into the estuaries can interfere with their productivity.

THE MANGROVE SWAMP AND THE COASTAL PRAIRIE. (click on image for an enlargement in a new window)

Natural disasters such as hurricanes can also bring about great changes in the mangrove ecosystem. Yet biologists do not necessarily view the destruction of mangroves by hurricanes as catastrophic. The hurricanes have been occurring as long as the mangroves have grown here, and are part of the complex of natural forces making the region what it is.

Fire does not seem to be a problem in the mangrove wilderness. The trees themselves are not especially fire-resistant, but it is not uncommon to see a glades fire burn to the edge of the mangroves and stop when it encounters the wet mangrove peat that forms the forest floor.

APPLE MUREX. A carnivorous mollusk that feeds on oysters.

The mangrove wilderness is a mecca for the majority of park visitors. Sportsmen take their motorboats into the bays and rivers to challenge the fighting tarpon. Bird lovers seek the roosts and rookeries of herons and wood storks. Canoeists, the only ones able to explore the secret depths, are drawn by the spell of labyrinthine channels under arching mangrove branches. Here one experiences a feeling of utter isolation from the machine world, in a wildness still thwarting man's efforts at destruction.

But the relentlessly rising sea of the past 5,000 years has belittled drought, fire, hurricane, and frost as it slowly inundated this land, 3 inches each hundred years. In compensation, the mangrove forest adds peat and rises with the sea. The sawgrass marshes retreat, and the mangrove ecosystem prevails, essentially unchanged.