A wetland is an area of land consisting of soil that is saturated with moisture, such as a swamp, marsh, or bog.
As defined in terms of physical geography, a wetland is an environment "at the interface between truly terrestrial ecosystems and aquatic systems making them inherently different from each other yet highly dependent on both". In essence, wetlands are ecotones. Wetlands often host considerable biodiversity and endemism. In many locations such as the United Kingdom, Norway and United States they are the subject of conservation efforts and Biodiversity Action Plans.
Wetlands are found under a wide range of hydrological
conditions, but at least some of the time water saturates
. The result is a hydric soil
, one characterized by an absence of free oxygen some or all of the time, and therefore called a "reducing environment
or just wetland plants) specifically adapted to the reducing conditions presented by such soils can survive in wetlands, whereas species intolerant of the absence of soil oxygen (called "upland" plants) cannot survive. Adaptations to low soil oxygen characterize many wetland species.
There are many types of vegetation in wetlands. There are plants such as cattails, bulrushes, sedges, arrowhead, water lilies, blue flag, and floaters like common duckweed. Pondweed is also another type of plant that grows in wetlands, but it is not easily seen. Peatland can be dominated by red maple, silver maple, and elm trees. Some types of trees in peatland can exhibit lower trunks and roots that have adapted to the wet surroundings by forming buttresses,like the cypress, enlarged root bases to better support the trees in the mucky soil. Trees can also form knees, raised roots that allow for gas exchange. Swamps can also have whitecedar, tamarack, and white pine. Below the canopy, there are often limited amounts of shrubs such as speckled alder, Winterberry, and sweet gale.
Mangroves are a species of plant which typically thrive in coastal wetlands (called marine or estuarine environments). They are a special tree taxon that can survive in salty wetland water. Mangroves also provide the base for the wetland food chain. They are the producers in the wetland environment. Because mangroves add sulfur to the wetlands, it makes the water more acidic, therefore allowing decomposed matter in the water to biodegrade faster than it normally would, which in turn, provides more food for the organisms in the wetland ecosystem.
Generally, the hydrology of a wetland is such that the area is permanently or periodically inundated or saturated at the soil surface for a period of time during the growing season. The presence (or absence) of water is not necessarily a good method for identifying wetlands because the amount of water generally fluctuates depending on such things as rainfall patterns, snow melt, dry seasons, longer droughts, and tidal patterns. Often the same wetland can appear to be an open body of water sometimes and a dry field at other times because of significant fluctuations in water levels. The three water sources that contribute to wetlands are:
Location determines which of these sources will be contributing water to a wetland.
Generally, wetlands are located within topographic features that are lower in elevation that the surrounding landscape such as depressions, valleys, and flat areas. Wetlands can form on sloping topography where there is a water source consistently saturating the soil such as a groundwater seep or adjacent creek. Topography plays an important role in determining the size and shape of a wetland by controlling where the water goes and how long it stays there.
Below are terms used for various types of wetlands:
- A bog or muskeg is acidic peat land (peat bog).
- A moor was originally the same as a bog but has come to be associated with this soil type on hill-tops.
- A moss is a raised bog in Scotland
- A fen is a freshwater peat land with chemically basic (which roughly means alkaline) ground water. This means that it contains a moderate or high proportion of hydroxyl ions (pH value greater than 7).
- A carr is a fen which has developed to the point where it supports trees. It is a European term, mainly applied in the north of the UK.
- A freshwater marsh's main feature is its openness, with only low-growing or "emergent" plants. It may feature grasses, rushes, reeds, typhas, sedges, and other herbaceous plants (possibly with low-growing woody plants) in a context of shallow water. It is an open form of fen.
- A vernal pool (or ephemeral) is a shallow, freshwater pond that is seasonal, having water during rainy months and drying up completely during the remainder of the year. One distinctive characteristic is the lack of fish species in the pool.
- A coastal salt marsh may be associated with estuaries and along waterways between coastal barrier islands and the inner coast. The plants may extend from reed in mildly brackish water to salicornia on otherwise bare marine mud. It may be converted to human use as pasture (salting) or for salt production (saltern).
- A swamp is wetland with more open water surface and deeper water than a marsh. In North America, it is used for wetlands dominated by trees and woody bushes rather than grasses and low herbs, but this distinction does not necessarily apply in other areas, for instance in Africa where swamps may be dominated by papyrus.
- A dambo is a shallow, grass-covered depression of the central and southern African plateau which is waterlogged in the rainy season and usually forms the headwaters of a stream or river. It is marshy at the edges and at the headwater but maybe swampy in the centre and downstream.
- A mangrove swamp or mangal is a salt or brackish water environment dominated by the mangrove species of tree, such as Sonneratia.
- A paperbark wetland is a fresh or brackish water environment dominated by the Melaleuca tree.
- A bayou or slough is a southern United States terms for a creek amongst swamp. In an Indian mangrove swamp, it would be called a creek.
- A constructed wetland is artificially contrived wetland, intended to absorb flash floods, clean sewage, enhance wildlife or for some other human reason.
- A pocosin is a bog-like wetland dominated by fire-adapted shrubs and trees, found mainly in the southeastern United States on the Atlantic Coastal Plain.
- Seasonally flooded basins or flats.
- Inland fresh meadows.
- Inland shallow fresh water.
The Hydrogeomorphic (HGM) Approach is a system developed by the United States Army Corps of Engineers
to classify all wetlands based on three factors that influence how they function: position in the landscape (geomorphic setting), water source (hydrology), and the flow and fluctuation of the water once in the wetland (hydrodynamics). There are seven classes (types) of wetlands in this system:
- mineral soil flats
- organic soil flats
- estuarine fringe
- lacustrine fringe
This approach also intends to develop subclasses of wetlands to account for specific conditions of various regions.
Wetlands in drylands
In contrast to wetlands in other biomes
(usually permanent and fresh water), wetlands in drylands are more diverse in their composition, depending on the local climate and other particularities of the surroundings. They can be fresh or saline, permanent, seasonal or temporary, filling intermittently or regularly.
Wetlands in drylands can be attributed all values and uses of wetlands found in other biomes. However, given the stark contrast to their dry surroundings, many of these values are enhanced. This applies to the water balance where gradual release and storage of rainwater by wetlands amid drylands is crucial because of the unpredictability and incalculability of rain. During dry seasons, wetlands in drylands are also pivotal as refugia for wildlife, livestock and people. Moreover, biodiversity levels are higher than in wetlands in other major biomes, in particular because of the accessibility of water amid an otherwise very dry environment.
In intertidal wetlands the majority of natural stress comes from salinity and tidal movements
. The intertidal wetlands must be able to survive extreme conditions of mainly salt water at high tide, fresh water at low tide and times of flood and brackish water at other times. The saline water is a very difficult condition for plants to survive in. The grey mangrove accomplishes this by excluding salt in the root system, salt glands in the leaf, and waxy leaves to minimize water loss. However it is vulnerable to changes in salinity levels. Changes to tidal movements through increased run-off or altered drainage can cause the roots of mangroves to be inundated for longer than normal periods affecting their pneumatophones. It can also be pushed past its threshold level if water quality is changed. Thus even healthy ecosystems are vulnerable to change. Some species such as oysters
have been used as indicator species
, with any decline in their numbers indicating the ecosystem is under stress. A change in nutrient levels may also affect primary productivity and thus bring about change.
Hydrologic functions include long term and short term water storage, subsurface water storage, energy dissipation, and moderation of groundwater flow or discharge.
By absorbing the force of strong winds and tides, wetlands protect terrestrial areas adjoining them from storms, floods, and tidal damage.
Nutrient cycling, retention of particulates, removal of imported elements and compounds, and the import and export of organic carbon are all biogeochemical functions of wetlands. Wetlands remove nutrients from surface and ground water by filtering and by converting nutrients to unavailable forms. Denitrification
is arguably the most important of these reactions because humans have increased nitrate
worldwide by applying fertilizers
. Increased nitrate availability can cause eutrophication
, but denitrification converts biologically available nitrogen back into nitrogen gas, which is biologically unavailable except to nitrogen fixing bacteria. Denitrification can be detected in many soils, but denitrification is fastest in wetlands soils.
Intertidal wetlands provide an excellent example of invasion, modification and succession. The invasion and succession process is establishment of seagrasses. These help stabilize sediment and increase sediment capture rates. The trapped sediment gradually develops into mud flats. Mud flat organisms become established encouraging other life forms changing the organic composition of the soil.
Wetland provide a safe and lush environment for many different species of fish, birds, and insects. It includes the mallard duck
, the Sickleback fish
, mangroves, and water moccasins
Like animals, there are many plant communities that will only survive in the unique environmental conditions of a wetland. In the continental U.S. wetlands account for only 5% of the total land area, but over 30% of the nation's vascular flora occur in wetlands.
Mangroves establish themselves in the shallower water upslope from the mudflats. Mangroves further stabilize sediment and over time increase the soil level. This results in less tidal movement and the development of salt marshes. (succession) The salty nature of the soil means it can only be tolerated by special types of grasses e.g. saltbush, rush and sedge. There is also changing species diversity in each succession.
Value to humans
While many of the functions above are directly or indirectly beneficial to humans and society, wetlands are specifically valuable to people as places for recreational and educational activities such as hunting, fishing, camping, and wildlife observation. Wetlands are often filled in to be used by humans for everything from agriculture to parking lots, in part because the economic value of wetlands has only been recognized recently: the shrimp and fish that breed in salt water marshes are generally harvested in deeper water, for example. Humans can maximize the area of healthy, functioning wetlands by minimizing their impacts and by developing management strategies that protect, and where possible rehabilitate those ecosystems at risk.
Wetlands are sometimes deliberately created to help with water reclamation. One example is Green Cay Wetlands in Boynton Beach, Florida, in the United States.
While the U. S. Environmental Protection Agency estimates that human activities are responsible for about 60 percent of methane emissions worldwide, wetlands are among the natural sources. A study by Ohio State University scientists suggests that river floods and storms that send water surging through swamps and marshes near rivers and coastal areas might cut in half the average greenhouse gas emissions from those affected wetlands.
By 1993 half the world wetlands had been drained.
Protection and rehabilitation
Historically, humans have made large-scale efforts to drain wetlands for development or to flood them for use as recreational lakes. Since the 1970s, more focus has been put on preserving wetlands for their natural function—sometimes also at great expense. One example is the project by the U.S. Army Corps of Engineers to control flooding and enhance development by taming the Everglades
, a project which has now been reversed to restore much of the wetlands as a natural habitat and method of flood control.
The creation of the treaty known as the Ramsar Convention (1971), or more properly
"The Convention on Wetlands of International Importance, especially as Waterfowl
Habitat", demonstrates the global concern regarding wetland loss
and degradation. The primary purposes of the treaty are to list wetlands of international importance and to promote their wise use, with the ultimate goal of preserving the world's wetlands.Exclusion: Those responsible for the management of wetland areas often facilitate public access to a small, designated area while restricting access to other areas. Provision of defined boardwalks and walkways is a management strategy used to restrict access to vulnerable areas, as is the issuing of permits whilst visiting. Education: In the past, wetlands were regarded as wastelands. Education campaigns have helped to change public perceptions and foster public support for the wetlands. Because of their location in the catchment area, education programs need to teach about total catchment management programs. Educational programs include guided tours for the general public, school visits, media liaison, and information centers.
Wetlands by country
Since European settlement over 90% of the wetlands in New Zealand
have been drained, predominantly to create farmland. Wetlands now have a degree of protection under the Resource Management Act
Wetlands of the United States are defined by the United States Army Corps of Engineers and the United States Environmental Protection Agency as "those areas that are inundated or saturated by surface or ground water at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetations typically adapted for life in saturated soils. Wetlands generally include swamps, marshes, bogs, and similar areas."