Wetlands

Wetlands are productive ecosystems that are formed when water is present for a sufficient period of time to create low-oxygen conditions in the soil, resulting in the creation of unique soil properties and a suite of wetland plants capable of living and reproducing in these environments. Most wetland definitions exclude permanently flooded areas such as lakes, rivers, and oceans or temporary puddles, such as those that occur on driveways, because of a lack of wetland plants.

Wetlands may be associated with streams and lakes, or they may be isolated from surface water. Wetlands include both fresh and saltwater systems and have a variety of local names such as marsh (dominated by emergent herbaceous vegetation), salt marsh (emergent herbaceous vegetation in saltwater), swamp (trees and shrubs), peatland (accumulating dead plant material), bog (peat accumulating wetlands with no significant water inflow or outflow), fen (peat wetland with some water flow), seep (sloped wetlands), wet meadow (herbaceous vegetation with waterlogged soils), oxbow (abandoned river channel), vernal pools (seasonally flooded pools in forested environments), and playa (arid or semi-arid wetlands). Wetlands may be completely natural systems, or highly altered and managed systems, such as rice (Oryza) fields and cranberry (Vaccinium) bogs. Although diverse in name and appearance, all wetlands generally have water, wetland vegetation, and unique soils.

Water or hydrology is central to the functioning of a wetland. The cumulative effects of water level, flow, seasonal pattern, and frequency of inundation dictates the plant communities and species composition, and the rate of chemical processing, which influences animal communities and other attributes within the wetland. Under most circumstances, wetlands must have standing water or be saturated within 11.8 inches (30 centimeters) of the soil surface for two weeks or more during the growing season, at least every other year. However, there is a large gradient in the hydroperiod (water regimes) of wetlands, from permanently to intermittently flooded. Permanently flooded wetlands are flooded throughout the year in all years, whereas in intermittently flooded wetlands, the surface is usually exposed with surface water present for variable periods, without detectable seasonal patterns. The overall hydroperiod or water budget results from the balance between inflows and outflows of water, which is dictated by the surface contour of the landscape, subsurface soil, geology, and groundwater conditions. Sources of water to a wetland include precipitation from rain and snowfall, surface inflows from overland or from streams and groundwater, and tidal inflow in coastal wetlands. Water is lost from a wetland from surface outflows, groundwater outflows, evaporation, plant transpiration, and tidal outflow.

Wetland Plants and Animals

Wetland plants, technically called hydrophytes or hydrophytic vegetation, are important components of wetland systems. Hydrophytic vegetation are plants that grow in water or in soil that at least periodically has low oxygen levels as a result of excessive water content. Hydrophytes have unique adaptations to survive in wetland environments. Aerenchyma, porous tissues in roots and stems that are filled with large gas-filled spaces, allow efficient storage, exchange, and movement of oxygen, carbon dioxide, and ethylene. Adventitious roots, roots that arise from tissues other than root tissues, spread onto the surface layer of the soil or grow above the soil surface to increase access to the air. Bald cypress (Taxodium distichum) has modified erect roots that grow from the roots above the water surface to aid in efficient carbon dioxide and oxygen exchange. Many wetland tree species have shallow root systems as a mechanism to grow in waterlogged soil, and take advantage of occasional dry periods to obtain atmospheric oxygen. However, shallow roots make trees susceptible to wind, so large trees must maintain a balance between remaining upright and rooted, and efficient respiration.

Many hydrophytes provide the basis for the complex wetland food web (the interwoven pathways by which the plant materials are consumed by other trophic levels of organisms) and also can contribute substantially to the food webs in terrestrial and aquatic systems. Many waterfowl species, as well as muskrats (Ondatra zibethicus) and beaver (Castor canadensis), consume the seeds or tubers of wetlands plants. Wetland managers manipulate water regimes to produce specific species of seeds, such as millets (Echinochloa) and smartweeds (Persicaria), to attract waterfowl. Much of the plant material, particularly from leaves and stems, enters the food chain as detritus as the plants break down and decompose at the end of the growing season. Detritus are small plant particles resulting from the breakdown and decomposition of the plants, and are subsequently consumed by various organisms such as invertebrates, which are then consumed by fish and wildlife. A variety of organisms also use plants as cover or habitat. Wetland plants enhance water quality by removing nutrients and some toxins from the water and storing them. Moreover, hydrophytes promote wetland function by reducing peak flood events and stabilizing soils.

Hydrophytes are classified into primary categories based on their growth form and appearance: emergent, floating, floating-leaved, scrub-shrub, trees, and submerged. Emergent wetland plants, including cattails (Typha), common reed (Phragmites australis), and common rush (Juncus effusus), are herbaceous species rooted in soil with basal portions that typically grow beneath the surface of the water, but with aerial leaves, stems (photosynthetic parts), and reproductive organs. Floating plants are not rooted in wetland soil, and instead float on the surface of the wetland. Roots may or may not be present, and there is no connection to the bottom substrate.

Water lettuce (Pistia stratiotes), duckweeds (Lemna), and water hyacinths (Eichhornia) are highly productive examples of the floating growth form. Water lilies (Nymphaea) and other floating-leaved plants have leaves that float on the water's surface and roots that are anchored in the substrate. Stems connect the leaves, which are circular or oval, and have a tough leathery texture designed to repel water to the bottom. Scrub-shrub plants are rooted in the soil, like emergent plants, but have a woody stem, growing up to six meters tall. Scrub-shrub plants may be true shrubs or small trees. Trees are large woody plants greater than 19 feet (6 meters) tall. Submerged plants, often referred to as submerged aquatic vegetation, spend their entire life cycle beneath the surface of the water. Nearly all are rooted in the substrate. Submerged plants take up dissolved oxygen and carbon dioxide from the water column and are well known for their water clearing abilities.

Hydric soils are unconsolidated organic and mineral matter on the Earth's surface where all pore space is filled with water, the soil surface is temporarily covered with flowing water, or water stands in a depression for a long enough period of time during the growing season to promote the absence of molecular oxygen in the soil. As a soil becomes saturated with water, it progresses through a series of chemical and physical steps involving inundation with water, promotion of anaerobic conditions, sequencing of a set series of biochemical reductions, slowing of organic matter decomposition, accumulation of distinct mottles or spots of different colors within the dominant soil color, and accumulation of organic matter. Many wetland soils are organic, but some are mineral. Soil color, texture, structure, and even smell all provide clues to the type of hydric soil present in a wetland. For example, some wetlands have a rotten-egg smell under anaerobic conditions because of the presence of sulfate-sulfur.

Location of Wetlands

Wetlands occur throughout the world. An estimated 5 to 8 percent of the land surface is covered by wetlands, with greater amounts in tropical (31 percent), subtropical (25 percent), and boreal (30 percent) regions, and lesser amounts in polar (2 percent) and subboreal (11 percent) regions. About 95 percent of wetlands are freshwater and only 5 percent are saltwater. The Pantanal in Brazil, Paraguay, and Bolivia, covering about 88,803 square miles (230,000 square kilometers), is often considered the largest wetland in the world. Wetlands are often viewed unfavorably as a hindrance to development and progress, or as disease reservoirs for mosquitoes carrying and spreading malaria, yellow fever, and other maladies. Wetlands have been drained and filled to facilitate agriculture, development of residential and industrial complexes, road building, mineral extraction, and other human infrastructure.

More than 50 percent of wetlands around the world have been destroyed because of human activities, and many more are affected by water pollution and groundwater withdrawal. The conterminous (lower 48) United States has lost 53 percent of wetlands, which is probably the most accurate estimate available for any region. New Zealand has lost 90 percent, Europe and China 60 percent, and Australia 50 percent of their wetlands. Wetland loss is most rapid during periods of growth and development in countries. Coastal areas and agricultural regions have suffered the most rapid and extensive wetland losses because of high human populations, high land use conversion rates, and a propensity of wetlands. In developed countries, wetland loss has slowed because of contemporary laws designed to protect wetlands and because wetlands easy to drain and fill are now gone. However, the United States still lost 5,590 hectares annually between 2004 and 2009. From 1998 to 2004, the United States actually gained 89,140 hectares of wetlands because of national policies. Wetland losses and gains only tell part of the story. Open-water wetlands often increased, whereas more ecologically valuable vegetated wetland types continued to be lost. Wetland loss rates in developing countries will likely continue to remain high as technological advances and human population growth continues.

Benefits of Wetlands

Wetlands produce disproportionately high benefits, compared to the size of their presence on the landscape. Wetlands provide numerous ecological services to people, including providing a source of clean water, reducing flood damage, enhancing biodiversity, providing recreational opportunities, and producing food and fiber. Wetlands are highly effective at purifying and storing water. Because of their ability to remove pollutants from the environment, they are often referred to as the kidneys of the landscape. Wetlands are effective at removing pollutants and sediment because of a reduction in water velocity based on their position in the landscape and vegetation, which slows water velocity, in addition to the numerous chemical processes occurring in wetlands during alternating wetting and drying cycles. Because of their effectiveness, wetlands are specifically constructed to treat industrial and household wastewater and mine land runoff. Generally, the slower the water passage and the higher the complexity in vegetation and substrate, the better the system will function. Wetlands substantially reduce the impacts of floods by intercepting storm runoff and storing it. The ability of wetlands to store flood water reduces the sharpness of peak water flows (peak flows cause the most damage to buildings and other infrastructure) and allows a slower discharge of water over a longer period of time. This results in less severe flooding and less flood damage. Wetlands associated with rivers generally provide the most benefit in regard to flood control.

Wetlands are the most productive systems in the world, with net primary productivity reaching up to 1,200 grams per square meter. This high productivity results in complex food webs and abundant plant and animal life. Waterfowl, which includes ducks, geese, swans, and other waterbirds including herons, egrets, and shorebirds, are the most widespread inhabitants of wetlands. The Prairie Pothole Region of the Northern Great Plains of North America is among the most productive waterfowl-producing areas in the world. It is known as the North American Duck Factory because of the large numbers of migratory dabbling and diving ducks that the prairie potholes (glacial wetlands) produce. Wetlands with a balanced ratio of open water to emergent vegetation are most productive for both breeding and wintering birds. Wetlands also provide habitat for numerous amphibian (frogs, toads, and salamanders), reptile (turtles, snakes, and crocodilians), mammals, fish, and invertebrate species. Wetlands provide a unique resource for recreational use, including hunting, birding, angling, hiking, exploring, and botanizing. An estimated $10 billion is spent annually by 50 million people observing and photographing wetland-dependent wildlife. In the United States, waterfowl alone attract over 15.4 million observers. There are over 1.3 million waterfowl hunters who spend over 13 million days hunting and $780 million in expenditures annually in the United States. Everglades National Park, a 6,500-square-kilometer complex of wetlands in Florida, receives around 1 million visitors annually.

Wetlands provide numerous economically valuable products, including peat, pelts, hides, fish, shellfish, timber, fiber, fruits, and grains. Peat is harvested extensively for use in the horticultural industry, and to a lesser degree as a fuel source. Fur-bearing mammals that depend principally on wetlands include beaver, mink (Neovision vision), muskrat, otter (Lutra), and nutria (Myocaster coypus). American alligator (Alligator mississippiensis), saltwater crocodile (Crocodylus porosus), and spectacled caiman (Caiman crocodilus) are harvested for their valuable hides, which are used to make boots, belts, and other leather items. An estimated 95 percent of commercially harvested fish and shellfish, including pink shrimp (Farfantepenaeus duorarum) and a variety of crabs, oysters, crayfish, salmon, catfish, and menhaden are wetland-dependent species. Frogs are also commercially grown and harvested for their legs in some regions. Bald cypress trees produce commercially valuable timber that is used in construction, and also are important sources of mulch. Numerous other trees, such as pecans (Carya illinoinensis), swamp white oak (Quercus bicolor), and sweet-gum (Liquidambar) are also important for timber, and pecan trees also produce valuable nut crops. The productivity of many herbaceous wetland plants, such as cattails, common reed, and paper reed (Cyperus papyrus), produce material useful for making baskets, mats, and even roofing material. Rice production in specialized, manipulated wetland systems called rice paddies provide 20 percent of the human diet worldwide. Wild rice is also harvested for consumption, primarily by indigenous cultures, and is also often packaged and sold for premium prices. Cranberries are commonly grown in northern wetland systems or upland systems converted into wetlands specifically for growing cranberries.

Protection of Wetlands

Because of the valuable ecosystem services and functions that many wetlands provide, political entities, including the United States, have passed laws and regulations to protect wetlands. The primary purpose of these laws is to avoid impacting wetlands. However, avoidance is not always possible. In many cases, permits are needed from local, state, and federal officials if a wetland is to be filled. In the United States, the federal Clean Water Act is the primary mechanism for protecting wetlands. Wetlands are protected under Sections 401 and 404 of the Clean Water Act, which regulates the discharge of dredged or fill material into the waters of the United States. If an area meets the technical definition of a wetland, it is illegal to fill it in, unless a federal and state permit is obtained. The U.S. Environmental Protection Agency has the technical lead for issues dealing with wetlands. However, the US Army Corp of Engineers is the primary agency responsible for enforcing wetland regulations. The US Department of Agriculture's Natural Resources Conservation Service (NRCS) deals with most wetland issues on farmlands, but it is not a regulatory agency.

Mitigation in the form of creating a new wetland is often required as a condition to receive the permit. This requirement has a spawned an industry built around the delineation, creation, and restoration of wetlands. Wetland delineators determine the boundary between wetland and uplands to determine the size of a wetland and to determine where development should and should not occur. Most created wetlands are built near the site of impact as a means of replacing the lost wetlands function within the same watershed. However, function is not readily assessed or easily measured. Most mitigation in the past has been performed on a size basis, but functional assessment mitigation is increasing in popularity. A size-based mitigated strategy results in the loss of more complex vegetated wetland types, requiring larger wetlands to be built. Hence, forested wetlands require a larger ratio of created to destroyed land than emergent wetlands, and both require more than open-water wetlands. Recently, more emphasis has been placed on mitigation banking. Mitigation banks have been designated as the preferred means of compensatory mitigation for unavoidable wetland losses. Wetland banks are generally built by companies willing to make long-term investments for future potential profit by creating wetland prior to destruction. Once a wetland is impacted, a company or individual then buys credits from the bank. Large public agencies also may produce wetland banks for their use. Mitigation banking is more cost effective than traditional compensatory mitigation, larger wetland sites provide more ecological value, mitigation occurs before impacts so functional success is already known, and project review time is reduced, which reduces permit processing and enhances regulatory agency effectiveness.

In an April 2001 court case (Solid Waste Agency in Northern Cook County v. U.S. Army Corps of Engineers) the Supreme Court ruled that the US Army Corp of Engineers does not have jurisdictional authority to enforce wetland regulations on wetlands that are not associated with navigable waters. Currently, there is no protection for these isolated wetlands, except for disincentive programs associated with wetlands on farmlands or individualized state laws. The “swampbuster” provision of the 1985 Farm Bill withholds farm program benefits if a farmer drains, dredges, or otherwise manipulates a wetland to make production possible. Wetlands manipulated before 1985 are termed prior converted wetlands and are exempt from the swampbuster provision. Internationally, some wetlands are protected and receive recognition through the Convention on Wetlands, which was signed in Ramsar, Iran, in 1971. The Ramsar Convention was signed as an international treaty for member nations to maintain the ecological character of wetlands of international importance. The Ramsar definition is broader than most wetland definitions, but is an effective means to help protect wetlands on international boundaries, and also provides additional recognition to wetlands within partner countries. There are 160 member nations to the convention, with 1,960 sites encompassing 190 million hectares of internationally significant wetlands.

The interest in wetlands has continued to grow. Formal academic courses in wetland science started in the early 1970s and continued steady growth through the 1980s and 1990s. After 2000, the number of wetland courses increased substantially, which is indicative of the popularity and acceptance of wetland science as a discipline. The Society of Wetland Scientists, an international organization of professional scientists and managers dedicated to promoting wetland science, education, and management, was formed in 1980. There are also more specialized groups interested in particular aspects of wetlands, such as the International Peat Society, which formed in 1968; its vision is to be the authoritative international organization on all peat and peatland issues. Since 1981, the Society of Wetland Scientists has published a scientific peer-reviewed journal called Wetlands. In 1989, the journal Wetlands Ecology and Management was first published by Springer. The journal Mangroves and Salt Marshes was published from 1996 to 1999, and in 2000 was merged into Wetlands Ecology and Management. Moreover, numerous ecological and environmental management journals publish articles on wetlands. The popularity of wetland science is expected to continue to grow over time because of its ecological importance.

Wetlands will continue to be lost in the future because of development pressures. Climate change has the potential to exacerbate wetland loss problems, causing additional losses and leading to degradation in others. Increased temperature results in higher evaporation and plant transpiration rates, reducing water levels and changing the hydroperiods of wetlands. Rainfall and snowfall amounts and intensity are also predicted to vary. These changes have important implications for species of plants and animals that depend on specific water regimes and hydroperiod lengths for completing their life cycles. Higher temperatures also impact species with specific temperature tolerances such as fish and amphibians. These species either have to adapt, move, be moved by humans, or become extirpated. However, wetlands may be a key to combating climate change in some regions. In the 2020s, US scientists began researching the potential of wetlands to capture and sequester carbon dioxide in the atmosphere. This would help protect coastal communities from rising sea levels and storms. It would also lead to the restoration of many degraded wetlands.

Climate alteration also will impact the suite of pathogen and insect stressors affecting particular wetland communities. Some of the greatest potential for wetland loss is in coastal areas. Increased melting rate of glaciers leads to sea-level rise and the subsequent loss of coastal salt marshes. There is also much concern in northern peat bogs about drying wetlands releasing carbon back to the atmosphere and increased methane emissions. Wetland managers are working on solutions to address climate change issues, many of which involve comprehensive land planning, increased human assistance, and adopting an adaptive management strategy.

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