Salt marshes are coastal wetlands found throughout the world on shorelines and on the edges of estuaries where freshwater mixes with seawater. An estuary is a partially enclosed body of water where freshwater from rivers and streams mixes with salt water from the ocean. Bays, inlets, harbors, and sounds can all be estuaries if they have a mixture of fresh and salt water. Salt marshes are transitional zones between the aquatic and terrestrial world. Further up the estuary there is a transition to fresh water marshes that are affected by tides, and still further up are non-tidal fresh water marshes. In the upper parts of estuaries where the salt water is more diluted with fresh water, they are referred to as “brackish” marshes.
Salt Marshes are conspicuous along the shorelines of the East Coast, Gulf Coast and West Coast of North America; they are also prominent European coastal features particularly in the British Isles, Romania, Spain, France, Denmark, Sweden and Norway. On the USA Atlantic coast, salt marshes are found in New England, become more extensive from New Jersey to northern Florida, and are most extensive on the coasts of the Carolinas and Georgia. Further south in Florida, they are replaced by mangrove swamps. Salt marshes are less common on the Pacific Coast, where the shoreline tends to be from along the Big Sur portion of the coast as well as the northern Sonoma and Mendocino portions of the California coast. There are large portions of the world coastline that have infrequent occurrence of salt marshes; for example, much of western Africa has little salt marsh incidence due to the prevalence of high dunes and an ongoing process of sand accumulation.
Salt marshes are often connected directly to a sea coast in the form of an embayment or estuary with shallow water features. In other cases the salt marsh is separated by a land barrier from the ocean body; examples of these brackish marshes abound in the Orkney Islands and the Black Sea plain of Romania. In some cases the separating land is a stabilized sand dune or raised beach landform. In many cases these coastal geometries change over millennia of time with dune accretion or coastal erosion. In a hybridized case of the Danube Delta, there are many hundreds of kilometres of braided estuarine salt marsh channels as well as hundreds of isolated pockets of salt marshes forming the entire delta wetland.
Physical and abiotic factors
Salt and brackish marshes are also sometimes called tidal marshes, since they occur in the zone between low and high tides. Marshes on the East Coast of the USA experience two high and two low tides a day, being alternately flooded and drained. The rise and fall of the tides, an obvious feature of shorelines, is caused by the gravitational pull of the sun and the moon on the Earth’s waters.
Salt marsh plants cannot grow where waves are strong, but thrive along quiet coasts. Salt marshes are periodically flooded by tides, so the plants living there must be able to deal with being submerged in salt water. This is stressful for two reasons: the salt and the water. The salinity, or salt content, varies depending upon whether the marshes are located directly adjacent to the ocean or further upstream in the estuary. Salt marsh soils tend to be waterlogged and low in oxygen which is also stressful to a plant. The water level and salinity level determine which species are found in a particular marsh.
The abiotic factors in salt marshes are highly variable, as the salinity can fluctuate greatly. The estuary is a place where incoming fresh water and ocean water mix, and the salinity can vary depending on the phase in the tidal cycle and the amount of rainfall. Salt marshes may experience salinities ranging from almost freshwater to full seawater, and anything living there must be able to tolerate these wide swings in environmental conditions. Temperature can also be widely variable, as the air in summer is much warmer than the water, and the air in winter is much colder than the water. Air temperature may be below freezing in winter and over 90° F in the summer. Because of these wide fluctuations, salt marshes do not have a great variety (biodiversity) of animals and plants; only a limited number of species can tolerate these conditions. Animals and plants in salt marshes include some that have terrestrial origins, like grasses, insects, birds and mammals, and others with marine origins like algae, mollusks, and fish.
Flora and fauna
Salt or brackish marshes are home to unique species of grasses, which are flowering plants found only in shallow intertidal areas. These plants are highly specialized and able to live in salt water and salty soil, and are therefore referred to as halophytes. They are also able to survive submerged in water part of the time, and are thus classified as hydrophytes. Tides play a major role in the lives of marsh animals as well, shaping their surroundings and behavior. Twice every day these marine creatures are exposed to the air and so must be able to cope with two quite different environments: an immersion in seawater during high tide, and exposure to air, sun, wind and possibly a dousing of fresh water in the form of rain during low tide. The higher up their location on the marsh, the longer their exposure to this alien environment. Marine animals must find a way to keep moist during low tide. When the tide is out, there is no food for those that obtain their food from the water. Many of these animals time their reproduction with the tides, often at the time of the full moon or new moon.