Wetlands come in different sizes and types. From top left: Upland vs. wetland vs. lacustrine zones; freshwaterswamp forest inBangladesh;peat bogs are freshwater wetlands that develop in areas withstanding water and lowsoil fertility; a freshwater cattail (Typha) marsh that develops with standing water and high soil fertility.
A simplified definition of wetland is "an area of land that is usually saturated with water".[14] More precisely, wetlands are areas where "water covers thesoil, 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".[15] A patch of land that develops pools of water after arain storm would not necessarily be considered a "wetland", even though the land is wet. Wetlands have unique characteristics: they are generally distinguished from otherwater bodies orlandforms based on theirwater level and on the types ofplants that live within them. Specifically, wetlands are characterized as having awater table that stands at or near theland surface for a long enough period each year to supportaquatic plants.[16][17]
Wetlands have also been described asecotones, providing a transition between dry land and water bodies.[18] Wetlands exist "...at the interface between trulyterrestrial ecosystems andaquatic systems, making them inherently different from each other, yet highly dependent on both."[11]
In environmental decision-making, there are subsets of definitions that are agreed upon to make regulatory and policy decisions.
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 orsalt, including areas of marine water the depth of which at low tide does not exceed six meters."
Article 2.1: "[Wetlands] may incorporateriparian and coastal zones adjacent to the wetlands, andislands or bodies of marine water deeper than six meters atlow 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".[1]
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.'[20]
For each of these definitions and others, regardless of the purpose, hydrology is emphasized (shallow waters, water-logged soils). The soil characteristics and the plants and animals controlled by the wetland hydrology are often additional components of the definitions.[21]
The following three groups are used withinAustralia to classify wetland by type: Marine and coastal zone wetlands, inland wetlands and human-made wetlands.[23] In the US, the best known classifications are theCowardin classification system[24] and the hydrogeomorphic (HGM) classification system. The Cowardin system includes five main types of wetlands:marine (ocean-associated),estuarine (mixed ocean- and river-associated),riverine (within river channels),lacustrine (lake-associated) andpalustrine (inland nontidal habitats).
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,Carolina bays andbaygalls[25][26] of the Southeastern US, mallines of Argentina, Mediterranean seasonal ponds of Europe and California,turloughs of Ireland,billabongs of Australia, among many others.
Wetlands are found throughout the world in different climates.[15] Temperatures vary greatly depending on the location of the wetland. Many of the world's wetlands are in thetemperate 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 theGulf of Mexico, average temperatures might be 11 °C (52 °F). Wetlands in thetropics are subjected to much higher temperatures for a large portion of the year. Temperatures for wetlands on theArabian Peninsula can exceed 50 °C (122 °F) and these habitats would therefore be subject to rapid evaporation. In northeasternSiberia, which has a polar climate, wetland temperatures can be as low as −50 °C (−58 °F).Peatlands in arctic and subarctic regions insulate thepermafrost, thus delaying or preventing its thawing during summer, as well as inducing its formation.[27]
The amount of precipitation a wetland receives varies widely according to its area. Wetlands inWales,Scotland, and westernIreland typically receive about 1,500 mm (59 in) per year.[citation needed] In some places inSoutheast Asia, where heavy rains occur, they can receive up to 10,000 mm (390 in).[citation needed] In some drier regions, wetlands exist where as little as 180 mm (7.1 in) precipitation occurs each year.[citation needed]
Wetlands vary widely due to local and regional differences intopography,hydrology,vegetation, and other factors, including human involvement. Other important factors include fertility, natural disturbance, competition,herbivory, burial and salinity.[1] Whenpeat accumulates,bogs andfens arise.
The most important factor producing wetlands is hydrology, orflooding. The duration of flooding or prolonged soil saturation bygroundwater determines whether the resulting wetland has aquatic,marsh orswampvegetation. Other important factors include soil fertility, natural disturbance, competition,herbivory, burial, and salinity.[1] Whenpeat from dead plants accumulates,bogs andfens 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 predominantlyprecipitation, surface water (saltwater or freshwater), and groundwater. Water flows out of wetlands byevapotranspiration, surface flows andtides, 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.[29]
Landscape characteristics control wetland hydrology and water chemistry. TheO2 andCO2 concentrations of water depend upontemperature,atmospheric pressure and mixing with the air (from winds or water flows). Water chemistry within wetlands is determined by thepH,salinity, nutrients,conductivity, soil composition,hardness, and the sources of water. Water chemistry varies across landscapes and climatic regions. Wetlands are generallyminerotrophic (waters contain dissolved materials from soils) with the exception ofombrotrophic bogs that are fed only by water from precipitation.
Because bogs receive most of their water from precipitation and humidity from theatmosphere, their water usually has lowmineral ionic composition. In contrast, wetlands fed by groundwater or tides have a higherconcentration 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 ofcalcium andmagnesium.[30]
Salinity has a strong influence on wetland water chemistry, particularly in coastal wetlands[1][31] and in arid and semiarid regions with large precipitation deficits. Natural salinity is regulated by interactions between ground and surface water, which may be influenced by human activity.[32]
Carbon is the majornutrient cycled within wetlands. Most nutrients, such assulfur,phosphorus,carbon, andnitrogen are found within the soil of wetlands.Anaerobic andaerobic respiration in the soil influences thenutrient cycling of carbon, hydrogen, oxygen, and nitrogen,[33] and the solubility of phosphorus[34] thus contributing to the chemical variations in its water. Wetlands with low pH and saline conductivity may reflect the presence of acidsulfates[35] and wetlands with average salinity levels can be heavily influenced by calcium or magnesium.Biogeochemical processes in wetlands are determined by soils with lowredox potential.[36]
The life forms of a wetland system includes its plants (flora) and animals (fauna) andmicrobes (bacteria, fungi). The most important factor is the wetland's duration of flooding.[1] Other important factors include fertility and salinity of the water or soils. The chemistry of water flowing into wetlands depends on the source of water, the geological material that it flows through[37] and the nutrients discharged from organic matter in the soils and plants at higher elevations.[38] Plants and animals may vary within a wetland seasonally or in response to flood regimes.
There are four main groups ofhydrophytes that are found in wetland systems throughout the world.[39]
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.[40] Submerged species provide a food source for native fauna, habitat for invertebrates, and also possess filtration capabilities. Examples includeseagrasses andeelgrass.
Floating water plants or floating vegetation are usually small, like those in theLemnoideae subfamily (duckweeds).Emergent vegetation like the cattails (Typha spp.), sedges (Carex spp.) and arrow arum (Peltandra virginica) rise above the surface of the water.
When trees and shrubs comprise much of the plant cover in saturated soils, those areas in most cases are calledswamps.[1] The upland boundary of swamps is determined partly by water levels. This can be affected by dams[41] Some swamps can be dominated by a single species, such assilver maple swamps around theGreat Lakes.[42] Others, like those of theAmazon basin, have large numbers of different tree species.[43] Other examples include cypress (Taxodium) andmangrove swamps.
Many species offrogs live in wetlands, while others visit them each year to lay eggs.Snapping turtles are one of the many kinds of turtles found in wetlands.
Many species offish are highly dependent on wetland ecosystems.[44][45] Seventy-five percent of the United States' commercial fish and shellfish stocks depend solely onestuaries to survive.[46]
Amphibians such asfrogs andsalamanders need both terrestrial and aquatic habitats in which to reproduce and feed. Because amphibians often inhabit depressional wetlands like prairie potholes and Carolina bays, the connectivity among these isolated wetlands is an important control of regional populations.[47] While tadpoles feed on algae, adult frogs forage on insects. Frogs are sometimes used as an indicator ofecosystem health because their thin skin permits absorption of nutrients and toxins from the surrounding environment resulting in increased extinction rates in unfavorable and polluted environmental conditions.[48]
Invertebrates of wetlands include aquatic insects such asdragonflies, aquatic bugs andbeetles, midges,mosquitos,crustaceans such as crabs, crayfish, shrimps, microcrustaceans, mollusks like clams, mussels, snails and worms. 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).[59]
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.
To replace these wetlandecosystem services, enormous amounts of money would need to be spent onwater purification plants, dams, levees, and other hard infrastructure, and many of the services are impossible to replace.
Floodplains and closed-depression wetlands can provide the functions of storage reservoirs and flood protection. The wetland system offloodplains is formed from major rivers downstream from theirheadwaters. "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 does not run straight off the land into water courses. This can help prevent sudden, damaging floods downstream."[46]
Notable river systems that produce wide floodplains include theNile 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 (US),Amazon River (South America),Yangtze River (China),Danube River (Central Europe) andMurray-Darling River (Australia).
Groundwater replenishment can be achieved for example bymarsh,swamp, and subterraneankarst and cave hydrological systems. Thesurface water visibly seen in wetlands only represents a portion of the overall water cycle, which also includes atmospheric water (precipitation) andgroundwater. Many wetlands are directly linked to groundwater and they can be a crucial regulator of both the quantity andquality of water found below the ground. Wetlands that havepermeable substrates likelimestone or occur in areas with highly variable and fluctuating water tables have especially important roles ingroundwater replenishment or water recharge.[62]
Substrates that areporous allow water to filter down through the soil and underlying rock intoaquifers which are the source of much of the world'sdrinking water. Wetlands can also act as recharge areas when the surrounding water table is low and as a discharge zone when it is high.
Mangroves,coral reefs,salt marsh can help with shoreline stabilization and storm protection. Tidal and inter-tidal wetland systems protect and stabilize coastal zones.[63]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 andstorm surges is the ability to reduce the speed and height of waves and floodwaters.
TheUnited 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.[64][65] 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.[66]
Water purification can be provided by floodplains, closed-depression wetlands,mudflat,freshwater marsh,salt marsh, mangroves. Wetlands cycle both sediments and nutrients, sometimes serving as buffers betweenterrestrial andaquatic ecosystems. A natural function of wetland vegetation is the up-take, storage, and (for nitrate) the removal of nutrients found inrunoff water from the surrounding landscapes.[67]
Precipitation and surface runoff inducessoil erosion, transporting sediment in suspension into and through waterways. All types of sediments whether composed of clay, silt, sand or gravel and rock can be carried into wetland systems through erosion. Wetland vegetation acts as a physical barrier to slow water flow and then trap sediment for both short or long periods of time. Suspended sediment can contain heavy metals that are also retained when wetlands trap the sediment.
The ability of wetland systems to store or remove nutrients and trap sediment 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 causeeutrophication. Upstream erosion fromdeforestation can overwhelm wetlands making them shrink in size and cause dramaticbiodiversity loss through excessive sedimentation load.
Constructed wetlands are built for wastewater treatment. An example of how a natural wetland is used to provide some degree ofsewage treatment is theEast Kolkata Wetlands inKolkata, 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.
Similar to natural wetlands, constructed wetlands also act as abiofilter and/or can remove a range ofpollutants (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).[70] All types of pathogens (i.e., bacteria, viruses,protozoans andhelminths) are expected to be removed to some extent in a constructed wetland. Subsurface wetlands provide greater pathogen removal than surface wetlands.[70]
Wetland systems' richbiodiversity has become a focal point catalysed by theRamsar Convention andWorld Wildlife Fund.[71] 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.
A key fish species which is overfished,[72] 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 plant seeds along the route.
Intertidal mudflats have a level of productivity similar to that of some wetlands even while possessing a low number of species. The abundantinvertebrates found within the mud are a food source formigratory waterfowl.[73]
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.
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 ofendemic species in the Selenga River Delta ofLake Baikal in Russia classifies it as a hotspot for biodiversity and one of the most biodiverse wetlands in the entire world.[74]
Wetland at the Broadmoor Wildlife Sanctuary in Massachusetts, United States, in February
Wetlands naturally produce an array of vegetation and other ecological products that can be harvested for personal and commercial use.[75] Many fishes have all or part of their life-cycle occurring within a wetland system. Fresh and saltwater fish are the main source of protein for about one billion people[76] and comprise 15% of an additional 3.5 billion people's protein intake.[77] 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%.[78] 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.[78]
Other mangrove-derived products include fuelwood, salt (produced by evaporating seawater), animal fodder, traditional medicines (e.g. from mangrove bark), fibers for textiles and dyes and tannins.[79]
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.[80] Pollination services are supported by many wetlands which may provide the only suitable habitat for pollinating insects, birds, and mammals in highly developed areas.[81]
Wetlands, the functions and services they provide as well as their flora and fauna, can be affected by several types of disturbances.[82] The disturbances (sometimes termed stressors or alterations) can be human-associated or natural, direct or indirect, reversible or not, and isolated or cumulative.
Disturbances can be further categorized as follows:
Minor disturbance: Stress that maintains ecosystem integrity.[10]
Moderate disturbance: Ecosystem integrity is damaged but can recover in time without assistance.[10]
Impairment or severe disturbance: Human intervention may be needed in order for ecosystem to recover.[10]
Nutrient pollution comes from nitrogen inputs to aquatic systems and have drastically effected the dissolved nitrogen content of wetlands, introducing higher nutrient availability which leads toeutrophication.[85]
To increase economic productivity, wetlands are often converted into dry land withdykes anddrains and used for agricultural purposes. The construction of dykes, and dams, has negative consequences for individual wetlands and entire watersheds.[1]: 497 Their proximity to lakes and rivers means that they are often developed for human settlement.[86] Once settlements are constructed and protected by dykes, the settlements then become vulnerable to land subsidence and ever increasing risk of flooding.[1]: 497 The Mississippi River Delta around New Orleans, Louisiana is a well-known example;[87] the Danube Delta in Europe is another.[88]
Water pollution is another key driver of the conversion of wetlands to dry land. Since wetlands tend to retain water with less influx or efflux compared to other bodies of water, they can quickly concentrate toxicants that originate from pollutants.[89] This accumulation of toxicants will cause the biodiversity of a wetland to change, particularly since toxicants will be harmful to native aquatic species.[90] The loss of wetlandbiodiversity is associated with wetland degradation, as the case of alpine wetlands demonstrates.[91]
Drainage offloodplains or development activities that narrow floodplain corridors (such as the construction oflevees) reduces the ability of coupled river-floodplain systems to control flood damage. That is because modified and less expansive systems must still 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 floodplain wetlands through the construction of artificial embankments such asdykes, bunds,levees,weirs, barrages anddams. All concentrate water into a main channel and waters that historically spread slowly over a large, shallow area are concentrated. 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 alevee breach in New Orleans caused by Hurricane Katrina. Human-made embankments along the Yangtze River floodplains have caused the main channel of the river to become prone to more frequent and damaging flooding.[92] Some of these events include the loss ofriparian 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 throughsiltation build-up in floodplain lakes.[46]
Overfishing is a major problem for sustainable use of wetlands. Concerns are developing over certain aspects of farm fishing, which uses natural wetlands and waterways to harvest fish for human consumption.Aquaculture is continuing to develop rapidly throughout the Asia-Pacific region especially in China where 90% of the total number of aquaculture farms occur, contributing 80% of global value.[78] Some aquaculture has eliminated massive areas of wetland through practices such as theshrimp 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 mitigate since other employment avenues for people are lacking. Also burgeoning demand for shrimp globally has provided a large and ready market.[93]
Fog rising over the Mukri bog nearMukri, Estonia. The bog has an area of 2,147 hectares (5,310 acres) and has been protected since 1992.
Wetlands have historically subjected to large draining efforts for development (real estate or agriculture), andflooding to create recreationallakes or generatehydropower. Some of the world's most important agricultural areas were wetlands that have been converted to farmland.[94][95][96][97] Since the 1970s, more focus has been put on preserving wetlands for their natural functions. Since 1900, 65–70% of the world's wetlands have been lost.[98] In order to maintain wetlands and sustain their functions, alterations and disturbances that are outside the normal range of variation should be minimized.
Balancing wetland conservation with the needs of people
Wetlands are vital ecosystems that enhance the livelihoods for the millions of people who live in and around them. Studies have shown 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 howdambos – wet, grassy valleys or depressions where water seeps to the surface – can be farmed sustainably. Project outcomes included a high yield of crops, development ofsustainable farming techniques, and water management strategies that generate enough water for irrigation.[99]
World map showing the coverage of wetlands as percentage of total land area defined as wetland according to the Ramsar convention by country[100]
TheRamsar Convention (full title:Convention on Wetlands of International Importance, especially as Waterfowl Habitat), is an internationaltreaty 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 some 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 (IOPs). These are:Birdlife International, theIUCN, theInternational Water Management Institute,Wetlands International and theWorld Wide Fund for Nature. The partners provide technical expertise, help conduct or facilitate field studies and provide financial support.
Restoration andrestoration ecologists intend to return wetlands to their natural trajectory by aiding directly with the natural processes of the ecosystem.[10] 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.[10] Restoration is needed after disturbance orperturbation of a wetland.[10] 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.[10]
Factors influencing selected approach may include[10] budget, time scale limitations, project goals, level of disturbance, landscape and ecological constraints, political and administrative agendas and socioeconomic priorities.
For this strategy, there is no biophysical manipulation and the ecosystem is left to recover based on the process ofsuccession alone.[10] The focus is to eliminate and prevent further disturbance from occurring and for this type of restoration requires prior research to understand the probability that the wetland will recover naturally. This is likely to be the first method of approach since it is the least intrusive and least expensive although some biophysical non-intrusive manipulation may be required to enhance the rate of succession to an acceptable level.[10] Example methods include prescribed burns to small areas, promotion of site specific soilmicrobiota and plant growth using nucleation planting whereby plants radiate from an initial planting site,[101] and promotion of niche diversity or increasing the range of niches to promote use by a variety of different species.[10] These methods can make it easier for the natural species to flourish by removing environmental impediments and can speed up the process of succession.
For this strategy, a mixture of natural regeneration and manipulated environmental control is used. This may require some engineering, and more intensive biophysical manipulations including ripping ofsubsoil,agrichemical applications of herbicides or insecticides, laying ofmulch, mechanical seed dispersal, and tree planting on a large scale.[10] In these circumstances the wetland is impaired and without human assistance it would not recover within an acceptable period of time as determined by ecologists. Methods of restoration used will have to be determined on a site by site basis as each location will require a different approach based on levels of disturbance and the local ecosystem dynamics.[10] Another form of partial reconstruction includes the utilization of semi-natural wetlands, such aspaddy fields that are agricultural plains covered by water during planting seasons.[102] Human assistance is required in order to maintain paddy fields, as they are agricultural in nature, but they have the capacity to reduce flooding in more inland regions.[102]
This most expensive and intrusive method of reconstruction requires 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 promoted will eventually return the ecosystem towards its natural trajectory.[10]
In many cases constructed wetlands are often designed to treat stormwater/wastewater runoff. They can be used in developments as part ofwater-sensitive urban design systems and have benefits such as flood mitigation, removing pollutants, carbon sequestration, providing habitat for wildlife and biodiversity in often highly urbanised and fragmented landscapes.[103]
The mechanism by which wetlands are able to support flood mitigation efforts is multifold. Due to their capacity to hold excess volumes of water during periods of heavy rainfall or inland water flow, wetlands are able to elicit reductions in flood area, flood depth, and flood duration.[104] Furthermore, wetlands are able to reduce the velocity of inland water flow, which is an additional mechanism by which wetlands reduce damages to local ecosystems and property found in surrounding regions.[105]
The ideas fromtraditional ecological knowledge can be applied as a holistic approach to the restoration of wetlands.[106] These ideas focus more on responding to the observations detected from the environment considering that each part of a wetland ecosystem is interconnected. Applying these practices on specific locations of wetlands increase productivity, biodiversity, and improve its resilience. These practices include monitoring wetland resources, planting propagules, and addition of key species in order to create a self-sustaining wetland ecosystem.[107]
In Southeast Asia,peat swamp forests and soils are being drained, burnt, mined, and overgrazed, contributing toclimate change.[108] As a result of peat drainage, the organic carbon that had built up over thousands of years and is normally under water is suddenly exposed to the air. The peat decomposes and is converted intocarbon dioxide (CO2), which is then released into the atmosphere. Peat fires cause the same process to occur rapidly and in addition create enormous clouds of smoke that cross international borders, which now happens almost yearly in Southeast Asia. While peatlands constitute only 3% of the world's land area, their degradation produces 7% of all CO2 emissions.
Wetlands are characterized bywater-loggedsoils and distinctive communities ofplant andanimalspecies that haveadapted to the constant presence ofwater. This high level of water saturation creates conditions conducive to methane production. Mostmethanogenesis, or methane production, occurs inoxygen-poor environments. Because themicrobes that live in warm, moist environments consume oxygen more rapidly than it candiffuse in from the atmosphere, wetlands are the ideal anaerobic environments forfermentation as well asmethanogen activity. However, levels of methanogenesis fluctuates due to the availability ofoxygen, soil temperature, and the composition of the soil. A warmer, more anaerobic environment with soil rich in organic matter would allow for more efficient methanogenesis.[113]
Some wetlands are a significant source of methane emissions[114][115] and some are also emitters ofnitrous oxide.[116][117] Nitrous oxide is agreenhouse gas with aglobal warming potential 300 times that of carbon dioxide and is the dominantozone-depleting substance emitted in the 21st century.[118] Wetlands can also act as a sink for greenhouse gases.[119]
Studies have favorably identified the potential for coastal wetlands (also calledblue carbon ecosystems) to provide some degree ofclimate change mitigation in two ways: by conservation, reducing the greenhouse gas emissions arising from the loss and degradation of such habitats, and by restoration, to increase carbon dioxide drawdown and its long-term storage.[120] However, CO2 removal using coastal blue carbon restoration has questionable cost-effectiveness when considered only as a climate mitigation action, either forcarbon-offsetting or for inclusion inNationally Determined Contributions.[120]
When wetlands are restored they have mitigation effects through their ability tosink carbon, converting a greenhouse gas (carbon dioxide) to solid plant material through the process ofphotosynthesis, and also through their ability to store and regulate water.[121][122]
Wetlands store approximately 44.6 million tonnes of carbon per year globally (estimate from 2003).[123] Insalt marshes and mangrove swamps in particular, the averagecarbon sequestration rate is210 g CO2 m−2 y−1 whilepeatlands sequester approximately20–30 g CO2 m−2 y−1.[123][124]
Coastal wetlands, such as tropicalmangroves and some temperatesalt marshes, are known to be sinks for carbon that otherwise contribute toclimate change in its gaseous forms (carbon dioxide and methane).[125] The ability of many tidal wetlands to store carbon and minimize methane flux from tidal sediments has led to sponsorship ofblue carbon initiatives that are intended to enhance those processes.[126][127]
The restoration of coastal blue carbon ecosystems is highly advantageous forclimate change adaptation, coastal protection, food provision and biodiversity conservation.[120]
Since the middle of the 20th century, human-causedclimate change has resulted in observable changes in the globalwater cycle.[128]: 85 A warming climate makes extremely wet and very dry occurrences more severe, causing more severe floods and droughts. For this reason, some of the ecosystem services that wetlands provide (e.g. water storage and flood control, groundwater replenishment, shoreline stabilization and storm protection) are important for climate change adaptation measures.[129] In most parts of the world and under allemission scenarios, water cycle variability and accompanying extremes are anticipated to rise more quickly than the changes of average values.[128]: 85
The value of a wetland to local communities typically involves first mapping a region's wetlands, then assessing the functions and ecosystem services the wetlands provide individually and cumulatively, and finally evaluating that information to prioritize or rank individual wetlands or wetland types for conservation, management, restoration, or development.[130] Over the longer term, it requires keeping inventories[131] of known wetlands and monitoring a representative sample of the wetlands to determine changes due to both natural and human factors.
Rapid assessment methods are used to score, rank, rate, or categorize various functions,ecosystem services, species, communities, levels of disturbance, and/orecological health of a wetland or group of wetlands.[132] 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 andgeographic information system (GIS) analyses of existing spatial data, but not detailed post-visit laboratory analyses of water or biological samples.
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.[133] Rapid assessment methods, partly because they often use dozens of indicators of 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.[13] A need for wetland assessments to be rapid arises mainly 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.
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.[134] 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. Identifying small, long, and narrow wetlands within the landscape is difficult. Many of today'sremote sensing satellites do not have sufficient spatial and spectral resolution to monitor wetland conditions, although multispectral IKONOS[135] and QuickBird[136] data may offer improved spatial resolutions once it is 4 m or higher. Most of the pixels are just mixtures of several plant species or vegetation types. They are difficult to isolate, translating into an inability to classify the vegetation that defines the wetland. The growing availability of 3D vegetation and topography data from LiDAR has partially addressed the limitation of traditional multispectral imagery, as demonstrated in some case studies worldwide.[137]
A wetland needs to be monitored[138] over time to assess whether it is functioning at an ecologically sustainable level or whether it is becoming degraded.[139] 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. Remote sensing such as aerial imagery and satellite imaging[140] provides effective tools to map and monitor wetlands across large geographic regions and over time. Many remote sensing methods can be used to map wetlands. The integration of multi-sourced data such asLiDAR and aerial photos proves more effective at mapping wetlands than the use of aerial photos alone,[137] especially with the aid of modern machine learning methods (e.g., deep learning). Overall, using digital data provides a standardized data-collection procedure and an opportunity for data integration within ageographic information system.
Every three years, representatives of the contracting parties meet as theConference of the Contracting Parties (COP), the policy-making organ of theconvention which adopts decisions (site designations, resolutions and recommendations) to administer the work of the convention and improve the way in which the parties are able to implement its objectives.[142] In 2022, COP15 was held in Montreal, Canada.
Each country and region tends to have a codified definition of wetlands 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".[143] This definition has been used in the enforcement of theClean Water Act. Some US states, such asMassachusetts andNew York, have separate definitions that may differ from the federal government's.
In theUnited 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 these legal definitions, "normal circumstances" are 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. Still, under normal environmental conditions, the soils will be inundated to the surface, creating anaerobic conditions persisting through the wet portion of the growing season.[144]
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