Off the coast of west central Florida, barriers enclose mangrove marsh areas, which progressively dominate the northern coast of western Florida. Farther northwest, where the coastal trend changes from north-south to east-west along the Florida Panhandle and Alabama coast, barriers re-develop due to the sediment available from Ochlockonee Bay. The quartz sand in the area drains from the Appalachian Mountains, and waves and currents re-work the sand into barriers that are separated by inlets.
West of the Mississippi River delta, different barriers have formed along the muddy coast. Offshore of western Louisiana are cheniers, which are barriers that form when waves push sandy or shelly sediment landward onto a marsh surface. The result is a low-lying feature with ridges of sand and silt overlaying marshes and lakes, blurring the distinction between coast and ocean. The Mississippi River provides the sediment to form these unique coastal features.
In the western Gulf of Mexico, long, sandy barriers form once again, including Padre Island and Mustang Island along the coast of eastern Texas. The barriers in this area feature large dune fields where wind shapes the sandy beaches into tall ridges.
Barriers along the Gulf coast are composed of different sediments and formed by a variety of mechanisms, but all are influenced by the large amount of sediment, relatively low wave energy and broad continental shelf found in the Gulf region.
Gulf Coral Coast
Live coral reefs surround the Florida Keys in southwestern Florida. Hardbottom habitats in this area feature rock colonized by calcifying algae, octocorals, stony corals and sponges. Patch reefs dominated by stony corals are found near Key Largo, Key West and Elliott Key. Bank reefs, also know as forereefs, extend seaward from other reef types.
Located immediately west of the Florida, the Tortugas Banks are extensive coral reefs growing on a foundation of Pleistocene-era limestone. These reefs have a high degree of coral cover but low coral diversity.
More reef areas are found on the West Florida Shelf in the Gulf of Mexico, extending west and north of the Florida Keys and the peninsula. Pulley Ridge is a rocky underwater feature located west of the Keys that is colonized by a vibrant reef ecosystem. Farther north are the Florida Middle Grounds, which comprise carbonate ledges westward of the bend in Florida's coastline. The Florida Middle Grounds are the northernmost coral reefs in the continental U.S. They support algae, sponges, urchins and many species of coral and fish.
Mangrove coasts are those where mangrove plants have rooted in the shallow water of bays, and sediments around their roots have built up to sea level, thus extending the coast. Mangroves include several species of low trees and shrubs that thrive in the warm, shallow, saltwater environments of the lower latitudes.
Mangroves have the ability to form unique intertidal forests that are characterized by dense entangled networks of arched roots, called prop roots, that facilitate trapping of fine sediments, thereby promoting accretion and the development of marshlands. The prop roots and upright exposed roots called pneumatophores also allow the plants to withstand occasional wave action and allow oxygen to reach the roots in anaerobic soils.
The most extensive mangrove coast in the United States is the southwest shore of Florida in the Everglades National Park and the region of Florida Bay. Mangrove islands and marshes continue at intervals along the western coast of Florida. Barriers replace the mangrove coast along the Florida Panhandle and Alabama coast, but farther west mangrove coastal areas and swamps redevelop in the Mississippi River delta region.
Gulf Delta Coast
The Mississippi River has built a series of deltas over time, resulting in overlapping lobes of deltaic sediment. The current delta, the Balize, or "Bird Foot," is approximately 1,000 years old. This delta is southeast of New Orleans, but the river is slowly shifting its course to the Atchafalaya distributory. This long-term process of river mouth shifting has taken place repeatedly in the Holocene period.
Most of the greater Mississippi River delta consists of marshland and mud flats with numerous shallow lakes and intertwining channels. The marsh features aquatic plants and an extensive amount of waterfowl. The large amount of sediment from the river has influenced the growth of barriers islands and cheniers. Mangrove coasts and marshes are also common in this area.