Wetland

In physical geography, a wetland is an environment "at the interface between truly terrestrial ecosystems...and truly aquatic systems...making them different from each yet highly dependent on both" (Mitsch & Gosselink, 1986). In essence, wetlands are ecotones.

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.

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 mangrove swamp or mangal is a salt or brackish water environment dominated by the mangrove species of tree, such as Sonneratia.
A marsh may be a fresh or a salt-water wetland. Its main feature is its openness, with only low-growing or "emergent" plants.
Coastal (salt)-marshes 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 mud.
A fresh-water marsh 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 a form of fen.
A fen is 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).
A carr is a fen which has developed to the point where it supports trees. It is more a north European term, as well as a southern.
A swamp is wetland dominated by trees rather than grasses and low herbs. The word tends to be used in warmer climates and in North America. It may be rather acidic.
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.
Constructed wetland is artificially contrived wetland. Intended to absorb flash floods, clean sewage, enhance wildlife or for some other human reason.

Wetland Types Found in the US

Inland Fresh Areas

Seasonally Flooded basins or flats
Inland Fresh Meadows
Inland Shallow Fresh Marshes* Mangrove swamps

Wetland functions

By absorbing the force of strong winds and tides, wetlands protect terrestrial areas adjoining them from storms, floods, and tidal damage. The plants in wetlands help to filter pollutants in the water. Fresh water marshes are often on river floodplains. 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.

Small wetland in Marshall County, Indiana.

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 decline in nutrient levels will 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 recognised 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.

Protecting or rehabilitating intertidal wetlands

A temperate wetland in Britain, with shallow open water and reedbeds.

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, information centres, conference presentations, interpretive signage, publications and facts sheets. Staff should also include education officers.
Action—Too little is known about the intertidal wetland system to successfully reinstate all natural conditions. Management plans focus on the rehabilitation of the site and the removal of human-induced stresses. For example, fox baiting, removal of weeds.
Design—Design interventions have proved successful in minimising sources of natural stress.
Legislation—Legislation and regulations are used to protect wetlands, in particular the Ramsar convention.

Negative human impacts

The two images show the loss of wetlands in Lafourche Parish, Louisiana from 1976 to 1999.

Changed wind patterns due to high-rise near some wetland areas. For example, Bicentennial Park.
Alteration of water flows through construction of roads. For example, Towra Point.
Removal—For urban and industrial land uses. These also increase turbidity and toxins in the water supplied to mangroves. (The removal can also result in changed energy flows and nutrient cycles, affecting food chains for both sedentary and migratory fauna.)
Replacement of wetland areas for parks, playing fields or pasture.
Destruction of sea grasses in areas adjoining wetlands can affect energy flows and nutrient cycles as species levels will be affected.
Introduction of exotic species. For example, foxes, rabbits, sheep, cattle, pigs, which change energy flows and nutrient cycles. Birds are particularly affected. For example, Little Tern.
Indirect influences from adjacent sites. For example, weed infestation (lantana, Towra Point), carried into the wetlands by horses from the nearby stables.
Trampling from illegal access.
Threat of oil spills. For example, Kurnell refinery on Towra Point.

Wetlands support a wide variety of wildlife (bird, plants, fish, mammals etc) and therefore the conservation of wetlands is of prime importance for the preservation of many species of wildlife. In 1962, the idea of wetlands conservation was born with a "List of Wetlands of International Importance". This was followed up in 1971 by the Ramsar Convention when conservationists from 23 countries met in the city of Ramsar, Iran on the shores of the Caspian Sea. There are now over 1,200 wetlands on the Ramsar List.

References

Mitsch, William J., and James G. Gosselink. 1986. Wetlands, New York: Van Nostrand Reinhold. 539 pp.
Campbell, Craig S., and Michael Ogden. 1999. Constructed Wetlands In The Sustainable Landscape. New York: John Wiley & Sons, Inc. 270 pp.

External links

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