Big Bend Coast

The Big Bend Coast is the marshy coast extending about 350 kilometres (220 mi) from the western end of Apalachee Bay down the west coast of peninsular Florida to the Anclote River or Anclote Key. It partially overlaps the coast line of the Big Bend region of Florida, and is coterminous with the coast line of the Nature Coast region of Florida. Most of the coast remains undeveloped, with extensive salt marshes, mangrove forests, seagrass meadows, and oyster reefs offshore, and coastal hammocks onshore.

The Big Bend Coast is variously defined as extending from the mouth of the Ochlockonee River, or of the St. Marks River, at the western end of Apalachee Bay, to the mouth of the Anclote River or to Anclote Key, just offshore of that river mouth. It is sometimes divided into two parts, the Big Bend Proper, from the Ochlockonee River to the Withlacoochee River, and the Springs Coast, from the Withlacoochee River to the Anclote River. The northern part of the coast, the Big Bend Proper, is sometimes further divided into four areas for study and discussion purposes: Apalachee Bay, Deadman's Bay (centered on the mouth of the Steinhatchee River), Suwannee Sound (centered on the mouth of the Suwannee River), and Wacasassa Bay. The Big Bend Coast is sometimes a component in the larger Wilderness Coast that also includes the coast west of Apalachee Bay to Cape San Blas. The Big Bend Proper includes the coasts of Wakulla, Jefferson, Taylor, Dixie, and Levy counties, while the Springs Coast includes the coasts of Citrus, Hernando and Pasco counties.

The Big Bend Coast is a drowned karst region, covered with salt marsh and mangrove forests. It includes freshwater springs, oyster reefs, and the delta of the Suwannee River. There are barrier islands west of the Ocklockonee River and south starting with Anclote Key, but there are no barrier islands between those places. The Big Bend Coast has little or no sand or mud. The karst topography has produced an irregular, frequently exposed, bedrock surface. The lack of sand has been a feature of the Big Bend Coast since at least the Pleistocene. Sediment of Holocene origin is generally limited to salt marshes and the nearshore zone, and is redistributed by tidal action and storm events. The Suwannee River carries about 73,000 metric tons of sediment to the coast each year. The Big Bend Coast is on the Gulf Coastal Lowlands of Florida, which has recently exposed ocean-smoothed terraces with Tertiary limestone at or just below the surface. The flatness of the Big Bend Coast is interrupted by an area of relic sand dunes just inland from the Cedar Keys. The presence of a high water table has produced a karst landscape. The limestone hosts the Florida aquifer, which reaches the surface near the coast. Steady discharge from the aquifer supports the discharge of the many springs feeding rivers and streams along the coast and maintains a high water table near the coast.

All of the rivers that reach the Gulf of Mexico along the Big Bend Coast are at least partly spring-fed. There are 14 rivers (and many smaller streams) reaching the coast between the Ochlockonee River at the western end of the Big Bend Coast and the Anclote River at the southern end, including the St. Marks, Aucilla, Econfina, Fenholloway, Steinhatchee, Suwannee, Waccasassa, Withlacoochee, Crystal, Homosassa, Chassahowitzka, Weeki Wachee, and Pithlachascotee rivers, and Spring Warrior Creek. Largest is the Suwannee River, with a small delta near the middle of the coast. The coast between the Withlacoochee and Anclote rivers is known as the Springs Coast. Four short (4 to 12 km long) rivers, the Crystal, Homosassa, Chassahowitzka, and Weeki Wachee rivers, are fed almost entirely by first-magnitude springs, as there is almost no surface runoff in the area. The entire lengths of the Crystal, Homosassa and Chassahowitzka rivers are subject to tidal influence. At the southern end of the Springs Coast, the Pithlachascotee and Anclote rivers arise further inland, but are almost entirely spring-fed.

The many rivers and smaller streams flowing to the Big Bend Coast lower the salinity of the nearshore water. The seasonality of rainfall produces seasonal variations in the salinity of the waters along the Big Bend Proper (the rivers of the Springs Coast are almost completely fed by springs, and have little or no seasonal variation in flow). Rainfall from tropical cyclones may also lower the salinity of nearshore waters. The shallowness of nearshore waters also mean that the water temperature is strongly affected by the air temperature. Tropical species may be killed by cold weather, or may migrate southward or to deeper water less subject to cooling in winter.

Besides rivers and streams, hundreds of springs (including submarine springs), fractures and seeps along the Big Bend Coast contribute to the flow of freshwater into the Gulf of Mexico. The close proximity of the Florida aquifer to the surface with only a shallow soil layer over the porous limestone bedrock means that groundwater can emerge in many locations. The water discharged by the Florida aquifer to the surface has a temperature of 70 to 72 °F (21 to 22 °C) year-round. On winter nights the waters of the Gulf are significantly cooler than the water emerging from the Florida aquifer. Thermal images were taken at night in March 2009 along 126 miles (203 km) of the Big Bend Coast from Jefferson County to Levy County. The thermal images revealed 874 "hot spots" along the coast which were at least 6 °F (3.3 °C) warmer than Gulf waters in the study area. One hundred ninety-three of the identified sources were under water in the Gulf. The authors estimated that the discharge from the identified inland sources is equivalent to that of one 1st-magnitude spring for every 2 miles (3.2 km) of coast.

The streams and springs flowing to the Big Bend Coast contribute approximately 1,000,000 US gallons (3,800,000 L; 830,000 imp gal) of freshwater to the Gulf of Mexico each day. Livingston describes the Big Bend Coast as a massive, open estuary.

The Big Bend Coast is subject to little or no wave energy. Tidal range is 73 to 75 centimetres (29 to 30 in) for all of the Big Bend Coast. In the mid-20th century the intertidal zone of the Big Bend Coast consisted of salt marshes up to 10 kilometres (6 mi) wide, dominated by herbaceous (non-woody) plants, including Juncus roemerianus, Distichlis spicata, Sporobolus pumilus (formerly Spartina patens), and Salicornia species. There were about 650 square kilometres (250 sq mi) of tidal marsh along the Big Bend Coast in 1997. The marshes grow on sediment that is usually no more than 2 metres (6.6 ft) thick, although some depressions in the karst bedrock may have thicker deposits. As of 2023, mangrove forests are replacing salt marshes in the intertidal zone along the southern part of the Big Bend Coast from the Cedar Keys to Anclote Key. Mussels, oysters, fiddler crabs, marsh periwinkles (the snail Littoraria irrorata), crown conch (Melongena corona), flathead grey mullet (Mugil cephalus), and blue crabs (Callinectes sapidus) are abundant in the salt marshes. The vegetation of the salt marshes have been called "perhaps the most productive in the world." Salinity levels and temperatures can seasonally become extreme in salt marshes, potentially killing many fish and invertebrates. Most species found in salt marshes have developed behavioral strategies, such as migration, to cope with the extremes. Marsh creeks carry freshwater runoff from the land to the gulf, and provide another habitat for many species. Glass shrimp and killifish are lifelong residents of marsh creeks. Pink shrimp, blue crabs, stone crabs, mullet, red drum, and Gulf flounder live in marsh creeks as juveniles. As a result of sea level rise, salt marshes have been retreating along the Big Bend Coast, but losses on the open water side have been more than offset by the replacement of coastal forest with new marshes, so that the area of salt marshes along the coast has increased by approximately 23% since the beginning of the 20th century.