Freshwater swamp forest in Bangladesh
Peat bogs are freshwater wetlands that develop in areas with standing water and low soil fertility.
Marshes develop along the edges of rivers and lakes.
A wetland is a distinct ecosystem that is inundated by water, either permanently or seasonally, where oxygen-free processes prevail. The primary factor that distinguishes wetlands from other land forms or water bodies is the characteristic vegetation of aquatic plants, adapted to the unique hydric soil.
Wetlands play a number of functions, including water purification,
water storage, processing of carbon and other nutrients, stabilization
of shorelines, and support of plants and animals. Wetlands are also considered the most biologically diverse
of all ecosystems, serving as home to a wide range of plant and animal
life. Whether any individual wetland performs these functions, and the
degree to which it performs them, depends on characteristics of that
wetland and the lands and waters near it. Methods for rapidly assessing these functions, wetland ecological health, and general wetland condition have been developed in many regions and have contributed to wetland conservation partly by raising public awareness of the functions and the ecosystem services some wetlands provide.
Wetlands occur naturally on every continent. The main wetland types are swamp, marsh, bog, and fen; sub-types include mangrove forest, carr, pocosin, floodplains, mire, vernal pool, sink, and many others. Many peatlands are wetlands. The water in wetlands is either freshwater, brackish, or saltwater.
Wetlands can be tidal (inundated by tides) or non-tidal. The largest wetlands include the Amazon River basin, the West Siberian Plain, the Pantanal in South America, and the Sundarbans in the Ganges-Brahmaputra delta.
The UN Millennium Ecosystem Assessment determined that environmental degradation is more prominent within wetland systems than any other ecosystem on Earth.
Constructed wetlands are used to treat municipal and industrial wastewater as well as stormwater runoff. They may also play a role in water-sensitive urban design.
Definitions
A patch of land that develops pools of water after a rain storm
would not necessarily be considered a "wetland", even though the land
is wet. Wetlands have unique characteristics: they are generally
distinguished from other water bodies or landforms based on their water level and on the types of plants that live within them. Specifically, wetlands are characterized as having a water table that stands at or near the land surface for a long enough period each year to support aquatic plants.
A more concise definition is a community composed of hydric soil and hydrophytes.
Wetlands have also been described as ecotones, providing a transition between dry land and water bodies. 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.
Technical definitions
Sunrise at Viru Bog, Estonia
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." 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. The largest wetlands in the world include the swamp forests of the Amazon and the peatlands of Siberia.
Ramsar Convention definition
Under the Ramsar international wetland conservation treaty, wetlands are defined as follows:
- Article 1.1: "...wetlands are areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt, including areas of marine water the depth of which at low tide does not exceed six 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."
Regional definitions
Although
the general definition given above applies around the world, 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.
Ecology
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.
Characteristics
Wetlands vary widely due to local and regional differences in topography, hydrology, vegetation, and other factors, including human involvement.
Hydrology
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.
Role of salinity
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.
Soil
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.
Biota
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.
Flora
Bud of Nelumbo nucifera, an aquatic plant.
There are four main groups of hydrophytes that are found in wetland systems throughout the world.
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.
Fauna
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Many species of frogs 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.
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.
Insects and invertebrates
total more than half of the 100,000 known animal species in wetlands.
Insects and invertebrates can be submerged in the water or soil, on the
surface, and in the atmosphere.
Algae
Algae
are diverse water plants 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.
Climates
Temperature
Wetlands contrast the hot, arid landscape around Middle Spring, Fish Springs National Wildlife Refuge, Utah
Wetlands are located in every climatic zone. 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.
Precipitation
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.
Temporal variation:
- Perennial systems
- Seasonal systems
- Episodic (periodic or intermittent) system of the down
- Surface flow may occur in some segments, with subsurface flow in other segments
- Ephemeral (short-lived) systems
- Migratory species
Uses of wetlands
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:
- Water storage (flood control)
- Groundwater replenishment
- Shoreline stabilisation and storm protection
- Water purification
- Reservoirs of biodiversity
- Pollination
- 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.
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.
Water storage (flood control)
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.
Groundwater replenishment
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.
Shoreline stabilization and storm protection
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.
Water purification
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. 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.
Constructed wetlands
Constructed wetland in an ecological settlement in Flintenbreite near Lübeck, Germany
The function of most natural wetland systems is not to manage wastewater.
However, their high potential for the filtering and the treatment of
pollutants has been recognized by environmental engineers that
specialize in the area of wastewater treatment.
These constructed wetland systems are highly controlled environments
that intend to mimic the occurrences of soil, flora, and microorganisms
in natural wetlands to aid in treating wastewater effluent. Constructed
wetlands can be used to treat raw sewage, storm water, agricultural and industrial effluent.
They are constructed with flow regimes, micro-biotic composition, and
suitable plants in order to produce the most efficient treatment
process. Other advantages of constructed wetlands are the control of retention times and hydraulic channels. The most important factors of constructed wetlands are the water flow processes combined with plant growth.
Constructed wetland systems can be surface flow systems with only free-floating macrophytes,
floating-leaved macrophytes, or submerged macrophytes; however, typical
free water surface systems are usually constructed with emergent
macrophytes.
Subsurface flow-constructed wetlands with a vertical or a horizontal
flow regime are also common and can be integrated into urban areas as
they require relatively little space.
Reservoirs of biodiversity
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. One of the most exciting discoveries was made by A. V. Shoshin
who registered about 300 species of free-living nematodes
using only six near-shore sampling localities in the Southern Baikal.
"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 products and productivity
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 (cattail, bulrush).
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.
Other mangrove-derived products:
- Fuelwood
- 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.
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.
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.
Additional functions and uses of wetlands
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.
Wetlands and climate change
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.
However, depending on their characteristics, some wetlands are a significant source of methane emissions and some are also emitters of nitrous oxide which 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.
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.
Peatswamps in Southeast Asia
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.
Wetland Disturbance
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
- Acidification
- Salinization
- Sedimentation
- Altered Solar Input (Turbidity/Shade)
- Vegetation Removal
- Thermal Alteration
- Dehydration/Aridification
- Inundation/Flooding
- 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
- Drainage
- Development
- Over-grazing
- Mining
- Unsustainable water use
- They can be manifested partly as:
- Water scarcity
- Impacts to Endangered species
- Disruption of wildlife breeding grounds
- Imbalance in sediment load and nutrient filtration
Water Chemistry
Anthropogenic
nitrogen inputs to aquatic systems have drastically effected the
dissolved nitrogen content of wetlands, introducing higher nutrient
availability which leads to eutrophication. Due to the low dissolved oxygen (DO) content, and relatively low
nutrient balance of wetland environments, they 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+)
- pH
- Conductivity- Conductivity increases with more dissolved ions in the water
- Turbidity
- Dissolved Oxygen
- Temperature
- Total Dissolved Solids
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.
Conservation
Fog rising over the Mukri bog near Mukri, Estonia. The bog has an area of 2,147 hectares (5,310 acres) and has been protected since 1992.
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.
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 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.
Ramsar Convention
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.
Valuation
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.
Assessment
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 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.
Inventory
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.
Monitoring
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.
Mapping
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.
Further improvements
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
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.
Levels of restoration
- Factors influencing selected approach may include:
- Budget
- Time scale limitations
- Project goals
- Level of disturbance
- Landscape and ecological constraints
- Political and administrative agendas
- Socioeconomic priorities
- Prescribed Natural Regeneration
- 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.
- Assisted Natural Regeneration
- 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.
- Partial Reconstruction
- 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.
- Complete Reconstruction
- 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.
Important considerations
- Constructed wetlands can take 10–100 years to fully resemble the vegetative composition of a natural wetland.
- Artificial wetlands do not have hydric soil. The soil has very low levels of organic carbon and total nitrogen compared to natural wetland systems, and this reduces the performance of several functions.
- Organic matter added to degraded natural wetlands can in some cases help restore their productivity.
Legislation
- International Efforts
- Canadian National Efforts
- The Federal Policy on Wetland Conservation
- Other Individual Provincial and Territorial Based Policies
List of wetland types
The following list is that used within Australia to classify wetland by type:
- A—Marine and Coastal Zone wetlands
- Marine waters—permanent shallow waters less than six metres deep at low tide; includes sea bays, straits
- Subtidal aquatic beds; includes kelp beds, seagrasses, tropical marine meadows
- Coral reefs
- Rocky marine shores; includes rocky offshore islands, sea cliffs
- Sand, shingle or pebble beaches; includes sand bars, spits, sandy islets
- Intertidal mud, sand or salt flats
- Intertidal marshes; includes saltmarshes, salt meadows, saltings, raised salt marshes, tidal brackish and freshwater marshes
- Intertidal forested wetlands; includes mangrove swamps, nipa swamps, tidal freshwater swamp forests
- Brackish to saline lagoons and marshes with one or more relatively narrow connections with the sea
- Freshwater lagoons and marshes in the coastal zone
- Non-tidal freshwater forested wetlands
- B—Inland wetlands
- Permanent rivers and streams; includes waterfalls
- Seasonal and irregular rivers and streams
- Inland deltas (permanent)
- Riverine floodplains; includes river flats, flooded river basins, seasonally flooded grassland, savanna and palm savanna
- Permanent freshwater lakes (more than 8 ha); includes large oxbow lakes
- Seasonal/intermittent freshwater lakes (more than 8 ha), floodplain lakes
- Permanent saline/brackish lakes
- Seasonal/intermittent saline lakes
- Permanent freshwater ponds (less than 8 ha), marshes and swamps on inorganic soils; with emergent vegetation waterlogged for at least most of the growing season
- Seasonal/intermittent freshwater ponds and marshes on inorganic soils; includes sloughs, potholes; seasonally flooded meadows, sedge marshes
- Permanent saline/brackish marshes
- Seasonal saline marshes
- Shrub swamps; shrub-dominated freshwater marsh, shrub carr, alder thicket on inorganic soils
- Freshwater swamp forest; seasonally flooded forest, wooded swamps; on inorganic soils
- Peatlands; forest, shrub or open bogs
- Alpine and tundra wetlands; includes alpine meadows, tundra pools, temporary waters from snow melt
- Freshwater springs, oases and rock pools
- Geothermal wetlands
- Inland, subterranean karst wetlands
- C—Human-made wetlands
- Water storage areas; reservoirs, barrages, hydro-electric dams, impoundments (generally more than 8 ha)
- Ponds, including farm ponds, stock ponds, small tanks (generally less than 8 ha)
- Aquaculture ponds; fish ponds, shrimp ponds
- Salt exploitation; salt pans, salines
- Excavations; gravel pits, borrow pits, mining pools
- Wastewater treatment; sewage farms, settling ponds, oxidation basins
- Irrigated land and irrigation channels; rice fields, canals, ditches
- Seasonally flooded arable land, farm land
Other classification systems for wetlands exist. In the US, the best known are the Cowardin classification system and the hydrogeomorphic (HGM) classification system.
