A wetland is an ecosystem in which water covers the ground or saturates soil at or near the surface for sufficient periods to shape soil development and biological communities. Wetlands include marshes, swamps, bogs, and fens, and may be permanently flooded, seasonally inundated, or intermittently wet. Their defining feature is not simply standing water but the influence of saturation on soils and vegetation. They often connect terrestrial and aquatic environments and support organisms adapted to both. (epa.gov)
Definition and distribution
Wetland definitions vary with scientific, management, and legal purposes. Ecological descriptions generally emphasize water regime, characteristic soils, and vegetation adapted to saturated conditions. The Convention on Wetlands, also called the Ramsar Convention, uses a broader definition encompassing natural and artificial, permanent and temporary waters. It includes lakes, rivers, rice paddies, reservoirs, tidal flats, and marine waters no deeper than six metres at low tide. Consequently, inventories using different definitions are not always directly comparable. (epa.gov)
Wetlands occur from tundra to tropical regions. Inland examples occupy floodplains, lake margins, poorly drained depressions, and places where groundwater reaches the surface. Coastal wetlands occur along sheltered shores and within estuaries, where freshwater and seawater interact. Water sources, topography, climate, and water chemistry produce substantial differences among wetlands, even within the same region. A wetland that appears dry during part of the year may still provide essential breeding habitat for specialized wildlife. (epa.gov)
Major types
Common wetland names describe vegetation, water sources, or soil characteristics rather than a single universal classification:
- Marshes are dominated by emergent, soft-stemmed vegetation, including reeds, sedges, and rushes. They can contain fresh, brackish, or saline water.
- Swamps are dominated by woody vegetation, either trees or shrubs. Many experience seasonal flooding.
- Bogs are peat-forming wetlands supplied mainly by precipitation. They commonly have acidic conditions and relatively low nutrient availability.
- Fens are peat-forming wetlands supplied by groundwater or drainage from surrounding mineral soils. They generally receive more nutrients and are less acidic than bogs.
- Mangrove wetlands contain salt-tolerant woody plants along tropical and subtropical coasts. (epa.gov)
These categories overlap with broader landscape classifications. The Cowardin system, used by the United States National Wetlands Inventory, distinguishes marine, estuarine, riverine, lacustrine, and palustrine systems. Hydrogeomorphic classifications instead emphasize landform, dominant water source, and water movement. Such approaches describe differences that vegetation names alone cannot capture. (epa.gov)
Hydrology and ecological processes
The timing, duration, frequency, and depth of inundation or saturation are central to wetland ecology. Water may arrive through precipitation, river flooding, groundwater discharge, or tides. Seasonal changes influence which plants establish, when animals reproduce, and how wetlands exchange materials with neighboring environments. Wetlands therefore function within a wider drainage basin, rather than as isolated patches of wet ground. (epa.gov)
Waterlogged soils can develop oxygen-poor conditions that alter microbial activity and decomposition. In peat-forming wetlands, incompletely decomposed plant material accumulates as peat. These deposits preserve organic matter over long periods and make peatlands important components of the carbon cycle. Not all wetlands accumulate peat: many have predominantly mineral soils. (epa.gov)
Wetland organisms also participate in the nitrogen cycle and other nutrient cycles. Shallow water, available nutrients, and plant production support food webs involving invertebrates, fish, amphibians, birds, and mammals. Different combinations of water depth, vegetation, and seasonal flooding create feeding, sheltering, and breeding opportunities, contributing to biodiversity across the surrounding landscape. (epa.gov)
Ecosystem services and climate
Wetlands provide several ecosystem services. They can store floodwater, slow its movement, and reduce erosion. Vegetation stabilizes soils, while reduced water velocity allows suspended sediment to settle. Plants and microorganisms take up nutrients, helping improve water quality. These functions depend on the wetland’s characteristics and connections to other parts of its drainage basin; they are not identical at every site. Wetlands also support fisheries, recreation, education, and other human activities. (epa.gov)
Their climatic role involves both carbon storage and gas emissions. Wetland vegetation and soils retain carbon that might otherwise enter the atmosphere as carbon dioxide. However, oxygen-poor conditions can also support production of methane, a greenhouse gas. Carbon storage alone therefore does not describe the complete atmospheric effect of a wetland. Draining or otherwise degrading peatlands can turn long-term carbon stores into significant sources of greenhouse gases. (epa.gov)
Threats, conservation, and restoration
Conversion to agriculture or development, drainage, pollution, and changes to water regimes can reduce wetland area or alter ecological functions. Removing trees from a forested wetland, for example, changes habitat structure even if the site remains wet. Peatlands also face pressures from urbanization, industrialization, land-use change, and climate change. Conservation consequently concerns both wetland extent and ecological condition. (epa.gov)
The Ramsar Convention was adopted in Ramsar, Iran, on 2 February 1971 and entered into force in 1975. Its framework combines the wise use of all wetlands, designation and management of internationally important sites, and international cooperation. Wise use means maintaining ecological character through ecosystem approaches within the context of sustainable development; it does not require excluding every human use. (ramsar.org)
Wetland ecological restoration seeks to recover natural functions by repairing physical, chemical, or biological conditions. Re-establishment rebuilds a former wetland, whereas rehabilitation repairs an existing degraded one. Restoring an appropriate water regime is often fundamental. Project assessment considers native communities, landscape connections, measurable objectives, and continuing monitoring. Restoration complements protection of intact wetlands rather than replacing it. (epa.gov)