Wetlands

(ecology) An area characterized by a high content of soil moisture, such as a swamp or bog.

Ecosystems that form transitional areas between terrestrial and aquatic components of a landscape. Typically they are shallow-water to intermittently flooded ecosystems, which results in their unique combination of hydrology, soils, and vegetation. Examples of wetlands include swamps, fresh- and salt-water marshes, bogs, fens, playas, vernal pools and ponds, floodplains, organic and mineral soil flats, and tundra. As transitional elements in the landscape, wetlands often develop at the interface between drier uplands such as forests and farmlands, and deep-water aquatic systems such as lakes, rivers, estuaries, and oceans. Thus, wetland ecosystems are characterized by the presence of water that flows over, ponds on the surface of, or saturates the soil for at least some portion of the year.

Wetland soils can be either mineral (composed of varying percentages of sand, silt, or clay) or organic (containing 12–20% organic matter). Through their texture, structure, and landscape position, soils control the rate of water movement into and through the soil profile (the vertical succession of soil layers). Retention of water and organic carbon in the soil environment controls biogeochemical reactions that facilitate the functioning of wetland soils. See also Biogeochemistry.

Vegetated wetlands are dominated by plant species, called hydrophytes, that are adapted to live in water or under saturated soil conditions. Adaptations that allow plants to survive in a water-logged environment include morphological features, such as pneumatophores (the “knees,” or exposed roots, of the bald cypress), buttressed tree trunks, shallow root systems, floating leaves, hypertrophied lenticels, inflated plant parts, and adventitious roots. Physiological adaptations also allow plants to survive in a wetland environment. These include the ability of plants to transfer oxygen from the root system into the soil immediately surrounding the root (rhizosphere oxidation); the reduction or elimination of ethanol accumulation due to low concentrations of alcohol dehydrogenase; and the ability to concentrate malate (a nontoxic metabolite) instead of ethanol in the root system. See also Root (botany).

Wetlands differ with respect to their origin, position in the landscape, and hydrologic and biotic characteristics. For example, work has focused on the hydrology as well as the geomorphic position of wetlands in the landscape. This hydrogeomorphic approach recognizes and uses the fundamental physical properties that define wetland ecosystems to distinguish among classes of wetlands that occur in riverine, depressional, estuarine or lake fringe, mineral or organic soil flats, and slope environments.

The extent of wetlands in the world is estimated to be 2–3 × 10⁶ mi² (5–8 × 10⁶ km²), or about 4–6% of the Earth's land surface. Wetlands are found on every continent except Antarctica and in every clime from the tropics to the frozen tundra. Rice paddies, which comprise another 500,000–600,000 mi² (1.3–1.5 × 10⁶ km²), can be considered as a type of domesticated wetland of great value to human societies worldwide. See also Bog; Mangrove; Muskeg; Playa; Salt marsh; Tundra.

Wetlands are often an extremely productive part of the landscape. They support a rich variety of waterfowl and aquatic organisms, and represent one of the highest levels of species diversity and richness of any ecosystem. Wetlands are an extremely important habitat for rare and endangered species.

Wetlands often serve as natural filters for human and naturally generated nutrients, organic materials, and contaminants. The ability to retain, process, or transform these substances is called assimilative capacity, and is strongly related to wetland soil texture and vegetation. The assimilative capacity of wetlands has led to many projects that use wetland ecosystems for wastewater treatment and for improving water quality. Wetlands also have been shown to prevent downstream flooding and, in some cases, to prevent ground-water depletion as well as to protect shorelines from storm damage. The best wetland management practices enhance the natural processes of wetlands by maintaining conditions as close to the natural hydrology of the wetland as possible. See also Ground-water hydrology.

The world's wetlands are becoming a threatened landscape. Loss of wetlands worldwide currently is estimated at 50%. Wetland loss results primarily from habitat destruction, alteration of wetland hydrology, and landscape fragmentation. Global warming may soon be added to this list, although the exact loss of coastal wetlands due to sea-level rise is not well documented. Worldwide, destruction of wetland ecosystems primarily has been through the conversion of wetlands to agricultural land.

Hydrologic modifications that destroy, alter, and degrade wetland systems include the construction of dams and water diversions, ground-water extraction, and the artificial manipulation of the amount, timing, and periodicity of water delivery. The primary impact of landscape fragmentation on wetland ecosystems is the disruption and degradation of wildlife migratory corridors, reducing the connectivity of wildlife habitats and rendering wetland habitats too small, too degraded, or otherwise irreversibly altered to support the critical life stages of plants and animals.

The heavy losses of wetlands in the world, coupled with the recognized values of these systems, have led to a number of policy initiatives at both the national and international levels.

Wetland restoration usually refers to the rehabilitation of degraded or hydrologically altered wetlands, often involving the reestablishment of vegetation. Wetland enhancement generally refers to the targeted restoration of one or a set of ecosystem functions over others, for example, the focused restoration of a breeding habitat for rare, threatened, or endangered amphibians. Wetland creation refers to the construction of wetlands where they did not exist before. Created wetlands are also called constructed or artificial wetlands. Restoring, enhancing, or creating a wetland requires a comprehensive understanding of hydrology and ecology, as well as engineering skills. See also Dam; Ecosystem; Estuarine oceanography; Hydrology; River engineering; Restoration ecology.