A wetland is a distinct ecosystem that is flooded by water, either permanently (for years or decades) or seasonally (for weeks or months). Flooding results in oxygen-free ( anoxic) processes prevailing, especially in the soils.  The primary factor that distinguishes wetlands from terrestrial land forms or water bodies is the characteristic vegetation of aquatic plants, adapted to the unique anoxic hydric soils.  Wetlands are considered among the most biologically diverse of all ecosystems, serving as home to a wide range of unique plant and animal species. Methods for assessing wetland functions, wetland ecological health, and general wetland condition have been developed for many regions of the world. These methods have contributed to wetland conservation partly by raising public awareness of the functions some wetlands provide. 
Wetlands occur naturally on every continent,  except for Antarctica. The water in wetlands is either freshwater, brackish or saltwater.  The main wetland types are classified based on the dominant plants and/or the source of the water. For example, marshes are wetlands dominated by emergent vegetation such as reeds, cattails and sedges; swamps are ones dominated by woody vegetation such as trees and shrubs (although reed swamps in Europe are dominated by reeds, not trees). Examples of wetlands classified by their sources of water include tidal wetlands ( oceanic tides), estuaries (mixed tidal and river waters), floodplains (excess water from overflowed rivers or lakes), springs, seeps and fens ( groundwater discharge out onto the surface), bogs and vernal ponds ( rainfall or meltwater).   Of course, some wetlands will have multiple types of plants and be fed by multiple sources of water, making them difficult to classify.
Wetlands contribute a number of functions that benefit people. These are called ecosystem services and include water purification, groundwater replenishment, stabilization of shorelines and storm protection, water storage and flood control, processing of carbon ( carbon fixation, decomposition and sequestration), other nutrients and pollutants, and support of plants and animals.  Wetlands are reservoirs of biodiversity and provide wetland products. They also place a role in climate change mitigation and adaptation. However, some wetlands are a significant source of methane emissions and some are also emitters of nitrous oxide.   Constructed wetlands are designed and built to treat municipal and industrial wastewater as well as to divert stormwater runoff. Constructed wetlands may also play a role in water-sensitive urban design.
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.  According to the UN Millennium Ecosystem Assessment, wetlands are more affected by environmental degradation that than any other ecosystem on Earth. 
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.  
Wetlands have also been described as ecotones, providing a transition between dry land and water bodies.  Mitsch and Gosselink write that 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.
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." : 2
- 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 metres."
- Article 2.1: "[Wetlands] may incorporate riparian and coastal zones adjacent to the wetlands, and islands or bodies of marine water deeper than six metres at low tide lying within the wetlands."
The water in wetlands is either freshwater, brackish, or saltwater.  There are four main kinds of wetlands – marsh, swamp, bog and fen (bogs and fens being types of mires). Some experts also recognize wet meadows and aquatic ecosystems as additional wetland types.  Sub-types include mangrove forest, carr, pocosin, floodplains,  mire, vernal pool, sink, and many others.  Many peatlands are wetlands. Wetlands can be tidal (inundated by tides) or non-tidal. 
The following three groups are used within Australia to classify wetland by type: Marine and coastal zone wetlands, inland wetlands and human-made wetlands.  Other classification systems for wetlands exist. In the US, the best known are the Cowardin classification system  and the hydrogeomorphic (HGM) classification system. The Cowardin system includes five main types of wetlands.
Peatlands are a unique kind of wetland where lush plant growth and slow decay of dead plants (under anoxic conditions) results in organic peat accumulating; bogs, fens, and mires are different names for peatlands.
Some wetlands have localized names unique to a region such as the prairie potholes of North America's northern plain, pocosins and Carolina bays of the Southeastern US, mallines of Argentina, Mediterranean seasonal ponds of Europe and California, turloughs of Ireland, billabongs of Australia, among many others.
The most important factor producing wetlands is 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 fertility, natural disturbance, competition, herbivory, burial and salinity.  When peat accumulates, bogs and fens arise.
Wetland hydrology is associated with the spatial and temporal dispersion, flow, and physio-chemical attributes of surface and ground water in its reservoirs. Based on hydrology, wetlands can be categorized as riverine (associated with streams), lacustrine (associated with lakes and reservoirs), and palustrine (isolated). Sources of hydrological flows into wetlands are predominantly precipitation, surface water, and groundwater. Water flows out of wetlands by evapotranspiration, surface runoff, and subsurface water outflow. Hydrodynamics (the movement of water through and from a wetland) affects hydro-periods (temporal fluctuations in water levels) by controlling the water balance and water storage within a wetland. 
Landscape characteristics control wetland hydrology and hydrochemistry. The O2 and CO2 concentrations of water depend on temperature and atmospheric pressure. Hydrochemistry within wetlands is determined by the pH, salinity, nutrients, conductivity, soil composition, hardness, and the sources of water. Water chemistry of wetlands varies across landscapes and climatic regions. Wetlands are generally minerotrophic with the exception of bogs.
Bogs receive most of their water from the atmosphere; therefore, their water usually has low mineral ionic composition. In contrast, groundwater has a higher concentration of dissolved nutrients and minerals.
The water chemistry of fens ranges from low pH and low minerals to alkaline with high accumulation of calcium and magnesium because they acquire their water from precipitation as well as ground water. 
Salinity has a strong influence on wetland water chemistry, particularly in wetlands along the coast.   and in regions with large precipitation deficits. In non-riverine wetlands, natural salinity is regulated by interactions between ground and surface water, which may be influenced by human activity. 
Carbon is the major nutrient cycled within wetlands. Most nutrients, such as sulfur, phosphorus, carbon, and nitrogen are found within the soil of wetlands. Anaerobic and aerobic respiration in the soil influences the nutrient cycling of carbon, hydrogen, oxygen, and nitrogen,  and the solubility of phosphorus  thus contributing to the chemical variations in its water. Wetlands with low pH and saline conductivity may reflect the presence of acid sulfates  and wetlands with average salinity levels can be heavily influenced by calcium or magnesium. Biogeochemical processes in wetlands are determined by soils with low redox potential.  Wetland soils are identified by redoxymorphic mottles or low chroma, as determined by the Munsell Color System.
Due to the low dissolved oxygen (DO) content, and relatively low nutrient balance of wetland environments, wetlands are very susceptible to alterations in water chemistry. Key factors that are assessed to determine water quality include:
- Major anion analysis: (HCO3−,Cl−,NO3−,SO42-)
- Major cation analysis (Ca2+, Mg2+, Na+, K+)
- Conductivity- conductivity increases with more dissolved ions in the water
- Dissolved oxygen
- Total dissolved solids
- Gas emissions ( carbon dioxide and methane; CO2 and CH4)
These chemical factors can be used to quantify wetland disturbances, and often provide information as to whether a wetland is surface water fed or groundwater fed due to the different ion characteristics of the two water sources.  Wetlands are adept at impacting the water chemistry of streams or water bodies that interact with them, and can withdraw ions that result from water pollution such as acid mine drainage or urban runoff.,  
The biota of a wetland system includes its flora and fauna as described below. The most important factor affecting the biota is the duration of flooding.  Other important factors include fertility and salinity. In fens, species are highly dependent on water chemistry. The chemistry of water flowing into wetlands depends on the source of water and the geological material in which it flows through  as well as the nutrients discharged from organic matter in the soils and plants at higher elevations in slope wetlands.  Biota may vary within a wetland due to season or recent flood regimes.
Submerged wetland vegetation can grow in saline and fresh-water conditions. Some species have underwater flowers, while others have long stems to allow the flowers to reach the surface.  Submerged species provide a food source for native fauna, habitat for invertebrates, and also possess filtration capabilities. Examples include seagrasses and eelgrass.
Floating water plants or floating vegetation is usually small, like arrow arum ( Peltandra virginica).
Trees and shrubs, where they comprise much of the cover in saturated soils, qualify those areas in most cases as swamps.  The upland boundary of swamps is determined partly by water levels. This can be affected by dams  Some swamps can be dominated by a single species, such as silver maple swamps around the Great Lakes.  Others, like those of the Amazon basin, have large numbers of different tree species.  Examples include cypress ( Taxodium) and mangrove.
Fish are more dependent on wetland ecosystems than any other type of habitat. Seventy-five percent of the United States' commercial fish and shellfish stocks depend solely on estuaries to survive.  Tropical fish species need mangroves for critical hatchery and nursery grounds and the coral reef system for food.
Amphibians such as frogs need both terrestrial and aquatic habitats in which to reproduce and feed. While tadpoles control algal populations, adult frogs forage on insects. Frogs are used as an indicator of ecosystem health due to their thin skin which absorbs both nutrient and toxins from the surrounding environment resulting in an above average extinction rate in unfavorable and polluted environmental conditions. 
Reptiles such as alligators and crocodiles are common in wetlands of some regions. Alligators occur in fresh water along with the fresh water species of the crocodile. The Florida Everglades is the only place in the world where both crocodiles and alligators coexist.  The saltwater crocodile inhabits estuaries and mangroves and can be seen in the coastline bordering the Great Barrier Reef in Australia.  Snakes, lizards and turtles also can be seen throughout wetlands. Snapping turtles are one of the many kinds of turtles found in wetlands.
Mammals include numerous small and medium-sized species such as voles, bats, and platypus in addition to large herbivorous and apex species such as the beaver, coypu, swamp rabbit, Florida panther, and moose. Wetlands attract many mammals due to abundant seeds, berries, and other vegetation components, as well as abundant populations of prey such as invertebrates, small reptiles and amphibians. 
Invertebrates of wetlands include aquatic insects (such as dragonflies, aquatic bugs and beetles, midges, mosquitoes), crustaceans (such as crabs, crayfish, shrimps, microcrustaceans), mollusks (such as clams, mussels, snails), and worms (such as polychaetes, oligochaetes, leeches), among others. Invertebrates comprise more than half of the known animal species in wetlands, and are considered the primary food web link between plants and higher animals (such as fish and birds).  The low oxygen conditions in wetland water and their frequent flooding and drying (daily in tidal wetlands, seasonally in temporary ponds and floodplains) prevent many invertebrates from inhabiting wetlands, and thus the invertebrate fauna of wetlands is often less diverse than some other kinds of habitat (such as streams, coral reefs, and forests). Some wetland invertebrates thrive in habitats that lack predatory fish. Many insects only inhabit wetlands as aquatic immatures (nymphs, larvae) and the flying adults inhabit upland habitats, returning to the wetlands to lay eggs. For instance, a common hoverfly Syritta pipiens inhabits wetlands as larvae (maggots), living in wet, rotting organic matter; these insects then visit terrestrial flowers as adult flies.
Algae are diverse plant-like organisms that can vary in size, color, and shape. Algae occur naturally in habitats such as inland lakes, inter-tidal zones, and damp soil and provide a dedicated food source for many animals, including some invertebrates, fish, turtles, and frogs. There are three main groups of algae:
- Plankton are algae which are microscopic, free-floating algae. This algae is so tiny that on average, if 50 of these microscopic algae were lined up end-to-end, it would only measure one millimetre. Plankton are the basis of the food web and are responsible for primary production in the ocean using photosynthesis to make food.
- Filamentous algae are long strands of algae cells that form floating mats.
- Chara and Nitella algae are upright algae that look like a submerged plant with roots. 
Depending on their characteristics, some wetlands are a significant source of methane emissions and some are also emitters of nitrous oxide.   This is a greenhouse gas with a global warming potential 300 times that of carbon dioxide and is the dominant ozone-depleting substance emitted in the 21st century.  Excess nutrients mainly from anthropogenic sources have been shown to significantly increase the N2O fluxes from wetland soils through denitrification and nitrification processes (see table below).    A study in the intertidal region of a New England salt marsh showed that excess levels of nutrients might increase N2O emissions rather than sequester them. 
|Wetland type||Location||N2O flux
(µmol N2O m−2 h−1)
|Mangrove||Shenzhen and Hong Kong||0.14 – 23.83|||
|Mangrove||Muthupet, South India||0.41 – 0.77|||
|Mangrove||Bhitarkanika, East India||0.20 – 4.73|||
|Mangrove||Pichavaram, South India||0.89 – 1.89|||
|Mangrove||Queensland, Australia||−0.045 – 0.32|||
|Mangrove||South East Queensland, Australia||0.091 – 1.48|||
|Mangrove||Southwest coast, Puerto Rico||0.12 – 7.8|||
|Mangrove||Isla Magueyes, Puerto Rico||0.05 – 1.4|||
|Salt marsh||Chesapeake Bay, US||0.005 – 0.12|||
|Salt marsh||Maryland, US||0.1|||
|Salt marsh||North East China||0.1 – 0.16|||
|Salt marsh||Biebrza, Poland||−0.07 – 0.06|||
|Salt marsh||Netherlands||0.82 – 1.64|||
|Salt marsh||Baltic Sea||−0.13|||
|Salt marsh||Massachusetts, US||−2.14 – 1.27|||
Data on nitrous oxide fluxes from wetlands in the southern hemisphere are lacking, as are ecosystem-based studies including the role of dominant organisms that alter sediment biogeochemistry. Aquatic invertebrates produce ecologically-relevant nitrous oxide emissions due to ingestion of denitrifying bacteria that live within the subtidal sediment and water column  and thus may also be influencing nitrous oxide production within some wetlands.
In Southeast Asia, peatswamp forests and soils are being drained, burnt, mined, and overgrazed, contributing severely to climate change.  As a result of peat drainage, the organic carbon that was built up over thousands of years and is normally under water is suddenly exposed to the air. It decomposes and turns into carbon dioxide (CO2), which is released into the atmosphere. Peat fires cause the same process to occur and in addition create enormous clouds of smoke that cross international borders, such as happens every year in Southeast Asia. While peatlands constitute only 3% of the world's land area, their degradation produces 7% of all fossil fuel CO2 emissions.
Through the building of dams, Wetlands International is halting the drainage of peatlands in Southeast Asia, hoping to mitigate CO2 emissions. Concurrent wetland restoration techniques include reforestation with native tree species as well as the formation of community fire brigades. This sustainable approach can be seen in central Kalimantan and Sumatra, Indonesia.
Wetlands, the functions and services they provide as well as their flora and fauna, can be affected by several types of disturbances.  The disturbances (sometimes termed stressors or alterations) can be human-associated or natural, direct or indirect, reversible or not, and isolated or cumulative. When exceeding levels or patterns normally found within wetlands of a particular class in a particular region, the predominant ones include the following:  
- Enrichment/ eutrophication
- Organic loading and reduced dissolved oxygen
- Contaminant toxicity
- Altered solar input ( turbidity/shade)
- Vegetation removal
- Thermal alteration
- Dehydration/ aridification
- Habitat fragmentation
- Other human presence
Disturbances can be further categorized as follows:
- Minor disturbance: Stress that maintains ecosystem integrity. 
- Moderate disturbance: Ecosystem integrity is damaged but can recover in time without assistance. 
- Impairment or severe disturbance: Human intervention may be needed in order for ecosystem to recover. 
Just a few of the many sources of these disturbances are: 
- Unsustainable water use
- Nutrient pollution ( anthropogenic nitrogen inputs to aquatic systems have drastically effected the dissolved nitrogen content of wetlands, introducing higher nutrient availability which leads to eutrophication. )
- Water pollution
They can be manifested partly as:
- Water scarcity
- Impacts to endangered species
- Disruption of wildlife breeding grounds
- Imbalance in sediment load and nutrient filtration
Due to their productivity, wetlands are often converted into dry land with dykes and drains and used for agricultural purposes. The construction of dykes, and dams, has negative consequences for individual wetlands and entire watersheds. : 497 Their closeness to lakes and rivers means that they are often developed for human settlement.  Once settlements are constructed and protected by dykes, the settlements then become vulnerable to land subsidence and ever increasing risk of flooding. : 497 The Louisiana coast around New Orleans is a well-known example;  the Danube Delta in Europe is another. 
Because wetlands are indicative of the amount of water in soil, they are found all throughout the world in different climates .  Temperatures vary greatly depending on the location of the wetland. Many of the world's wetlands are in temperate zones, midway between the North or South Pole and the equator. In these zones, summers are warm and winters are cold, but temperatures are not extreme. In a subtropical zone wetland, such as one along the Gulf of Mexico, a typical temperature might be 11 °C (52 °F). Wetlands in the tropics are much warmer for a larger portion of the year. Wetlands on the Arabian Peninsula can reach temperatures exceeding 50 °C (122 °F) and would therefore be subject to rapid evaporation. In northeastern Siberia, which has a polar climate, wetland temperatures can be as low as −50 °C (−58 °F). Peatlands insulate the permafrost in subarctic regions, thus delaying or preventing thawing of permafrost during summer, as well as inducing the formation of permafrost. 
The amount of precipitation a wetland receives varies widely according to its area. Wetlands in Wales, Scotland, and western Ireland typically receive about 1,500 mm (59 in) per year. In some places in Southeast Asia, where heavy rains occur, they can receive up to 10,000 mm (390 in). In some drier regions, wetlands exist where as little as 180 mm (7.1 in) precipitation occurs each year.[ citation needed]
Temporal variation: 
- Perennial systems
- Seasonal systems
- Episodic (periodic or intermittent) systems
- Surface flow may occur in some segments, with subsurface flow in other segments
- Ephemeral (short-lived) systems
- Migratory species
Depending partly on a wetland's geographic and topographic location,  the functions it performs can support multiple ecosystem services, values, or benefits. United Nations Millennium Ecosystem Assessment and Ramsar Convention described wetlands as a whole to be of biosphere significance and societal importance in the following areas, for example:[ citation needed]
- Water storage (flood control)
- Groundwater replenishment
- Shoreline stabilisation and storm protection
- Water purification
- Wastewater treatment (in constructed wetlands)
- Reservoirs of biodiversity
- Wetland products
- Cultural values
- Recreation and tourism
- Climate change mitigation and adaptation
According to the Ramsar Convention:
The economic worth of the ecosystem services provided to society by intact, naturally functioning wetlands is frequently much greater than the perceived benefits of converting them to 'more valuable' intensive land use – particularly as the profits from unsustainable use often go to relatively few individuals or corporations, rather than being shared by society as a whole.
Unless otherwise cited, ecosystem services information is based on the following series of references. 
To replace these wetland ecosystem services, enormous amounts of money would need to be spent on water purification plants, dams, levees, and other hard infrastructure, and many of the services are impossible to replace.
Major wetland type: floodplain and closed-depression wetlands
Storage reservoirs and flood protection: The wetland system of floodplains is formed from major rivers downstream from their headwaters. "The floodplains of major rivers act as natural storage reservoirs, enabling excess water to spread out over a wide area, which reduces its depth and speed. Wetlands close to the headwaters of streams and rivers can slow down rainwater runoff and spring snowmelt so that it doesn't run straight off the land into water courses. This can help prevent sudden, damaging floods downstream."  Notable river systems that produce large spans of floodplain include the Nile River, the Niger river inland delta, the Zambezi River flood plain, the Okavango River inland delta, the Kafue River flood plain, the Lake Bangweulu flood plain (Africa), Mississippi River (USA), Amazon River (South America), Yangtze River (China), Danube River (Central Europe) and Murray-Darling River (Australia).
Human impact: Converting wetlands to upland through drainage and development forces adjoining or downstream water channels into narrower corridors. This accelerates watershed hydrologic response to storm events and this increases the need in some cases for alternative means of flood control. That is because the newly formed channels must manage the same amount of precipitation, causing flood peaks to be [higher or deeper] and floodwaters to travel faster.
Water management engineering developments in the past century have degraded these wetlands through the construction of artificial embankments. These constructions may be classified as dykes, bunds, levees, weirs, barrages and dams but serve the single purpose of concentrating water into a select source or area. Wetland water sources that were once spread slowly over a large, shallow area are pooled into deep, concentrated locations. Loss of wetland floodplains results in more severe and damaging flooding. Catastrophic human impact in the Mississippi River floodplains was seen in death of several hundred individuals during a levee breach in New Orleans caused by Hurricane Katrina. Ecological catastrophic events from human-made embankments have been noticed along the Yangtze River floodplains since the middle of the river has become prone to more frequent and damaging flooding. Some of these events include the loss of riparian vegetation, a 30% loss of the vegetation cover throughout the river's basin, a doubling of the percentage of the land affected by soil erosion, and a reduction in reservoir capacity through siltation build-up in floodplain lakes. 
The surface water which is the water visibly seen in wetland systems only represents a portion of the overall water cycle which also includes atmospheric water and groundwater. Wetland systems are directly linked to groundwater and a crucial regulator of both the quantity and quality of water found below the ground. Wetland systems that are made of permeable sediments like limestone or occur in areas with highly variable and fluctuating water tables especially have a role in groundwater replenishment or water recharge. Sediments that are porous allow water to filter down through the soil and overlying rock into aquifers which are the source of 95% of the world's drinking water. Wetlands can also act as recharge areas when the surrounding water table is low and as a discharge zone when it is too high. Karst (cave) systems are a unique example of this system and are a connection of underground rivers influenced by rain and other forms of precipitation. These wetland systems are capable of regulating changes in the water table on upwards of 130 m (430 ft).
Human impact: Groundwater is an important source of water for drinking and irrigation of crops. Over 1 billion people in Asia and 65% of the public water sources in Europe source 100% of their water from groundwater. Irrigation is a massive use of groundwater with 80% of the world's groundwater used for agricultural production. 
Unsustainable abstraction of groundwater has become a major concern. In the Commonwealth of Australia, water licensing is being implemented to control use of water in major agricultural regions. On a global scale, groundwater deficits and water scarcity is one of the most pressing concerns facing the 21st century. 
Tidal and inter-tidal wetland systems protect and stabilize coastal zones. Coral reefs provide a protective barrier to coastal shoreline. Mangroves stabilize the coastal zone from the interior and will migrate with the shoreline to remain adjacent to the boundary of the water. The main conservation benefit these systems have against storms and storm surges is the ability to reduce the speed and height of waves and floodwaters.
Human impact: The sheer number of people who live and work near the coast is expected to grow immensely over the next fifty years. From an estimated 200 million people that currently live in low-lying coastal regions, the development of urban coastal centers is projected to increase the population by fivefold within 50 years.  The United Kingdom has begun the concept of managed coastal realignment. This management technique provides shoreline protection through restoration of natural wetlands rather than through applied engineering. In East Asia, reclamation of coastal wetlands has resulted in widespread transformation of the coastal zone, and up to 65% of coastal wetlands have been destroyed by coastal development.   One analysis using the impact of hurricanes versus storm protection provided naturally by wetlands projected the value of this service at US$33,000/hectare/year. 
Nutrient retention: Wetlands cycle both sediments and nutrients balancing terrestrial and aquatic ecosystems. A natural function of wetland vegetation is the up-take, storage, and (for nitrate) the removal of nutrients found in runoff from the surrounding soil and water.  In many wetlands, nutrients are retained until plants die or are harvested by animals or humans and taken to another location, or until microbial processes convert soluble nutrients to a gas as is the case with nitrate.
Sediment and heavy metal traps: Precipitation and surface runoff induces soil erosion, transporting sediment in suspension into and through waterways. These sediments move towards larger and more sizable waterways through a natural process that moves water towards oceans. All types of sediments which may be composed of clay, sand, silt, and rock can be carried into wetland systems through this process. Wetland vegetation acts as a physical barrier to slow water flow and trap sediment for short or long periods of time. Suspended sediment often contains heavy metals that are retained when wetlands trap the sediment. In some cases, certain metals are taken up through wetland plant stems, roots, and leaves. Many floating plant species, for example, can absorb and filter heavy metals. Water hyacinth (Eichhornia crassipes), duckweed (Lemna) and water fern (Azolla) store iron and copper commonly found in wastewater, these plants also reduce pathogens. Many fast-growing plants rooted in the soils of wetlands such as cattail (Typha) and reed (Phragmites) also aid in the role of heavy metal up-take. Animals such as the oyster can filter more than 200 litres (53 US gal) of water per day while grazing for food, removing nutrients, suspended sediments, and chemical contaminants in the process. On the other hand, some types of wetlands facilitate the mobilization and bioavailability of mercury (another heavy metal), which in its methyl mercury form increases the risk of bioaccumulation in fish important to animal food webs and harvested for human consumption.
Capacity: The ability of wetland systems to store or remove nutrients and trap sediment and associated metals is highly efficient and effective but each system has a threshold. An overabundance of nutrient input from fertilizer run-off, sewage effluent, or non-point pollution will cause eutrophication. Upstream erosion from deforestation can overwhelm wetlands making them shrink in size and cause dramatic biodiversity loss through excessive sedimentation load. Retaining high levels of metals in sediments is problematic if the sediments become resuspended or oxygen and pH levels change at a future time. The capacity of wetland vegetation to store heavy metals depends on the particular metal, oxygen and pH status of wetland sediments and overlying water, water flow rate (detention time), wetland size, season, climate, type of plant, and other factors.
Human impact: The capacity of a wetland to store sediment, nutrients, and metals can be diminished if sediments are compacted such as by vehicles or heavy equipment, or are regularly tilled. Unnatural changes in water levels and water sources also can affect the water purification function. If water purification functions are impaired, excessive loads of nutrients enter waterways and cause eutrophication. This is of particular concern in temperate coastal systems.   The main sources of coastal eutrophication are industrially made nitrogen, which is used as fertilizer in agricultural practices, as well as septic waste runoff.  Nitrogen is the limiting nutrient for photosynthetic processes in saline systems, however in excess, it can lead to an overproduction of organic matter that then leads to hypoxic and anoxic zones within the water column.  Without oxygen, other organisms cannot survive, including economically important finfish and shellfish species.
Examples: An example of how a natural wetland is used to provide some degree of sewage treatment is the East Kolkata Wetlands in Kolkata, India. The wetlands cover 125 square kilometres (48 sq mi), and are used to treat Kolkata's sewage. The nutrients contained in the wastewater sustain fish farms and agriculture.
A constructed wetland (CW) is an artificial wetland to treat sewage, greywater, stormwater runoff or industrial wastewater. It may also be designed for land reclamation after mining, or as a mitigation step for natural areas lost to land development. Constructed wetlands are engineered systems that use natural functions vegetation, soil, and organisms to provide secondary treatment to wastewater. The design of the constructed wetland has to be adjusted according to the type of wastewater to be treated. Constructed wetlands have been used in both centralized and decentralized wastewater systems. Primary treatment is recommended when there is a large amount of suspended solids or soluble organic matter (measured as BOD and COD). 
Similarly to natural wetlands, constructed wetlands also act as a biofilter and/or can remove a range of pollutants (such as organic matter, nutrients, pathogens, heavy metals) from the water. Constructed wetlands are designed to remove water pollutants such as suspended solids, organic matter and nutrients (nitrogen and phosphorus).  All types of pathogens (i.e., bacteria, viruses, protozoan and helminths) are expected to be removed to some extent in a constructed wetland. Subsurface wetland provide greater pathogen removal than surface wetlands. There are two main types of constructed wetlands: subsurface flow and surface flow constructed wetlands. The planted vegetation plays an important role in contaminant removal. The filter bed, consisting usually of sand and gravel, has an equally important role to play.  Some constructed wetlands may also serve as a habitat for native and migratory wildlife, although that is not their main purpose. Subsurface flow constructed wetlands are designed to have either horizontal flow or vertical flow of water through the gravel and sand bed. Vertical flow systems have a smaller space requirement than horizontal flow systems.
Wetland systems' rich biodiversity is becoming a focal point at International Treaty Conventions and within the World Wildlife Fund organization due to the high number of species present in wetlands, the small global geographic area of wetlands, the number of species which are endemic to wetlands, and the high productivity of wetland systems. Hundred of thousands of animal species, 20,000 of them vertebrates, are living in wetland systems. The discovery rate of fresh water fish is at 200 new species per year. The impact of maintaining biodiversity is seen at the local level through job creation, sustainability, and community productivity. A good example is the Lower Mekong basin which runs through Cambodia, Laos, and Vietnam. Supporting over 55 million people, the sustainability of the region is enhanced through wildlife tours. The U.S. state of Florida has estimated that US$1.6 billion was generated in state revenue from recreational activities associated with wildlife.
Biodiverse river basins: The Amazon holds 3,000 species of freshwater fish species within the boundaries of its basin, whose function it is to disperse the seeds of trees. One of its key species, the Piramutaba catfish, Brachyplatystoma vaillantii, migrates more than 3,300 km (2,100 mi) from its nursery grounds near the mouth of the Amazon River to its spawning grounds in Andean tributaries, 400 m (1,300 ft) above sea level, distributing plants seed along the route.
Productive intertidal zones: Intertidal mudflats have a level of productivity similar to that of some wetlands even while possessing a low number of species. The abundance of invertebrates found within the mud are a food source for migratory waterfowl.
Critical life-stage habitat: Mudflats, saltmarshes, mangroves, and seagrass beds have high levels of both species richness and productivity, and are home to important nursery areas for many commercial fish stocks.
Genetic diversity: Populations of many species are confined geographically to only one or a few wetland systems, often due to the long period of time that the wetlands have been physically isolated from other aquatic sources. For example, the number of endemic species in Lake Baikal in Russia classifies it as a hotspot for biodiversity and one of the most biodiverse wetlands in the entire world. Evidence from a research study by Mazepova et al. suggest that the number of crustacean species endemic to Baikal Lake (over 690 species and subspecies) exceeds the number of the same groups of animals inhabiting all the fresh water bodies of Eurasia together. Its 150 species of free-living Platyhelminthes alone is analogous to the entire number in all of Eastern Siberia. The 34 species and subspecies number of Baikal sculpins is more than twice the number of the analogous fauna that inhabits Eurasia. In southern Baikal, about 300 species of free-living nematodes were found in only six near-shore sampling localities. "If we will take into consideration, that about 60% of the animals can be found nowhere else except Baikal, it may be assumed that the lake may be the biodiversity center of the Eurasian continent." 
Human impact: Biodiversity loss occurs in wetland systems through land use changes, habitat destruction, pollution, exploitation of resources, and invasive species. Vulnerable, threatened, and endangered species number at 17% of waterfowl, 38% of fresh-water dependent mammals, 33% of freshwater fish, 26% of freshwater amphibians, 72% of freshwater turtles, 86% of marine turtles, 43% of crocodilians and 27% of coral reef-building species. Introduced hydrophytes in different wetland systems can have devastating results. The introduction of water hyacinth, a native plant of South America into Lake Victoria in East Africa as well as duckweed into non-native areas of Queensland, Australia, have overtaken entire wetland systems suffocating the wetlands and reducing the diversity of other plants and animals. This is largely due to their phenomenal growth rate and ability to float and grow on the surface of the water.
Wetland productivity is linked to the climate, wetland type, and nutrient availability. Low water and occasional drying of the wetland bottom during droughts (dry marsh phase) stimulate plant recruitment from a diverse seed bank and increase productivity by mobilizing nutrients. In contrast, high water during deluges (lake marsh phase) causes turnover in plant populations and creates greater interspersion of element cover and open water, but lowers overall productivity. During a cover cycle that ranges from open water to complete vegetation cover, annual net primary productivity may vary 20-fold.  The grasses of fertile floodplains such as the Nile produce the highest yield including plants such as Arundo donax (giant reed), Cyperus papyrus (papyrus), Phragmites (reed) and Typha,[ citation needed]
Wetlands naturally produce an array of vegetation and other ecological products that can be harvested for personal and commercial use.  The most significant of these is fish which have all or part of their life-cycle occur within a wetland system. Fresh and saltwater fish are the main source of protein for one billion people and comprise 15% of an additional two billion people's diets. In addition, fish generate a fishing industry that provides 80% of the income and employment to residents in developing countries. Another food staple found in wetland systems is rice, a popular grain that is consumed at the rate of one fifth of the total global calorie count. In Bangladesh, Cambodia and Vietnam, where rice paddies are predominant on the landscape, rice consumption reach 70%.  Some native wetland plants in the Caribbean and Australia are harvested sustainably for medicinal compounds; these include the red mangrove ( Rhizophora mangle) which possesses antibacterial, wound-healing, anti-ulcer effects, and antioxidant properties. 
Food converted to sweeteners and carbohydrates include the sago palm of Asia and Africa (cooking oil), the nipa palm of Asia (sugar, vinegar, alcohol, and fodder) and honey collection from mangroves. More than supplemental dietary intake, this produce sustains entire villages. Coastal Thailand villages earn the key portion of their income from sugar production while the country of Cuba relocates more than 30,000 hives each year to track the seasonal flowering of the mangrove Avicennia.[ citation needed]
Other mangrove-derived products:[ citation needed]
- Salt (produced by evaporating seawater)
- Animal fodder
- Traditional medicines (e.g. from mangrove bark)
- Fibers for textiles
- Dyes and tannins
Human impact: Over-fishing is the major problem for sustainable use of wetlands. Concerns are developing over certain aspects of farm fishing, which uses natural waterways to harvest fish for human consumption and pharmaceuticals. This practice has become especially popular in Asia and the South Pacific. Its impact upon much larger waterways downstream has negatively affected many small island developing states. 
Aquaculture is continuing to develop rapidly throughout the Asia-Pacific region specifically in China with world holdings in Asia equal to 90% of the total number of aquaculture farms and 80% of its global value.  Some aquaculture has eliminated massive areas of wetland through practices seen such as in the shrimp farming industry's destruction of mangroves. Even though the damaging impact of large scale shrimp farming on the coastal ecosystem in many Asian countries has been widely recognized for quite some time now, it has proved difficult to check in absence of other employment avenues for people engaged in such occupation. Also burgeoning demand for shrimps globally has provided a large and ready market for the produce.[ citation needed]
Threats to rice fields mainly stem from inappropriate water management, introduction of invasive alien species, agricultural fertilizers, pesticides, and land use changes. Industrial-scale production of palm oil threatens the biodiversity of wetland ecosystems in parts of southeast Asia, Africa, and other developing countries.[ citation needed]
Over-exploitation of wetland products can occur at the community level as is sometimes seen throughout coastal villages of Southern Thailand where each resident may obtain for themselves every consumable of the mangrove forest (fuelwood, timber, honey, resins, crab, and shellfish) which then becomes threatened through increasing population and continual harvest.[ citation needed]
Wetlands perform two important functions in relation to climate change. They have mitigation effects through their ability to sink carbon, converting a greenhouse gas ( carbon dioxide) to solid plant material through the process of photosynthesis, and also through their ability to store and regulate water.   Wetlands store approximately 44.6 million tonnes of carbon per year globally.  In salt marshes and mangrove swamps in particular, the average carbon sequestration rate is 210 g CO2 m−2 y−1 while peatlands sequester approximately 20–30 g CO2 m−2 y−1.   Coastal wetlands, such as tropical mangroves and some temperate salt marshes, are known to be sinks for carbon that otherwise contributes to climate change in its gaseous forms (carbon dioxide and methane). The ability of many tidal wetlands to store carbon and minimize methane flux from tidal sediments has led to sponsorship of blue carbon initiatives that are intended to enhance those processes. 
Some types of wetlands can serve as fire breaks that help slow the spread of minor wildfires. Larger wetland systems can influence local precipitation patterns. Some boreal wetland systems in catchment headwaters may help extend the period of flow and maintain water temperature in connected downstream waters. Pollination services are supported by many wetlands which may provide the only suitable habitat for pollinating insects, birds, and mammals in highly developed areas. It is likely that wetlands have other functions whose benefits to society and other ecosystems have yet to be discovered.[ citation needed]
Wetlands have historically been the victim of large draining efforts for real estate development, or flooding for use as recreational lakes or hydropower generation. Some of the world's most important agricultural areas are wetlands that have been converted to farmland.     Since the 1970s, more focus has been put on preserving wetlands for their natural function yet by 1993 half the world's wetlands had been drained. [ full citation needed]
In order to maintain wetlands and sustain their functions, alterations and disturbances that are outside the normal range of variation should be minimized.
Wetlands are vital ecosystems that provide livelihoods for the millions of people who live in and around them. The Millennium Development Goals (MDGs) called for different sectors to join forces to secure wetland environments in the context of sustainable development and improving human wellbeing. A three-year project carried out by Wetlands International in partnership with the International Water Management Institute found that it is possible to conserve wetlands while improving the livelihoods of people living among them. Case studies conducted in Malawi and Zambia looked at how dambos – wet, grassy valleys or depressions where water seeps to the surface – can be farmed sustainably to improve livelihoods. Mismanaged or overused dambos often become degraded, however, using a knowledge exchange between local farmers and environmental managers, a protocol was developed using soil and water management practices. Project outcomes included a high yield of crops, development of sustainable farming techniques, and adequate water management generating enough water for use as irrigation. Before the project, there were cases where people had died from starvation due to food shortages. By the end of it, many more people had access to enough water to grow vegetables. A key achievement was that villagers had secure food supplies during long, dry months. They also benefited in other ways: nutrition was improved by growing a wider range of crops, and villagers could also invest in health and education by selling produce and saving money. 
The Convention on Wetlands of International Importance, especially as Waterfowl Habitat, or Ramsar Convention, is an international treaty designed to address global concerns 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. Methods include restricting access to the majority portion of wetland areas, as well as educating the public to combat the misconception that wetlands are wastelands. The Convention works closely with five International Organisation Partners. These are: Birdlife International, the IUCN, the International Water Management Institute, Wetlands International and the World Wide Fund for Nature. The partners provide technical expertise, help conduct or facilitate field studies and provide financial support. The IOPs also participate regularly as observers in all meetings of the Conference of the Parties and the Standing Committee and as full members of the Scientific and Technical Review Panel.
The value of a wetland to local communities, as well as the value of wetland systems generally to the earth and to humankind, is one of the most important valuations that can be conducted for sustainable development. This typically involves first mapping a region's wetlands, then assessing the functions and ecosystem services the wetlands provide individually and cumulatively, and evaluating that information to prioritize or rank individual wetlands or wetland types for conservation, management, restoration, or development. Over a longer period, it requires keeping inventories of known wetlands and monitoring a representative sample of the wetlands to determine changes due to both natural and human factors. Such a valuation process is used to educate decision-makers such as governments of the importance of particular wetlands within their jurisdiction.
This section needs additional citations for verification. (April 2020)
Rapid assessment methods are used to score, rank, rate, or categorize various functions, ecosystem services, species, communities, levels of disturbance, and/or ecological health of a wetland or group of wetlands. This is often done to prioritize particular wetlands for conservation (avoidance) or to determine the degree to which loss or alteration of wetland functions should be compensated, such as by restoring degraded wetlands elsewhere or providing additional protections to existing wetlands. Rapid assessment methods are also applied before and after a wetland has been restored or altered, to help monitor or predict the effects of those actions on various wetland functions and the services they provide. Assessments are typically considered to be "rapid" when they require only a single visit to the wetland lasting less than one day, which in some cases may include interpretation of aerial imagery and geographic information system (GIS) analyses of existing spatial data, but not detailed post-visit laboratory analyses of water or biological samples. Due to time and cost constraints, the levels of various wetland functions or other attributes are usually not measured directly but rather are estimated relative to other assessed wetlands in a region, using observation-based variables, sometimes called "indicators", that are hypothesized or known to predict performance of the specified functions or attributes.
To achieve consistency among persons doing the assessment, rapid methods present indicator variables as questions or checklists on standardized data forms, and most methods standardize the scoring or rating procedure that is used to combine question responses into estimates of the levels of specified functions relative to the levels estimated in other wetlands ("calibration sites") assessed previously in a region.  Rapid assessment methods, partly because they often use dozens of indicators pertaining to conditions surrounding a wetland as well as within the wetland itself, aim to provide estimates of wetland functions and services that are more accurate and repeatable than simply describing a wetland's class type.  A need for wetland assessments to be rapid arises mostly when government agencies set deadlines for decisions affecting a wetland, or when the number of wetlands needing information on their functions or condition is large.
In North America and a few other countries, standardized rapid assessment methods for wetlands have a long history, having been developed, calibrated, tested, and applied to varying degrees in several different regions and wetland types since the 1970s. However, few rapid assessment methods have been fully validated. Done correctly, validation is a very expensive endeavor that involves comparing rankings of a series of wetlands based on results from rapid assessment methods with rankings based on less rapid and considerably more costly, multi-visit, detailed measurements of levels of the same functions or other attributes in the same series of wetlands.
Although developing a global inventory of wetlands has proven to be a large and difficult undertaking, many efforts at more local scales have been successful. Current efforts are based on available data, but both classification and spatial resolution have sometimes proven to be inadequate for regional or site-specific environmental management decision-making. It is difficult to identify small, long, and narrow wetlands within the landscape. Many of today's remote sensing satellites do not have sufficient spatial and spectral resolution to monitor wetland conditions, although multispectral IKONOS and QuickBird data may offer improved spatial resolutions once it is 4 m or higher. Majority of the pixels are just mixtures of several plant species or vegetation types and are difficult to isolate which translates into an inability to classify the vegetation that defines the wetland. Improved remote sensing information, coupled with good knowledge domain on wetlands will facilitate expanded efforts in wetland monitoring and mapping. This will also be extremely important because we expect to see major shifts in species composition due to both anthropogenic land use and natural changes in the environment caused by climate change.
A wetland needs to be monitored over time to assess whether it is functioning at an ecologically sustainable level or whether it is becoming degraded. Degraded wetlands will suffer a loss in water quality, loss of sensitive species, and aberrant functioning of soil geochemical processes.
Practically, many natural wetlands are difficult to monitor from the ground as they quite often are difficult to access and may require exposure to dangerous plants and animals as well as diseases borne by insects or other invertebrates..Therefore, mapping using aerial imagery is one effective tool to monitor a wetland, especially a large wetland, and can also be used to monitor the status of numerous wetlands throughout a watershed or region. Many remote sensing methods can be used to map wetlands. Remote-sensing technology permits the acquisition of timely digital data on a repetitive basis. This repeat coverage allows wetlands, as well as the adjacent land-cover and land-use types, to be monitored seasonally and/or annually. Using digital data provides a standardized data-collection procedure and an opportunity for data integration within a geographic information system. Traditionally, Landsat 5 Thematic Mapper (TM), Landsat 7 Enhanced Thematic Mapper Plus (ETM+), and the SPOT 4 and 5 satellite systems have been used for this purpose. More recently, however, multispectral IKONOS and QuickBird data, with spatial resolutions of 4 by 4 m (13 by 13 ft) and 2.44 by 2.44 m (8.0 by 8.0 ft), respectively, have been shown to be excellent sources of data when mapping and monitoring smaller wetland habitats and vegetation communities.
For example, Detroit Lakes Wetland Management District assessed area wetlands in Michigan, USA, using remote sensing. Through using this technology, satellite images were taken over a large geographic area and extended period. In addition, using this technique was less costly and time-consuming compared to the older method using visual interpretation of aerial photographs. In comparison, most aerial photographs also require experienced interpreters to extract information based on structure and texture while the interpretation of remote sensing data only requires analysis of one characteristic (spectral).
However, there are a number of limitations associated with this type of image acquisition. Analysis of wetlands has proved difficult because to obtain the data it is often linked to other purposes such as the analysis of land cover or land use.
Methods to develop a classification system for specific biota of interest could assist with technological advances that will allow for identification at a very high accuracy rate. The issue of the cost and expertise involved in remote sensing technology is still a factor hindering further advancements in image acquisition and data processing. Future improvements in current wetland vegetation mapping could include the use of more recent and better geospatial data when it is available.
Restoration and restoration ecologists intend to return wetlands to their natural trajectory by aiding directly with the natural processes of the ecosystem.  These direct methods vary with respect to the degree of physical manipulation of the natural environment and each are associated with different levels of restoration.  Restoration is needed after disturbance or perturbation of a wetland.  Disturbances include exogenous factors such as flooding or drought.  Other external damage may be anthropogenic disturbance caused by clear-cut harvesting of trees, oil and gas extraction, poorly defined infrastructure installation, over grazing of livestock, ill-considered recreational activities, alteration of wetlands including dredging, draining, and filling, and other negative human impacts.   Disturbance puts different levels of stress on an environment depending on the type and duration of disturbance.  There is no one way to restore a wetland and the level of restoration required will be based on the level of disturbance although, each method of restoration does require preparation and administration. 
Factors influencing selected approach may include 
- Time scale limitations
- Project goals
- Level of disturbance
- Landscape and ecological constraints
- Political and administrative agendas
- Socioeconomic priorities
There are no biophysical manipulation and the ecosystem is left to recover based on the process of succession alone.  The focus of this method is to eliminate and prevent further disturbance from occurring.  In order for this type of restoration to be effective and successful there must be prior research done to understand the probability that the wetland will recover with this method.  Otherwise, some biophysical manipulation may be required to enhance the rate of succession to an acceptable level determined by the project managers and ecologists.  This is likely to be the first method of approach for the lowest level of disturbance being that it is the least intrusive and least costly. 
There are some biophysical manipulations however they are non-intrusive.  Example methods that are not limited to wetlands include prescribed burns to small areas, promotion of site specific soil microbiota and plant growth using nucleation planting whereby plants radiate from an initial planting site,  and promotion of niche diversity or increasing the range of niches to promote use by a variety of different species.  These methods can make it easier for the natural species to flourish by removing competition from their environment and can speed up the process of succession. 
Here there is a mix between natural regeneration and manipulated environmental control.  These manipulations may require some engineering and more invasive biophysical manipulation including ripping of subsoil, agrichemical applications such as herbicides and insecticides, laying of mulch, mechanical seed dispersal, and tree planting on a large scale.  In these circumstances the wetland is impaired and without human assistance it would not recover within an acceptable period of time determined by ecologists.  Again these methods of restoration will have to be considered on a site by site basis as each site will require a different approach based on levels of disturbance and ecosystem dynamics. 
The most expensive and intrusive method of reconstruction requiring engineering and ground up reconstruction.  Because there is a redesign of the entire ecosystem it is important that the natural trajectory of the ecosystem be considered and that the plant species will eventually return the ecosystem towards its natural trajectory. 
Each county and region tends to have its own definition for legal purposes. In the United States, wetlands are defined as "those areas that are inundated or saturated by surface or groundwater 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".  This definition has been used in the enforcement of the Clean Water Act. Some US states, such as Massachusetts and New York, have separate definitions that may differ from the federal government's.
In the United States Code, the term wetland is defined "as land that (A) has a predominance of hydric soils, (B) is inundated or saturated by surface or groundwater at a frequency and duration sufficient to support a prevalence of hydrophytic vegetation typically adapted for life in saturated soil conditions and (C) under normal circumstances supports a prevalence of such vegetation." Related to this legal definitions, the term "normal circumstances" are conditions expected to occur during the wet portion of the growing season under normal climatic conditions (not unusually dry or unusually wet), and in the absence of significant disturbance. It is not uncommon for a wetland to be dry for long portions of the growing season. Wetlands can be dry during the dry season and abnormally dry periods during the wet season, but under normal environmental conditions the soils in a wetland will be saturated to the surface or inundated such that the soils become anaerobic, and those conditions will persist through the wet portion of the growing season. 
- The Federal Policy on Wetland Conservation 
- Other Individual Provincial and Territorial Based Policies 
Variations of names for wetland systems:
The largest wetlands include the swamp forests of the Amazon River basin, the peatlands of the West Siberian Plain,  the Pantanal in South America,  and the Sundarbans in the Ganges- Brahmaputra delta. 
- A Directory of Important Wetlands in Australia
- Converted wetland
- Groundwater-dependent ecosystems
- List of Ramsar wetland sites in Pakistan
- Mediterranean wetlands
- All pages with titles containing wetland
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