Wetland



A wetland is a distinct semi-aquatic ecosystem whose groundcovers are flooded or saturated in water, either permanently, for years or decades, or only seasonally. Flooding results in oxygen-poor processes taking place, especially in the soils. Wetlands form a transitional zone between waterbodies and dry lands, and are different from other terrestrial or aquatic ecosystems due to their vegetation's roots having adapted to oxygen-poor waterlogged soils. They are considered among the most biologically diverse of all ecosystems, serving as habitats to a wide range of aquatic and semi-aquatic plants and animals, with often improved water quality due to plant removal of excess nutrients such as nitrates and phosphorus.
Wetlands exist on every continent, except Antarctica. The water in wetlands is either freshwater, brackish or saltwater. The main types of wetland are defined based on the dominant plants and the source of the water. For example, marshes are wetlands dominated by emergent herbaceous vegetation such as reeds, cattails and sedges. Swamps are dominated by woody vegetation such as trees and shrubs. Mangrove forest are wetlands with mangroves and halophytic woody plants that have evolved to tolerate salty water.
Examples of wetlands classified by the sources of water include tidal wetlands, where the water source is ocean tides; estuaries, water source is mixed tidal and river waters; floodplains, water source is excess water from overflowed rivers or lakes; and bogs and vernal ponds, water source is rainfall or meltwater, sometimes mediated through groundwater springs. The world's largest wetlands include the Amazon River basin, the West Siberian Plain, the Pantanal in South America, and the Sundarbans in the Ganges-Brahmaputra delta.
Wetlands contribute many ecosystem services that benefit people. These include for example water purification, stabilization of shorelines, storm protection and flood control. In addition, wetlands also process and condense carbon, and other nutrients and water pollutants. Wetlands can act as a sink or a source of carbon, depending on the specific wetland. If they function as a carbon sink, they can help with climate change mitigation. However, wetlands can also be a significant source of methane emissions due to anaerobic decomposition of soaked detritus, and some are also emitters of nitrous oxide.
Humans are disturbing and damaging wetlands in many ways, including oil and gas extraction, building infrastructure, overgrazing of livestock, overfishing, alteration of wetlands including dredging and draining, nutrient pollution, and water pollution. Wetlands are more threatened by environmental degradation than any other ecosystem on Earth, according to the Millennium Ecosystem Assessment from 2005. Methods exist for assessing wetland ecological health. These methods have contributed to wetland conservation by raising public awareness of the functions that wetlands can provide. Since 1971, work under an international treaty seeks to identify and protect "wetlands of international importance."

Definitions and terminology

Technical definitions

A simplified definition of wetland is "an area of land that is usually saturated with water". More precisely, wetlands are areas where "water covers the soil, or is present either at or near the surface of the soil all year or for varying periods of time during the year, including during the growing season". A patch of land that develops pools of water after a rain storm would not necessarily be considered a "wetland", even though the land is wet. Wetlands have unique characteristics: they are generally distinguished from other water bodies or landforms based on their water level and on the types of plants that live within them. Specifically, wetlands are characterized as having a water table that stands at or near the land surface for a long enough period each year to support aquatic plants.
A more concise definition is a community composed of hydric soil and hydrophytes.
Wetlands have also been described as ecotones, providing a transition between dry land and water bodies. Wetlands exist "...at the interface between truly terrestrial ecosystems and aquatic systems, making them inherently different from each other, yet highly dependent on both."
In environmental decision-making, there are subsets of definitions that are agreed upon to make regulatory and policy decisions.
Under the Ramsar international wetland conservation treaty, wetlands are defined as follows:
  • Article 1.1: "...wetlands are areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six meters."
  • Article 2.1: " may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six meters at low tide lying within the wetlands."
An ecological definition of a wetland is "an ecosystem that arises when inundation by water produces soils dominated by anaerobic and aerobic processes, which, in turn, forces the biota, particularly rooted plants, to adapt to flooding".
Sometimes a precise legal definition of a wetland is required. The definition used for regulation by the United States government is: 'The term "wetlands" means those areas that are inundated or saturated by surface or ground water at a frequency and duration to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions. Wetlands generally included swamps, marshes, bogs, and similar areas.'
For each of these definitions and others, regardless of the purpose, hydrology is emphasized. The soil characteristics and the plants and animals controlled by the wetland hydrology are often additional components of the definitions.

Types

Wetlands can be tidal or non-tidal. The water in wetlands is either freshwater, brackish, saline, or alkaline. There are four main kinds of wetlands – marsh, swamp, bog, and fen. Some experts also recognize wet meadows and aquatic ecosystems as additional wetland types. Sub-types include mangrove forests, carrs, pocosins, floodplains, peatlands, vernal pools, sinks, and many others.
The following three groups are used within Australia to classify wetland by type: Marine and coastal zone wetlands, inland wetlands and human-made wetlands. In the US, the best known classifications are the Cowardin classification system and the hydrogeomorphic classification system. The Cowardin system includes five main types of wetlands: marine, estuarine, riverine, lacustrine and palustrine.

Peatlands

are a unique kind of wetland where lush plant growth and slow decay of dead plants results in organic peat accumulating; bogs, fens, and mires are different names for peatlands.

Wetland names

Variations of names for wetland systems:
Some wetlands have localized names unique to a region such as the prairie potholes of North America's northern plain, pocosins, Carolina bays and baygalls of the Southeastern US, mallines of Argentina, Mediterranean seasonal ponds of Europe and California, turloughs of Ireland, billabongs of Australia, among many others.

Locations

By temperature zone

Wetlands are found throughout the world in different climates. Temperatures vary greatly depending on the location of the wetland. Many of the world's wetlands are in the temperate zones, midway between the North or South Poles and the equator. In these zones, summers are warm and winters are cold, but temperatures are not extreme. In subtropical zone wetlands, such as along the Gulf of Mexico, average temperatures might be. Wetlands in the tropics are subjected to much higher temperatures for a large portion of the year. Temperatures for wetlands on the Arabian Peninsula can exceed and these habitats would therefore be subject to rapid evaporation. In northeastern Siberia, which has a polar climate, wetland temperatures can be as low as. Peatlands in arctic and subarctic regions insulate the permafrost, thus delaying or preventing its thawing during summer, as well as inducing its formation.

By precipitation amount

The amount of precipitation a wetland receives varies widely according to its area. Wetlands in Wales, Scotland, and western Ireland typically receive about per year. In some places in Southeast Asia, where heavy rains occur, they can receive up to. In some drier regions, wetlands exist where as little as precipitation occurs each year.
Temporal variation:
Surface flow may occur in some segments, with subsurface flow in other segments.

Processes

Wetlands vary widely due to local and regional differences in topography, hydrology, vegetation, and other factors, including human involvement. Other important factors include fertility, natural disturbance, competition, herbivory, burial and salinity. When peat accumulates, bogs and fens arise.

Hydrology

The most important factor producing wetlands is hydrology, or flooding. The duration of flooding or prolonged soil saturation by groundwater determines whether the resulting wetland has aquatic, marsh or swamp vegetation. Other important factors include soil fertility, natural disturbance, competition, herbivory, burial, and salinity. When peat from dead plants accumulates, bogs and fens develop.
Wetland hydrology is associated with the spatial and temporal dispersion, flow, and physio-chemical attributes of surface and ground waters. Sources of hydrological flows into wetlands are predominantly precipitation, surface water, and groundwater. Water flows out of wetlands by evapotranspiration, surface flows and tides, and subsurface water outflow. Hydrodynamics affects hydro-periods by controlling the water balance and water storage within a wetland.
Landscape characteristics control wetland hydrology and water chemistry. The O2 and CO2 concentrations of water depend upon temperature, atmospheric pressure and mixing with the air. Water chemistry within wetlands is determined by the pH, salinity, nutrients, conductivity, soil composition, hardness, and the sources of water. Water chemistry varies across landscapes and climatic regions. Wetlands are generally minerotrophic with the exception of ombrotrophic bogs that are fed only by water from precipitation.
Because bogs receive most of their water from precipitation and humidity from the atmosphere, their water usually has low mineral ionic composition. In contrast, wetlands fed by groundwater or tides have a higher concentration of dissolved nutrients and minerals.
Fen peatlands receive water both from precipitation and ground water in varying amounts so their water chemistry ranges from acidic with low levels of dissolved minerals to alkaline with high accumulation of calcium and magnesium.