Dynamics of the Salt Marsh - Where Are Marshes Located? What Are Their Features?

Dynamics of the Salt Marsh

Where are Marshes Located? What are Their Features?

South Carolina has more marsh acreage than any other Atlantic coast state (344,500 acres of salt marsh). Because salt marshes in South Carolina are influenced by the twice daily rise and fall of tides, they are subject to rapid changes in salinity, temperature and water depth.

Salinity, frequency and extent of flooding of the marsh determine the types of plants and animals found there. The low marsh zone floods twice daily in South Carolina, while the high marsh floods only during storms and unusually high tides. Animals and plants live in these zones of the marsh, depending on how well they can withstand the drier conditions of the upper marsh or the wet conditions that regularly occur in the lower marsh.

Salt Marsh Cordgrass

One plant, smooth cordgrass (Spartina alterniflora), dominates the regularly flooded lowmarsh. Smooth cordgrass is the most abundant salt marsh plant in South Carolina and is responsible for much of the marsh's productivity. Spartina's successful adaptations enable it to live where few other plants could survive. It has narrow, tough blades and special glands that secrete excess salt, making it ideal to withstand the high heat and daily exposure to salt water. Few animals eat this plant, but many animals and plants live on it or on the marsh surface protected by its roots and stalks. Spartina stalks are thick and are very tough and well anchored by a root system.

From a distance, the low marsh appears to be uniform; however, there are two forms of Spartina. A tall form grows along creek banks and can reach heights of 9 feet. A short form of Spartina occurs in interior parts of the low marsh and ranges from 2 to 3 feet in height. In contrast to the low marsh which has one major species of plant, the high marsh contains a mixture of several species including black needlerush, salt meadow cordgrass and short-form smooth cordgrass. This high marsh area grades into a marsh-upland border which is a transitional zone between the salt marsh and the maritime shrub community that consists of wax myrtle, yaupon and cedar.

South Carolina also has brackish marshes which occur in water of lower salt content. Salinity, however, still controls what species of plants and animals occur in brackish marshes. Common plants in these marshes include a mixture of black needlerush and big cordgrass. Brackish marshes are also a transitional habitat between freshwater marshes and salt marshes and contain species from both.

Salt Marsh Ecology: What Lives in the Salt Marsh?

Live Spartina is not a source of food but dead marsh plants are a source of nourishment for many species. Decaying Spartina breaks into small pieces called detritus that fuels the marsh and its animals. In spring and summer, marshes are lush green, highly productive and grow in height. In late fall, the green Spartina begins to turn brown as leaves die and decomposition begins. Water, waves, wind and storms dislodge and break up decaying leaves, and transport them to mud flats and other locations around the marsh. This dead plant matter, or detritus, forms an attachment site for microscopic organisms such as bacteria, fungi and small algae. These organisms colonize the broken bits of plant material and break down portions of the detritus that are not digestible by animals.

For the most part, this decomposition occurs on or in the sediments where bottom-dwelling scavengers such as worms, fishes, shrimps and crabs live. These animals eat the decaying plant material, along with the bacteria, fungi and attached organisms. They then digest the material and excrete the undigested plant remains in feces that can be colonized again by microorganisms. As the microorganisms utilize detritus and reduce it to smaller and smaller pieces, the remaining detritus becomes fertilizer for the next Spartina crop. In this way, the whole food web cycle is repeated.

Microscopic animals associated with detritus also cover the surface of mud in the salt marsh. They help stabilize sediments, are food for larger organisms and contribute to an enrichment of the sediments. Large numbers of more sizable invertebrates (animals without backbones) inhabit salt marshes. Rapid changes in salinity, temperature and exposure create stressful conditions and thereby limit how many species occur in this habitat. Fiddler crabs, marsh snails and marsh mussels are typical invertebrate species which live in salt marshes. The popular and highly prized oyster generally borders salt marshes. Fiddler crabs and marsh snails shred dead plant material during feeding, aiding the decomposition process.

Insects are also abundant in the salt marsh. Most of these salt marsh invertebrates consume living plants, or fluids secreted by the plants. Some insects also feed on detritus, though the importance of their role in the food web as grazers and detritus feeders is small compared to their importance to the abundant species of birds who depend on them for food. The undigested grass eaten by insects is deposited as feces on the marsh surface where it becomes part of the detrital food web. Many fish species living near the salt marsh rely on insects for food during part of the year.

Fishes, crabs, and shrimps live in salt marshes where stems, leaves, and roots provide food and shelter from predators. The young of many species, such as the blue crab, white shrimp and spot tail bass utilize the salt marsh as a nursery. Without benefit of an abundance of food and protection given by marsh plants, few younger animals would survive to adulthood. Many fishes which inhabit marshes move on and off the marsh surface with the tide. Once they leave the protection of the marsh surface to enter the adjacent tidal creek, they become more susceptible to being eaten by large predators living in creeks. Some marsh-dwelling fishes and shrimps remain on the marsh surface after the tide recedes. They live in potholes and standing pools of water. These common marsh inhabitants include mummichogs and grass shrimp.

Few reptiles live in salt marsh habitats. Diamond back terrapins are probably the most common species in the marsh, where they lay eggs and forage during high tide. American alligators do occur in brackish salt marshes but are not often found in high salinity marshes. The regularly flooded salt marshes of South Carolina provide excellent habitat for birds, with many places for feeding, reproducing and roosting. Species such as the red-winged black bird alternately eat insects and seeds depending on the season. Other birds, such as herons and egrets, feed on fishes, shrimps and fiddler crabs. These graceful predators are year-round residents