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Salt marshes are the dominant coastal habitat along the East and Gulf Coasts of the United States.
Salt marshes occur along the edge of the ocean in areas where wave energy is mild enough that fine sand and silt can build up.
Salt marshes occur in the temperate zone, but are replaced in the tropics by mangrove forests. Mangroves can’t grow in the temperate zone because they can’t tolerate freezing.
Salt marshes are threatened by sea level rise, which will drown the marshes if they don’t add sediment fast enough to keep up with sea level.
Small islands of terrestrial habitat surrounded by salt marsh are called marsh “hammocks”, and are home to many bird and animal species.
Historically, many salt marshes in the United States were destroyed in order to create ports, marinas, and housing. Today, federal law protects salt marshes from too much development.
Salt marsh habitats are created by the plants that live in them. The plants trap sediment and cause soil to rapidly accumulate.
Salt marshes are covered by the ocean during high tide, and exposed to the air during low tide.
Tides vary from one day to the next in how high they reach. Some high tides cover the whole salt marsh; others cover only part of it.
The flow of the tides on and off of the marsh carries food and nutrients between the marsh and the ocean.
Salt marshes often have a rotten egg smell, due to hydrogen sulfide gas released from sediments that have little oxygen.
Salt marsh plants are very productive, growing as fast as tropical rain forests.
The growth of salt marsh plants is affected by sea level and river discharge, and can vary by three times from one year to the next.
Salt marshes provide important nursery grounds for many fish and shrimp that we eat.
Salt marshes provide important feeding grounds for many fish that recreational anglers like to catch.
Salt marshes provide important feeding grounds for many bird species, including migratory birds and rare species like Whooping Cranes.
Only a small amount of the plant growth in a salt marsh is eaten by herbivores (like grasshoppers or deer). Most of it dies, is decomposed by fungi and bacteria, and enters the “detrital” food web.
Because dead plant material gets buried in the soil as marshes accumulate more sediment, salt marshes may be able to store large amounts of carbon, and may be an important component of the global carbon cycle.
Salt marsh plants differ in how tolerant they are of tidal flooding, and in how good they are at competing for light and nutrients. The interplay between these two factors creates distinct “zones” in salt marshes, with different species of plants in each zone.
Plants have to be adapted to high salt levels in order to live in salt marshes. Some salt marsh plants can exclude salt at their roots, and some excrete salt from their leaves.
Although salt marsh plants have to be able to tolerate high salt levels in order to survive, most actually grow better at lower salt levels. This reveals the physiological effort that it takes for plants to survive in a salt marsh.
Plants have to be adapted to waterlogged soils in order to live in salt marshes. Some salt marsh plants are able to transport oxygen from their leaves to their roots in order to allow their roots to breathe.
Only a few species of plants live in salt marshes, because very few plant species can tolerate being flooded with salty water.
Most salt marsh plants can reproduce both from underground runners and from seeds.
Marsh mussels, Geukensia, have rings on their shells that can be used to tell their age, and can live for over 15 years.
Marsh snails, Littoraria, can live for over 15 years.
At high tide, Littoraria snails move up the stalks of marsh plants to escape the rising water and blue crab predators.
Marsh snails, Littoraria, feed primarily on fungi that colonize dying or wounded Spartina plants.
Only a few species of snails live in salt marshes, because very few snail species can live underwater some of the time and in the air some of the time.
Fiddler crabs, Uca, dig so many burrows in salt marshes that the burrows increase the flow of oxygen into the soil, and increase plant growth.
Several species of birds, such as the marsh wren and seaside sparrow, build their nests in the salt marsh.
Salt marshes are crisscrossed by tidal creeks. These are channels through which ocean water flows as it moves on and off the marsh with the tides.
At low tide, marsh creek banks have a greenish color from microscopic algae that live on the mud surface. These small algae can be just as productive as the much larger marsh grasses.
During the Great Depression, people were put to work digging ditches to drain ponds in saltmarshes, which were thought to be breeding grounds for mosquitos. By 1938, about 90% of marshes between Maine and Virginia had been artificially ditched.
Salt marshes help buffer inland areas from wave damage during storms.
Salt marshes help protect coastal waters by filtering fertilizers and pollutants that come from upland areas.
The roots of salt marsh plant bind silts and sands, making the soil stronger. A large part of the soil is made up of dead roots, which are preserved in the soil after they die.
Marsh mussels, Geukensia, often occur in dense clumps. Clumps of mussels improve growth of marsh plants by adding nutrients to the soil.
Fiddler crabs, Uca, dig so many burrows in the marsh that they completely turn over the upper marsh soils after several years.
Marshes can be disturbed by clumps of dead plant stems that float into the marsh at high tide. These clumps, called “wrack” also transport seeds of marsh plants that colonize the disturbed areas.
Sandy areas lacking vegetation are called salt pans. In these areas, the soil is so salty that not even salt marsh plants can tolerate it.
The salt marsh plants most tolerant of high salinities are succulents from the genera Batis, Salicornia and Sarcocornia.
The salt marsh plant most tolerant of flooding is the grass Spartina alterniflora. It occurs along the entire Atlantic Coast and Gulf Coast of the United States, dominating the lowest zone of the marsh.
The salt marsh grass Spartina alterniflora has been accidentally or deliberately introduced to a number of parts of the world that lack similar plants. In these areas, it colonizes intertidal mudflats and converts them into marshes. This is very harmful to the species that normally live on those mudflats.
Several species of crabs live in U.S. salt marshes. The squareback marsh crab, Armases cinereum, is an omnivorous species that lives at the upper marsh edge. This crab eats plants and climbs bushes to eat insects.
The sap-feeding plant hopper Prokelisia is a common insect in U.S. salt marshes. They are small (about 4 mm long) but abundant. Their density can exceed 1,000 adults per square meter.
Several species of birds live in U.S. salt marshes. The marsh wren feeds on insects and spiders.
The salt marsh grasshopper, Orchelimum, will dive into the water to escape bird predators.
How large the tides are varies geographically. In some regions there is less than 1 meter difference between high and low tide. In other regions, the difference is more than 3 meters.
The sea level in the marsh is controlled not just by the tides but also by the wind. In places where the tides are small, the wind may have the biggest effect on the water level.
The level of the marsh surface is controlled by several factors, including the amount of plants and roots, the amount of mud and sand suspended in the water, and how much the soils compact over time.
A salt marsh is will remove three times more energy from storm waves than a bare mud flat, because the vegetation baffles the waves. 
Marshes at high latitudes may be entirely covered by snow and ice in the winter.
At high latitudes, floating ice can carry chunks of marsh away into the ocean, or carry chunks of mud up on top of the marsh. This is known as ice rafting.
In winter the marsh vegetation moves nutrients out of the shoots into the roots, and the shoots die back. This is called senescence. In the spring, the plants move nutrients from the roots to support rapid growth of new shoots, and a new growing season begins.
Many saltmarshes form in estuaries and deltas because rivers carry large amounts of mud and sand that create the right conditions for marsh plants.
At high tide, when the marsh platform is flooded, fish swim out of the channels and through the vegetation to feed on the marsh surface.
Marshes are very different. Some have firm sediments and are easy to walk on, others are soft and you can sink in past your knees!
Marshes are divided into distinct zones, the high marsh and the low marsh. The low marsh floods daily at high tide. The high marsh floods only during very high tides associated with new and full moons.
The primary source of water to the marsh is seawater that floods the marsh at each high tide. But the upper marsh sometimes receives lots of freshwater from terrestrial habitats during heavy rains.
Tidal marshes serve many important functions. They buffer stormy seas, slow shoreline erosion, and absorb excess nutrients before they reach the oceans and estuaries.
High elevation parts of salt marshes that are not flooded by every high tide can become very salty as evaporation concentrates salts in the soil. This is especially true in arid regions where there is little rainfall that might dilute the salt.
Microscopic organisms like bacteria, small algae, and fungi help decompose the detritus resulting from salt marsh plants. These microorganisms and the remaining decomposing plant material become an ideal source of food for bottom-dwellers in salt marshes like worms, fishes, crabs, and shrimps.
Shortly following California’s 1849 Gold Rush population boom, many salt marshes of San Francisco Bay were surrounded by levees and converted to shallow evaporation ponds for salt production. To protect the endangered plants, birds, fish, and mammals that rely on a healthy salt marsh ecosystem, some of these salt ponds are now being restored back to tidal marshes.
Many salt marshes in the United States have been damaged or destroyed by dikes, ditches, or filling. Now that we realize how valuable salt marshes are, many groups are working to restore damaged marshes and create new ones.
The base of the salt marsh food web consists of microscopic algae living on the mud surface and the higher plants (angiosperms) that are more obvious at first glance. 
Microscopic algae living on the marsh surface are eaten by fiddler crabs.
Salt marsh plants are eaten by a variety of insects, including planthoppers and grasshoppers.
In Europe it is common to graze livestock, like sheep, on salt marshes. The meat from sheep raised on the marsh has a special flavor and sells for a premium price.
Salt marsh insects have lots of predators. They are eaten by predatory insects, spiders, fish and birds, including the marsh wren and seaside sparrow.
Fiddler crabs living in the marsh have lots of predators. They are eaten by mud crabs, blue crabs, large fish, wading birds, and raccoons.
Salt marsh snails, Littoraria, have lots of predators. They are eaten by blue crabs, mud crabs, and the salt marsh terrapin, which is a turtle that lives in salt marshes.
At high tide, large fish swim up onto the marsh to eat fiddler crabs, shrimp, snails and small fish living on the marsh.
The American alligator is one of the top predators found in the salt marsh. It was historically extremely common, but became very rare due to hunting. It is now protected, and its numbers are increasing.
Raccoons that live near salt marshes often go out into the marsh at low tide to feed on fiddler crabs. You can sometimes see them in the marsh, but more often you just see their footprints and their scat (feces) full of fiddler crab shells.
Male fiddler crabs have one large claw and one small one. The large claw is used to display to females. Because only the small claw can be used for feeding, male fiddler crabs take longer to feed than do females, which have two small claws.
Salt marshes have very few reptiles and amphibians, because it is hard for these animals to tolerate the salty water. Fresh water marshes, in contrast, have abundant reptiles and amphibians.
Salt marshes at high latitudes (like in New England) typically have soils that are mostly “peat” that is formed from dead plant material. In contrast, salt marshes at low latitudes (like in Georgia or Florida or Texas) typically have soils that are mostly “mineral” material—sand and clay. This happens for two reasons. First, decomposition is slower in cold climates, so dead plant material accumulates in the soil at high latitudes. Second, rivers deliver more sand and clay to the marshes at low latitudes compared to high latitudes.
Salt marsh plants are more productive and grow larger at low latitudes (like in Georgia and Texas) than high latitudes (like in New England). This happens because of the longer growing season and warmer temperatures at low latitudes.