Areas inundated by fresh, brackish, and salt water are all considered wetlands; among many wetland types are fens and bogs, tidal marshes, riparian zones, and lakeshores. Wetlands, which cover less than 9% of the Earth's surface, can be extremely productive and many are disproportionately large providers of ecosystem services. Three of the key services that wetlands provide are flood mitigation, water purification, and biodiversity support.
In the upper part of a watershed, many wetlands store water that flows overland toward rivers and streams. They can release this water into the main channel slowly, reducing and delaying flood peaks. Downstream, wetlands can absorb and reduce peak flood levels, providing area into which flood waters can spread, dissipating flood energy by slowing water movement, and removing flood water through transpiration and infiltration.
The same physical characteristics of wetlands that slow and absorb overland flow related to flooding can also provide a mechanism for storing and detoxifying urban and agricultural wastewater before it discharges directly into a main channel. Wetlands filter out various nutrients, other pollutants, and sediment: they support anaerobic bacteria that denitrify waste; the plants take up and store nutrients; and by slowing and redirecting water flow, wetlands enhance sedimentation - the accreting sediments can effectively bury pollutants. While many wetlands can purify water very economically, their effectiveness depends on many factors, including rate of inflow, amount of sediment and organics in the wastewater, residence time of wastewater in the wetland, and total surface area.
A wide variety of animals rely on wetlands for survival. Plant species that deliver flood abatement and water purification can also support biodiversity, providing varied food and shelter. A riparian wetland, for example, might provide food plants and underground burrows for muskrats; seeds, food plants, and nest-building materials for ducks; and food and shelter for fish and invertebrates.
Wetlands provide a variety of other services as well. Major products associated with wetlands are peat, timber, and mulch. Regulating services in addition to flood mitigation and water purification include waste detoxification, carbon storage, and control of pests and diseases. Wetlands provide many cultural services as well, particularly recreation services such as bird watching, boating, and hunting. Wetlands also provide key supporting services, such as soil formation and buffering freshwater aquifers from saltwater intrusion.
Worldwide, wetlands are estimated to provide many billions of dollars in services each year. They are recognized by the international treaty, the Ramsar Convention on Wetlands, and regulated by domestic law in many countries. Nonetheless, they have historically undergone widespread losses in favor of other land uses; worldwide, 50% of wetlands are estimated to have been lost since 1900.
While the services provided by wetlands are widely recognized, simultaneously maximizing multiple services may not be possible. In some cases this is related to location: upland watersheds may be very important for flood control but may be too far upstream to have an impact on water purification. In other cases one service may thrive to the detriment of another: a wetland that is absorbing a heavy nutrient load may be overtaken by a single, aggressive plant species and thus fail to be an effective reservoir for biodiversity. Finally, it can be costly to measure function and hence difficult to judge how effectively a wetland is performing a given service or how to manage for that particular service.
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