Tidal Marshes

Almost a third of the Atlantic Coast's tidal salt marshes are located in Georgia's Lower Coastal Plain, as are thousands of acres of rare tidal freshwater marshes. Immortalized in poet Sidney Lanier's poem "The Marshes of Glynn," these wide expanses of salt marsh are the most visible physical feature along Georgia's 100-mile-long coast. The salt marshes stretch in a band four to six miles wide between the mainland and coastal barrier islands. A major reason for Georgia's extensive expanses of salt marsh is that its coast experiences the highest tides along the southeastern shoreline. In general, the higher the tidal range, the greater the production potential of the marsh.
Estimates of Georgia's total tidal marshland acreage vary. In 1977 the National Aeronautics and Space Administration reported that Georgia had 429,294 acres of tidal marshland, more than any other state on the East Coast. The study, however, did not distinguish between salt marsh and freshwater marsh. The Georgia Department of Natural Resources estimates that the state encompasses more than 378,000 acres of salt marsh.
Georgia's salt marshes are some of the most biologically productive natural systems on Earth. They produce nearly twenty tons of biomass to the acre—which makes them four times more productive than the most carefully cultivated cornfields, according to the Georgia Department of Natural Resources. The enormous productivity helps to make the salt marshes primary nursery areas for blue crabs, oysters, shrimp, and other economically important fish and shellfish. Young shrimp and other marine organisms also use salt marshes as shelters and hiding places from predators. In addition, salt marshes help filter pollutants from the water and act as buffers against offshore storms. The potential damage from large storm-spawned waves and tides is greatly reduced when they pass over the marshes.
The origin of the salt marshes can be traced to the Pleistocene geologic epoch that began about 18,000 years ago. Rising sea levels from melting continental glaciers created shallow lagoons behind young barrier islands. Ocean currents and tidal rivers flowing into the quiet lagoons deposited large amounts of clay and sand sediments there. Gradually, the sediments built up to the degree that they were no longer underwater at low tide.
The muddy soils then became fertile ground for one of the world's most salt-tolerant plants, Spartina alterniflora, or smooth cordgrass. Spartina took root in the lagoonal sediments and flourished. Today, vast expanses of Spartina dominate Georgia's salt marshes.
Georgia's twice-a-day tides are the lifeblood of the salt marshes. Incoming tides bring in nutrients from estuaries connected by tidal creeks to the marshes. The nutrients nourish and feed the grasses of the marsh. Outgoing tides carry nutritious marsh products—including detritus produced from decaying Spartina —back into the estuaries. There, the products help to sustain large numbers of other marine organisms. The outgoing tides also remove wastes from the marsh.

Salt Marsh Zones

The salt marsh may appear to be a homogenous, table-flat expanse of grass. But biologists usually divide the salt marsh into ecological zones, which are based on increasing elevation from creek banks to the landward edge of the marsh. With each change in elevation, tide inundation is to a lesser depth and for a shorter period of time. The height of Spartina and the length of time it is exposed to saltwater also vary in each zone.
Zones in Georgia's salt marshes usually include creek bank, low marsh, high marsh, salt pan, marsh hammock, and marsh border community. Spartina is at its most luxuriant, growing as tall as ten feet, along creek banks. There, the tides bring in abundant nutrients and clay and sand sediments and efficiently wash away salt, dead matter, and other waste.
Adjacent to creek banks are natural levees, which build up when the high tides overflow the banks and deposit sediments. Spartina grows three to four feet atop the levees. Behind the levees is the low marsh zone, where Spartina also grows about three to four feet tall. Though the tides flood this zone several hours a day, it does not get as many nutrients and sediments as the upper creek bank. The only other plants in the low marsh zone are algae, especially blue-green algae and diatoms. The sand content of the soil is less than 10 percent.
The high marsh zone is closer to land. Only a few inches of elevation may separate it from the low marsh zone. Spartina is only a foot or so tall in the high marsh. Soil sand content is about 10-70 percent. Seawater inundates only an hour or so each day in the high marsh. The soil usually is saltier than the low marsh because of poor tidal flushing. In the areas of high marsh closest to land, however, freshwater runoff may have considerable influence on the vegetation. Here, Spartina must compete with other salt-tolerant plants like glasswort, needlerush, saltbush, and sandwort. About 18 percent of Georgia's salt marshes are composed primarily of needlerush (Juncus roemerianus).
Salt pans are small barren areas of the high marsh too saline for any vegetation to grow. They form where evaporation concentrates large amounts of salts in the marsh soil.
Marsh hammocks are actually marsh islands, the only dry land in the marsh. Living there are trees like red cedar and wax myrtle and other plants like cactus, saw palmetto, yaupon holly, and yucca. About 1,200 hammocks dot Georgia's salt marshes. Some are only a fraction of an acre in size; some cover more than 1,000 acres and support maritime forests.
Only the highest of tides that occur once or twice a month reach the marsh border community, a transition zone between marsh and upland areas. Growing here are groundsel bush, marsh elder, sea ox-eye daisy, and other plants and shrubs able to withstand strong wind, salt spray, and an occasional inch or so of saltwater.

Salt Marsh Processes

All marsh soils are anaerobic, or without oxygen, except for the first few millimeters of the surface and around Spartina roots and crab and worm burrows. Anaerobic bacteria living in the soil are responsible for the breakdown of accumulated organic matter. The bacteria break down the organic matter into ammonium, hydrogen sulfide, methane, and other products. Hydrogen sulfide gives the salt marsh its characteristic rotten-egg odor. Red streaks in marsh mud also indicate the presence of oxidized iron, a common and important element in the marsh.
Spartina is responsible for most of the food produced in the salt marsh. Less than 10 percent of the living plant is consumed directly by insects, snails, and other organisms. But when Spartina dies in the fall, bacteria and fungi break it down into minute particles called detritus. Washed into the tidal creeks and estuaries at high tide, the detritus becomes the most important part of the estuarine food web. The minute, decaying grass particles create a nutritious "soup" that feeds clams, crabs, small fish, oysters, plankton, and shrimp.
Phytoplankton, which are tiny free-floating green plants in the water column, and microalgae known as diatoms, which coat the mud's surface, also contribute significantly to marsh and estuarine food production. They produce their own food through photosynthesis and are consumed by other organisms, including larval forms of marine creatures, which support even larger animals.

Salt Marsh Animals

Most of the organisms that live in the salt marsh are transients that spend only part of their life cycles there. The marsh has few resident plants and animals because of the harsh environment. The major conditions that severely limit resident life are the intermittent exposure to air and saltwater as the tides rise and fall, the rapid changes of water temperature and salinity with the inflow and outflow of estuarine waters, and saturated, anaerobic soils.
Many species of crabs live in the marsh. The brown squareback crab (Sesarma cinereum) is found on the landward side of the marsh. The purple squareback crab (S. reticulatum) and mud crab (Eurytium limosum) are found in the mud area on the levees in the dense Spartina. Two abundant fiddler crab species are the mud fiddler (Uca pugnax), found in both high and low marsh, and the sand fiddler (U. pugilator), occurring in great numbers near the landward edges of the marsh. The largest but least common of the fiddlers is the red-jointed fiddler (U. minax), which has a high tolerance for freshwater.
Three species of snails are also commonly found in the salt marshes: the marsh periwinkle (Littorina irrorata), the mud snail (Ilynassa obsoleta), and the air-breathing coffeebean snail (Melampus bidentatus). The snails feed on detritus and algae and are a food source for many larger animals.
More than 100 insect species have been identified in Georgia's salt marshes. Two dominant species are salt marsh grasshopper (Orchelimum fidicinium) and the planthopper (Prokelisia marginata). An ant (Crematogasterclara) lives in the stems of Spartina. Two species of salt marsh mosquito are Aedes taeniorhynchus and A. sollicitans, both of which attack humans. Three blood-sucking midges, Culicoides furens, C. hollensis, and C. melleus, breed in the salt marsh. These noxious insects, commonly called "no-see-ums," are very abundant in the summer. Other annoying marsh-breeding insects are deer flies (Chrysops spp.).
The only reptile inhabiting the salt marsh is the diamondback terrapin (Malaclemys terrapin). Alligators (Alligator mississippiensis) occasionally feed in the marsh. Three bird species nest in the marsh—the clapper rail, or marsh hen (Rallus longirostris); seaside sparrow (Ammodramus maritimus); and long-billed marsh wren (Telmatodytes palustris). Great blue herons (Ardea herodias), common and snowy egrets (Egretta spp.), and other wading birds commonly forage in the marsh at low tide. The willet (Catoptrophorus semipalmatus), though common on beaches, is more common in the salt marsh.
Several mammal species also feed in the salt marsh. Raccoons are one of the most abundant. Marsh rabbits are common along the marsh edges adjacent to high ground. Mink and otter are common but seldom seen. The rice rat is common along the tidal creek levees.

Marsh Protection

In 1970 Georgia legislators, fearing that the state's coastal salt marshes would be irrevocably damaged by a proposed phosphate mining operation and other industrial activities, passed the Coastal Marshlands Protection Act. The jurisdiction of the act includes marshlands, intertidal areas, mudflats, tidal water bottoms, and salt marshes. They were spurred on by scientific studies showing the immense value of the marshes for storm protection, for pollution filtering, and as a nursery area for more than 70 percent of Georgia's economically important crustaceans, fish, and shellfish.
The law provides the state government with the authority to protect tidal wetlands. The government manages certain activities and structures in marsh areas and requires permits for other activities and structures. Erecting structures, dredging, or filling marsh areas requires a permit from the Marshlands Protection Committee, administered through the Coastal Resources Division of the Georgia Department of Natural Resources.

Research

Much of the world's pioneering research on salt marshes took place at the University of Georgia Marine Institute on Sapelo Island. Established in 1953, the institute was the country's first and is one of its most renowned centers dedicated to the exploration of salt-marsh and estuarine ecosystems.
The remote location of the institute provides researchers unparalleled access to largely undisturbed salt marshes, which offer an ideal laboratory for studying how natural systems function as a whole. This ecosystems approach, pioneered by University of Georgia ecologist Eugene Odum, was based on the relationship between the biological (e.g., plants and animals) and physical (e.g., tides and geology) components of the environment.
Among other things, early scientists at the institute began charting the local food web. They studied how microorganisms broke down the marsh grass into particles small enough to be consumed by tiny organisms that were, in turn, eaten by the larger fish, birds, shrimp, and crabs. The researchers also helped to establish the importance of coastal areas as nurseries for shrimp, oysters, and other ocean organisms. Their studies were the early building blocks of ecosystem and landscape ecology, as well as of the emerging fields of conservation and restoration ecology.
Today, several institutions on the Georgia coast are involved in salt-marsh research. One focus of current research is understanding the causes of an unprecedented salt marsh die-off that occurred in Georgia in early 2002. More than 1,200 acres have been affected. The mysterious marsh die-offs have laid bare large swaths of Spartina and needlerush. The problem has significant implications for fisheries, navigation, water quality, and wildlife.

Freshwater Tidal Marshes

Georgia's freshwater marshes occur upstream of estuaries, primarily along rivers that flow into the estuaries. The most extensive are those at the mouth of the Altamaha River and in the Savannah National Wildlife Refuge along the Savannah River. Tides influence water levels, but the water in the marshes is fresh. The marshes may extend for some distance up the rivers before being replaced by cypress-gum or hardwood swamps. Much of the area now covered by freshwater marsh was cypress swamp before it was cleared and diked for rice culture.
The lack of salt stress allows a greater diversity of plants to thrive. Shallow freshwater marshes contain cattail, wild rice, pickerelweed, bulrushes, smartweeds, arrowhead, and arrow arum and help to support a large and diverse range of bird and fish species, among other wildlife. The deeper freshwater marshes are abundant, occupying about 25,000 acres, and consist almost exclusively of giant cutgrass (Zizaniopsis miliacea).
Tidal freshwater marshes are now relatively rare natural communities because vast tracts of them were lost to salt-water intrusion, drainage for development and agriculture, and other reasons. The U.S. Fish and Wildlife Service believes that the remaining freshwater marshes in the Savannah National Wildlife Refuge are threatened by the deepening of the Savannah Harbor.
Another concern among scientists and conservationists is that rising sea levels brought on by global warming could result in freshwater marshes being converted into salt marshes.
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Further Reading
Mark D. Bertness, The Ecology of Atlantic Shorelines (Sunderland, Mass.: Sinauer Associates, 1999).

Georgia Conservancy, A Guide to the Georgia Coast, 2d rev. ed. (Savannah: The Conservancy, [1993]).

A. Sydney Johnson and Hillburn O. Hillestad, An Ecological Survey of the Coastal Region of Georgia, National Park Service Scientific Monograph Series, no. 3 (Atlanta: National Park Service, 1974).

Barbara Kinsey, A Sapelo Island Handbook (Athens: University of Georgia Marine Institute, 1982).

Charles Seabrook, The World of the Salt Marsh (Athens: University of Georgia Press, 2012).

John Teal and Mildred Teal, Life and Death of the Salt Marsh (Boston: Little, Brown, [1969]).

Mildred Teal and John Teal, Portrait of an Island (New York: Atheneum, 1964; reprint, Athens: University of Georgia Press, 1997).
Cite This Article
Seabrook, Charles. "Tidal Marshes." New Georgia Encyclopedia. 01 April 2014. Web. 30 October 2014.
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