Wetland
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In physical geography, a wetland is an environment "at the interface between truly terrestrial ecosystems and truly aquatic systems making them inherently different from each other yet highly dependent on both" (Mitsch & Gosselink, 1986). In essence, wetlands are ecotones. Wetlands often host considerable biodiversity and endemism. In many locations such as the United Kingdom and USA they are the subject of conservation efforts and Biodiversity Action Plans.
The United States Army Corps of Engineers and the Environmental Protection Agency jointly define wetlands 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 vegetation typically adapted for life in saturated soil conditions. Wetlands generally include swamps, marshes, bogs, and similar areas. [1]
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[edit] Characteristics
Wetlands are found under a wide range of hydrological conditions, but at least some of the time water saturates the soil. 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." Plants (called hydrophytes 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) can not survive. Adaptations to low soil oxygen characterize many wetland species.
Intertidal wetlands are found in coastal areas where air temperature, wave action, salinity levels, and sediment movements are moderated by the locational features of the estuarine environment and ecosystem.
[edit] Wetland types
- 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 fresh-water 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 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 are 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.
[edit] Wetland types found in the US
[edit] Inland fresh areas
- Seasonally flooded basins or flats
- Inland fresh meadows
- Inland shallow fresh marshes
- mangrove swamps
The U.S. Fish and Wildlife Service's National Wetlands Inventory (NWI) produces and provides information on the characteristics, extent, and status of U.S. wetlands and deepwater habitats and other wildlife habitats. The NWI also produces periodic reports on the status and trends of wetlands in the conterminous U.S. The NWI website includes a Wetlands Mapper in which users can view, download, or print maps of digital wetlands information.
[edit] Wetland functions
By absorbing the force of strong winds and tides, wetlands protect terrestrial areas adjoining them from storms, floods, and tidal damage. 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 (for an example, see Ullah and Faulkner 2006). Many wetlands also provide habitats for resident and migratory fish and wildlife.
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 soils.
The 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.
In the salt marshes there is greater species diversity, nutrient recycling, and niche specialisation making it one of the most productive ecosystems on Earth.
[edit] Adjustments to natural stress
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 and molluscs 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.
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 intertidal wetlands by minimising their impacts and by developing management strategies that protect, and where possible rehabilitate those ecosystems at risk.
[edit] Protecting or rehabilitating wetlands
Historically, humans have made large-scale efforts to drain wetlands for development or 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.
- 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. Due to 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.
[edit] Politics and Regulation of Wetlands Protection
[edit] In the USA
In the United States, some wetlands are regulated by the federal government under the Clean Water Act. Determining the boundary between regulated wetlands and non-regulated lands therefore can be contentious. Deciding where that boundary lies is as easy as determining the boundary between rock-n-roll music and country music, or between legal highway speeds and illegal highway speeds. In reality, there is no natural boundary between the classes that humans define on these gradients (wetland/upland, rock-n-roll/country, speeding/not speeding), and this issue is highlighted by the US. Fish and Wildlife Service’s definition from Classification of Wetlands and Deepwater Habitats of the United States, which defines wetlands as “lands transitional between terrestrial and aquatic systems.” Regulations to protect water quality and highway safety require that we create arbitrary boundaries within those gradients, but these boundaries are scientifically definable, and consist of areas where three criterion of the presence of hydric soils, the presence of wetland vegetation, and the presence of appropriate hydrology. Such regulations must be predictable, reproducible, and enforced. Otherwise, we will sacrifice clean water for development in the case of wetlands regulation (or vice versa), or sacrifice safe travel for quick travel (or vice versa) in the case of speed limits. Determining which wetlands are regulated under section 404 of the clean water act or section 10 of the rivers and harbors act is termed “jurisdictional determination.” Determining the boundary of wetland, whether jurisdictional under sections 404 or 10, or not jurisdictional but still meeting the technical definition of a wetland, that is having the soils, vegetation and hydrology criterion met is called a "wetland delineation," and generally is performed by college graduates with science or engineering degrees working for engineering firms or environmental consulting firms who are familiar with the 1987 U.S. Army Corps of Engineers Wetland delineation manual. Defining a boundary depends upon soil and vegetation characteristics; it is easier to do where the slope of the land is steeper. Deciding if a wetland is a regulated wetland depends on classifying the water in it as “water of the United States” or not. Classifying water as “of the U.S.” or “not of the U.S.” for purposes of enforcing the Clean Water Act suggests a natural boundary that probably does not exist in nature, and one that was not created regarding air for purposes of enforcing the Clean Air Act.
Wetlands are the focus of the US National Wetlands Coalition, which in turn has become the focus of some controversy over "false fronts," a form of political camouflage.
[edit] Notes
- ^ Definition from Corps of Engineers Wetlands Delineation Manual by Environmental Laboratory U.S. Army Corps of Engineers Waterways Experiment Station 3909 Halls Ferry Road Vicksburg, MS 39180-6199
[edit] See also
National Wetlands Research Center, United States
[edit] References
Ullah, S; Faulkner, SP. 2006. Denitrification potential of different land-use types in an agricultural watershed, lower Mississippi valley. ECOLOGICAL ENGINEERING 28 (2): 131-140.
