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North of Key Biscayne there are long white sandy beaches, but to the south the beaches disappear and are replaced by a string of mangrove-lined islands that enclose a series of shallow bays: Biscayne Bay, Card Sound, Barnes Sound, and Florida Bay. A rich diversity of marine plants, sponges, sea whips, and small corals cover the bottom of these bays. The circulation within the bays is characteristically weak and water exchange with the ocean is primarily tidal. When the bay bottom is undisturbed, the water is usually clear and supports abundant populations of shrimp, crab, lobster, and fish.

Seaward of the Keys, the Florida Current moves northward at about 3 1/2 knots, bathing the southeastern edge of Florida with warm water from the Gulf of Mexico and the Caribbean. Because the water is clear and warm, coral reefs grow along the continental shelf between the keys and the continental slope. Abundant sea fans, corals, and rainbow-colored fish make this one of the Nation's most beautiful underwater regions. But the white sandy beaches, the mangroves, the crystal water, and the coral gardens are disappearing. Hotels push out onto the beaches and new homes occupy former mangrove islands. The bays are dragged, dredged, filled, and channeled, and the coral reefs plundered for their shells and tropical fish.

Biscayne Bay from Miami to the Keys was once mangrove coastline. Now the majority of it is developed, leaving only the southern stretch in a natural state. [larger image]		Called walking trees, red mangroves have stilt-like roots. These trees, which are tolerant of salt, live in a transition zone between fresh water and the ocean. (Photo by R. Rothwell) [larger image]

Biscayne Bay's clear crystal waters and coral reefs provide the skin diver with a whole new world to explore. [larger image]		South Florida's sandy beaches are a prime attraction to tourists. Most are crowded. [larger image]

The growing population in south Dade County creates a need for more homes, apartments, and hotels. Because the water table is high, and much of the land subject to flooding during the wet season or hurricanes, building sites are limited. Most of Miami and its suburbs lies atop the Coastal Ridge, with wetlands to the west and south. To get more land for development, wetlands can either be drained or filled. To drain the marshes the water must be channeled somewhere; in Dade County this is usually to one of the shallow bays. Similarly, filling requires that the fill be dredged from shallow bays.

Removal of the dredged fill from the bay bottom eliminates the habitat of living plants and animals. There is evidence that the disturbed bay bottom requires a long time to recover. If dredging and bulkheading are uncontrolled, an entire bay, its plants and animals, and its beauty can be destroyed. Suspended sediment created by dredging is often carried by currents far from the excavation site. This sediment eventually settles out, often in amounts that coat the bottom and smother the plants and animals. Turbidity that accompanies dredging reduces light that is important for the growth of marine plants. The cost of dredging to the environment is high and should be considered before, not after, dredging is allowed.

Silt from turbid or muddy waters caused by dredge and fill operations smothers and kills coral reefs. [larger image]		Dredging of coral. [larger image]

Mangroves are an important part of the system that keeps the water in the bay clear and organically rich. Runoff filters through the mangroves where it mixes with salt water, and organic nutrients are added from decaying leaves, fungi, and microscopic animals that live in the rich mangrove soil. Where mangroves have been destroyed, the brackish water zone and the nutrients are reduced, with loss of health and productivity of the area. Removal of the mangrove fringe also exposes the upland urban developments to increased hurricane damage and greater tidal surges during heavy storms.

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