Coastal wetlands

Coastal wetlands are flooded periodically by tides, so that resident plants and animals spend part of the time in the water and part of the time in the air. Some of the plants and animals that live in these areas have origins as land species, such as grasses, insects, birds, and mammals. Others, such as mollusks, crabs, and fishes, come from the sea. The plants must be able to live in saltwater and in soil that tends to be waterlogged and low in oxygen. Twice a day, many of the marine animals must be able to survive exposure to the air, sun, wind, and possibly rain at low tide. These are harsh conditions, in which drastic changes occur regularly. Bays, inlets, harbors, and sounds are all places where freshwater from rivers and streams mixes with saltwater from the ocean; these areas are called estuaries. Salt marshes are found on protected shorelines and on the edges of estuaries. In the United States, they span the entire East Coast, and are extensive along the Gulf of Mexico, but are less common on the West Coast.

On the Pacific Coast, most of the shoreline tends to be rocky, so salt marshes are relatively scarce, except in the major estuary/marsh systems of California's San Francisco Bay and Washington's Puget Sound. They are also found in other coastal regions, from the Arctic tundra to the tropics, and on every continent, except Antarctica. Salt marshes are extensive between New Jersey and northern Florida, particularly along the coasts of the Carolinas and Georgia. Farther south, in Florida, they are replaced by mangrove swamps, which are the tropical equivalents of salt marshes. Mangroves are not grasses, but trees, and the aerial prop roots of red mangroves provide special habitat for juvenile fishes, some of which, as they mature, move to coral reefs.

The lower portions of marshes on the East Coast of the United States are alternately flooded and drained twice a day by the tides, caused by the gravitational pull of the sun and the moon on the water. The term flood tide refers to the incoming tide, while ebbtide indicates the falling, receding tide. The exact height and timing of the tides at any given place is determined by the phase of the moon and many other factors, such as the shape of the shoreline and the strength and direction of the wind. One extreme case is the Bay of Fundy in Nova Scotia, where there is a difference of almost 50 feet between high and low tides. In some places in the Caribbean, the difference between high and low water is less than one foot. The environmental conditions in a salt marsh are highly variable because the incoming freshwater and ocean water are always mixing in the estuary. If a marsh is located right on the ocean, its water will be more saline than the water in a marsh located farther upstream in an estuary that receives much freshwater. The ratio of saltwater to fresh predicts the habitat and resident species. A brackish marsh, for example, occurs in the portion of an estuary where the saltwater is more diluted with freshwater. Farther up the estuary, there is a transition to freshwater marshes that are still affected by the tides, although the concentration of salt is low. Still farther upstream are freshwater marshes that are untouched by the tides.

There is a parallel transition to less salt-tolerant plants and animals as the water in the marsh becomes fresher. Salinity also fluctuates, depending on the phase in the tidal cycle and the amount of recent rainfall. The same salt marsh may experience salinities ranging from almost freshwater to full strength seawater, and anything that lives there must be able to tolerate these wide swings in environmental conditions. The amount of dissolved oxygen undergoes similar swings as water alternately covers and uncovers the marsh. Temperature also varies widely across the seasons. The air in summer is much warmer than the water, but in winter, the water is warmer than the air. Plants and animals must cope with these variations on a daily basis, but despite these inhospitable and inconsistent conditions, salt marshes are thriving ecosystems.

Coastal Wetland Plant and Animal Species

Certain species of grasses found only in the shallow intertidal areas are highly specialized to deal with saltwater and salty soil. They are called halophytes (Greek hals, meaning salt, and phyton, meaning plant). Because they have to deal with being submerged in water part of the time, they are also considered hydrophytes (hydro, meaning water). In addition to having to tolerate salt and being immersed in water, the lack of oxygen in the waterlogged marsh soil also makes the environment stressful to plants. In East Coast marshes, this zone is occupied by cord grass, Spartina alterniflora. The low marsh zone in some other regions frequently lacks vegetation altogether; these areas are mudflats. The diversity of plant species increases as one moves higher in the marsh to areas that are only affected by the saltwater during very high tides. In the high marsh, there are grasses, rushes, herbs, and shrubs, with different species adapted to different zones. Zonation is a typical feature of salt marshes, with different plant species found at different elevations, depending on their ability to tolerate immersion and salt, and their ability to compete for space.

Marine animals must find a way to keep moist during low tide, and when the tide is out, there is no food for those that obtain their food directly from the water. Most marsh animals are inactive during low tide, with a few exceptions. Fiddler crabs (genus Uca) forage at low tide and remain in their burrows at high tide, safe from predatory fish. Smaller crustaceans, called amphipods and isopods, may also remain active on the marsh surface during low tide, and the salt marsh mussel (Geukensia demissa) frequently “air gapes” (gape their shells open for air) at low tide. Virtually all residents of the salt marsh time their lives and their reproductive cycles according to the tides and the phases of the moon. On the highest tides (at the full and new moon), fiddler crabs release their young where tidal currents can carry them out to the ocean. Horseshoe crabs come up onto the beach to lay their eggs at the high tides before the full moon.

Marsh Accretion

Salt marshes develop gradually when tides deposit sediments (soil, fine-grained clay, sand, and silt from rivers and shorelines) across low-lying land, creating wet mudflats or sand flats. Salt marshes develop in sheltered areas that are flat and drain slowly, where mats of microscopic algae and bacteria form and stabilize the sediment so that its elevation can increase. This process speeds up when the plants take root as the marsh ecosystem develops. Salt-tolerant grasses arrive when plants raft in on the tides and seeds are carried in by the air or water. The plants slowly take hold and spread, usually by means of underground horizontal stems or rhizomes, which further stabilize the sediment through the growth of dense root systems and add volume, helping to raise elevations. Once the grasses are in place, the force of wave action is reduced, thus permitting additional layering and settling of the sediment to occur. As the plants decay, fine sediment builds up, causing marsh accretion. As sea levels rise, the marsh pushes landward and seaward, and freshwater plants on the land are replaced by marsh grasses that can tolerate the salt. Underneath the marsh, peat accumulates from the belowground (root and rhizome) plant debris. Peat formation eventually raises the elevation of the sediment surface so that it is flooded less of the time. At that point, other somewhat less salt- and flood- tolerant plant species are able to survive. Eventually, this leads to the development of high marsh areas that are flooded less often by the incoming tides. The bulk of the roots and rhizomes contribute to both sediment deposition and marsh accretion. The rate that matches marshland growth to changing sea level has been about 4 to 10 inches (10.16 to 25.4 centimeters) per century. With climate change causing thermal expansion of water and melting of glaciers, sea level is rising faster than the sedimentation rate in many areas. Unless marsh accretion can keep pace with sea-level rise, marshes may be replaced by open water.

The result of the natural sedimentation is a general spreading of the marsh system and development of the pattern of zonation of plants, in which particular species are located in bands at different elevations. A fully developed marsh includes creeks that form a network over the marsh and are routes used by tidal water as it enters and leaves the marsh, the primary link of the estuary to the land. The general ecology of salt marshes and the roles played by different species of animals and plants are seen worldwide. Salt marshes are important for many reasons. Many species of migratory shorebirds depend on tidal wetlands as stop-over points during their migration between summer and winter habitats, and some birds overwinter in the marsh. Wading birds such as egrets and great blue herons feed in the productive salt marshes during summer months.

About 85 percent of waterfowl and migratory birds use coastal areas for resting, feeding, and breeding habitat. Estuarine animals in various stages of life can be found among the salt marsh plants. At high tide, juveniles of small fishes move up and swim among the stems of the marsh grasses, where they are protected from larger predators that cannot penetrate the shallow waters of the creeks. It is estimated that over two-thirds of the commercially harvested fish on the East Coast spend some portion of their life cycle in the marsh-estuary ecosystem. Salt marshes are thus considered nurseries, where many species of fish and shellfish live during the early stages of their lives, depending on the marsh for food and shelter. The plants provide shelter for spawning and protect the juveniles of many different species of commercial and ecological importance, including blue crabs, croakers, flounders, and spot. Bluefish spawn out in the ocean, for example, but the juveniles (snappers) move into estuaries in the spring and remain there through the summer, feasting on the smaller fish that live in the marsh/estuarine system. Some fish, like the mummichog or common killifish (Fundulus heteroclitus), spend their entire lives in the marsh/estuary system.

Functional Nature of Marshes

Salt marshes also serve to protect coastal areas from storms and floods. They can stabilize shorelines because they can take the brunt of storm waves, buffering the shore from flood and storm damage. One common source of flood damage occurs following storms when runoff from the land hits the coastal plain. Marshes are natural sponges that can absorb much of this runoff, reducing its impact on coastal environments and real estate. Established marsh grasses are also very effective against erosion. The roots and rhizomes of marsh plants help the sediments cohere and consolidate, resulting in less erosion in vegetated areas. When marshes are removed, the effects of storms on coastal communities are much more severe and devastating, and therefore more costly. In places where marshes are strong, functional systems, storm damage to the land and coastal communities is much less severe. In Sri Lanka, for example, the in December 2004 caused much less devastation in areas where mangroves were present than in areas where they had been removed. Hurricane Katrina in 2005 produced a catastrophe in Louisiana in part because of the loss of tidal wetlands.

Marshes also purify water, filtering out pollutants, sediments, and nutrients from the water that washes down from the land. Marshes along the shore slow the movement of runoff from land into the estuary while they increase sedimentation and the removal of wastes. Salt marsh sediments retain or sequester many kinds of toxic contaminants, helping to reduce the degree of contamination of coastal waters, and intercept nutrients that arrive in runoff from the land, protecting adjacent estuarine areas from excess nutrients that can be harmful. Resident microbes in marshes can remove wastes from the water. They break down and process pollutants in their normal metabolism. In some areas, engineers have constructed wetlands specifically to treat wastewater, a process that capitalizes on this natural ability.

Marshes are highly productive, rivaling tropical rainforests in producing the most basic food energy per acre. They are highly productive because they are rich in nutrients and cycle the nutrients very efficiently. They also have many different types of plants that perform photosynthesis: rooted grasses, seaweeds, and microscopic algae that are found in the water (plankton) and living on the surface of the mud. There have been some attempts to attach a dollar value to the services provided by salt marshes. For example, it has been estimated that the amount of annual storm surge protection services provided to areas most vulnerable to hurricane and tropical storms is $23 billion. However, monetizing their benefits and then making decisions about the conservation or destruction of that region in purely economic terms is an overly simplistic approach. Many of the important functions of a marsh are intangible and its benefits accrue to everyone, so fixing its value is unrealistic, if not impossible. Unfortunately, U.S. coastal wetlands are in decline, and are further threatened by increasing coastal development and rising sea levels.

Coastal Disturbances

Marshes can be disturbed by both natural factors and human activities. Natural disturbance by ice and floating debris can damage temperate marshes, particularly in the colder northern areas. Tidal action can lift up pieces of marsh and deposit them on higher areas, where they literally suffocate the underlying vegetation. Floating masses of dead plant material (wrack) are produced in the fall, when some plants die and collapse, and then are swept away by the tides. Winter ice can amass and destroy large chunks of low marsh vegetation. Fire and overgrazing by animals like geese can also destroy large areas of marshes. However, these effects are trivial compared with the damage that results from human activities. Early settlers in America found marshes important for raising livestock, but when agriculture moved inland because of expanded settlement, salt marshes were regarded as useless wastelands that were extensively filled for expanding towns and cities. In the twenty-first century rising sea levels caused by global warming threaten marshes and other coastal wetlands.

Since the 1950s, an average of over 25 square miles (65 square kilometers) per year of Louisiana's coastal wetlands has been lost because of numerous factors, including development and oil and gas exploration and production. The engineering of channels and the system of levees along the Mississippi River prevents regular tidal flooding, stopping the flow and process of salt marsh sedimentation and vertical growth. The normal accumulation of sediments washing down into the marsh cannot occur, and damage from hurricanes is exacerbated. If the area had been undisturbed, accumulated sediments would have elevated the marshes and reduced the effects of the storm surge. The 2005 hurricanes caused the additional loss of 215 square miles (557 square kilometers) of salt marsh, an order of magnitude greater than the average annual loss. According to the Army Corps of Engineers, for every 3 square miles of wetlands, storm surge can be reduced by a foot. Among other lessons, Hurricane Katrina and its aftermath demonstrate the importance that wetlands play in protecting communities from floods.

As more and more people choose to live in coastal areas, impacts on tidal marshes have increased. Today, larger coastal cities and towns hug the shores of estuaries, continuing to encroach on these vital habitats. Cities developed in these locations because they were ideal for trade when goods were carried by ships. Forest and agricultural products could be easily moved downriver to a coastal port. Ports were often dredged out of marshes, and networks of roadways and railroads were built that crossed the marshes in every direction. This construction disrupted the flow of water, impeded tidal exchange, and slowly strangled the marshes.

Marshes have been filled, drained, diked, ditched, grazed, and harvested. They have become depositories for pollutants that wash into streams and rivers from the land, as well as for contaminants introduced from coastal waters. They have been sprayed with insecticides for mosquito control, chemicals that have a wide range of negative effects on desirable estuarine animals and plants. A range of nonnative species have been introduced or arrived accidentally and have altered their ecology. East Coast marshes have been affected by a strain of the common reed, Phragmites australis, which has taken over many lower salinity marshes, reducing the diversity of native plant species, while the native Spartina alterniflora of East Coast marshes has become an undesired invasive species on the West Coast and in China.

Salt marshes have been treated as something devalued and “reclaimed”—that is, converted to a use that is considered to have more value. There was previously a widespread perception that wetlands were wastelands and pestilent swamps. They were drained, dredged, and filled with dredge materials for urban development, fenced in for livestock grazing, or polluted through use as garbage dumps. In the past several decades, however, people have begun to understand wetlands, to appreciate the “ecosystem services” they provide, free of charge.

Mudflats and salt marshes lack the public appeal and mystique of tropical rainforests and coral reefs, although they are interesting and ecologically valuable. While protective legislation in the 1970s slowed the rate of loss, marshes are still degraded by the invasion of exotic species, poor management practices, development of urban and suburban areas, and the rise in sea level concomitant with climate change. It is estimated that 65 percent of all coastal marshlands and swamps in the contiguous United States would be inundated with 3 feet of sea-level rise. As marshes are destroyed and altered, even if at a slower rate than in the past, the vulnerability to floods and storms, the purity of water systems, the vitality of communities, and the health of populations are all at risk. A myriad of plant and animal species face dire consequences if coastal marshlands continue to be lost or exploited.

In the late 1980s, it was estimated that about 60,000 acres (249 square kilometers) of wetlands were lost per year. Since President George H. W. Bush established a national policy of no-net-loss-of-wetlands in 1989, the overall rate of loss has slowed down. However, from 1996 to 2006, 431 square miles (1,116 square kilometers) of coastal marshes were lost from the United States. Numerous conservation and restoration projects have taken place, with varying degrees of success. The practice of creating new salt marsh habitats to replace ones that have been destroyed elsewhere is widespread. But in many cases, there has been little, if any, follow-up to measure the success of the project. Despite increased public awareness of the value of wetlands, they are still losing ground to development, and some people still view them merely as obstacles in the way of urban expansion. Salt marshes and mangroves also provide unique opportunities for fishing, kayaking, and educational field trips. They are fascinating places to study, and should be valued for their beauty, wilderness, and the sense of peace and tranquility that they provide.