Wetland

• 1. Introduction

  ›

  A wetland is an area of land usually saturated with water, where water covers the soil or sits at or near the surface for part or all of the year. Flooding starves the soil of oxygen, producing anoxic conditions that ordinary land plants cannot survive. Yet wetlands rank among the most biologically diverse ecosystems on Earth. They exist on every continent except Antarctica, holding freshwater, brackish water, or saltwater. The Millennium Ecosystem Assessment from 2005 found them more threatened by environmental degradation than any other ecosystem. What makes a flooded, oxygen-poor patch of ground so valuable, and so vulnerable? Why have humans spent centuries draining the very places that purify their water and blunt their floods? The world's largest wetlands include the Amazon River basin, the West Siberian Plain, the Pantanal in South America, and the Sundarbans in the Ganges-Brahmaputra delta. To understand them, start with the water itself.

• 2. What Counts As A Wetland

  ›

  A patch of land that pools water after a rainstorm is not necessarily a wetland, even though it is wet. The defining trait is a water table standing at or near the surface long enough each year to support aquatic plants. One concise definition calls a wetland a community of hydric soil and hydrophytes, plants adapted to flooding. Ecologists describe wetlands as ecotones, transition zones sitting between truly terrestrial ecosystems and aquatic systems, dependent on both yet identical to neither.

  The Ramsar treaty defines wetlands broadly. Its Article 1.1 covers areas of marsh, fen, peatland, or water, whether natural or artificial, permanent or temporary, including marine water no deeper than six meters at low tide. Article 2.1 allows them to take in riparian and coastal zones and islands lying within the wetland.

  The United States government uses a stricter legal definition for regulation under the Clean Water Act. It covers areas inundated or saturated by surface or ground water often enough to support vegetation adapted to saturated soil, generally including swamps, marshes, and bogs. The United States Code adds the requirement of hydric soils and hydrophytic vegetation under normal circumstances. Those normal circumstances are tied to the wet portion of the growing season, and it is not uncommon for a wetland to be dry for long stretches of that season. Across every definition, hydrology comes first, with soil and living things following from the water.

• 3. Marsh, Swamp, Bog, And Fen

  ›

  Marshes are wetlands dominated by emergent herbaceous plants such as reeds, cattails, and sedges. Swamps, by contrast, are ruled by woody vegetation, trees and shrubs, though reed swamps in Europe break the pattern by being dominated by reeds rather than trees. Mangrove forests round out the picture, filled with mangroves and other salt-tolerant woody plants. There are four main kinds of wetland: marsh, swamp, bog, and fen, with bogs and fens both classed as peatlands or mires.

  Water source offers another way to sort them. Tidal wetlands draw their water from ocean tides; estuaries from a mix of tidal and river water; floodplains from rivers and lakes that overflow; and bogs and vernal ponds from rainfall or meltwater. In the United States, the Cowardin classification names five main types: marine, estuarine, riverine, lacustrine, and palustrine, the last covering inland nontidal habitats. Australia groups wetlands into marine and coastal, inland, and human-made.

  Peatlands are a category of their own. There, lush plant growth paired with the slow decay of dead plants under anoxic conditions builds up organic peat. Many wetlands also carry local names: the prairie potholes of North America's northern plains, the billabongs of Australia, the turloughs of Ireland, and the mallines of Argentina, among many others.

• 4. The Chemistry Of Flooded Ground

  ›

  Hydrology is the most important factor that produces a wetland. The duration of flooding, or how long groundwater keeps the soil saturated, decides whether the result carries aquatic, marsh, or swamp vegetation. Water enters mainly through precipitation, surface water, and groundwater, and leaves through evapotranspiration, surface flows and tides, and subsurface outflow.

  Water chemistry shifts with the source. Most wetlands are minerotrophic, meaning their waters carry dissolved materials from soils. The exception is ombrotrophic bogs, fed only by precipitation, whose water tends toward low mineral content. Fen peatlands take water from both rain and ground water, so their chemistry ranges from acidic and mineral-poor to alkaline and rich in calcium and magnesium.

  Salinity exerts a strong pull, especially in coastal wetlands and in arid regions with large precipitation deficits. Carbon is the major nutrient cycled within these systems, with sulfur, phosphorus, and nitrogen also held in the soil. Anaerobic and aerobic respiration in that soil governs how those nutrients move, and biogeochemical processes are shaped by soils with low redox potential, the hallmark of ground that has been deprived of oxygen by standing water.

• 5. Life In The Waterlogged World

  ›

  Hydrophytes fall into four main groups in wetlands worldwide. Submerged vegetation grows in salt and fresh water alike, some species flowering underwater and others sending long stems up to the surface; seagrasses and eelgrass are examples. Floating plants tend to be small, like the duckweeds of the Lemnoideae subfamily. Emergent plants such as cattails, sedges, and arrow arum rise above the water. Where trees and shrubs cover saturated soil, the area is usually a swamp, sometimes dominated by a single species like the silver maple swamps around the Great Lakes, sometimes rich in many, like those of the Amazon basin.

  Fauna depend on these places heavily. Seventy-five percent of the United States' commercial fish and shellfish stocks rely solely on estuaries to survive. Frogs serve as indicators of ecosystem health, because their thin skin absorbs nutrients and toxins from the surrounding environment. The Florida Everglades is the only place in the world where American crocodiles and American alligators coexist, while the saltwater crocodile inhabits estuaries and mangroves.

  Mammals range from voles, bats, and muskrats to the platypus, the beaver, the Florida panther, the jaguar, and the moose. Invertebrates make up more than half of the known animal species in wetlands. Aquatic insects, crustaceans, mollusks, and worms form the primary food web link between plants and higher animals such as fish and birds.

• 6. Services Money Cannot Replace

  ›

  To replace what wetlands do for free, enormous sums would have to be spent on water purification plants, dams, and levees, and many of those services cannot be replaced at all. The Ramsar Convention notes that the worth of intact wetlands often far exceeds the benefits of converting them, especially since the profits from unsustainable use flow to a few individuals or corporations rather than to society.

  Floodplains act as natural storage reservoirs. They let excess water spread over a wide area, reducing its depth and speed, while wetlands near the headwaters slow rainwater runoff and snowmelt before it rushes downstream. The river systems that build wide floodplains include the Nile, the Zambezi, the Mississippi, the Amazon, the Yangtze, the Danube, and the Murray-Darling.

  Coastal wetlands guard the shoreline. Mangroves, coral reefs, and salt marsh reduce the speed and height of waves and floodwaters, and mangroves migrate with the shoreline to stay at the water's edge. One analysis valued the storm protection that wetlands provide naturally at US$33,000 per hectare per year. The United Kingdom has begun managed coastal realignment, protecting shorelines by restoring natural wetlands rather than building engineering works. In East Asia, by contrast, up to 65% of coastal wetlands have been destroyed by coastal development.

  Water purification rounds out the list. Wetland vegetation slows water and traps sediment, which can carry heavy metals, while taking up nutrients from runoff. Every system has a threshold, though. An overabundance of nutrients from fertilizer, sewage, or pollution causes eutrophication. The East Kolkata Wetlands in India show the principle at work, covering 125 square kilometers and treating Kolkata's sewage while sustaining fish farms and agriculture.

• 7. The Cost Of Draining Them

  ›

  Since 1900, between 65 and 70 percent of the world's wetlands have been lost, many drained for real estate or agriculture or flooded to create recreational lakes and hydropower. Some of the world's most important farmland was once wetland. The construction of dykes and dams harms individual wetlands and whole watersheds, and once settlements behind dykes are built, they grow vulnerable to land subsidence and rising flood risk. The Mississippi River Delta around New Orleans is a well-known example, the Danube Delta another.

  Narrowing floodplains carries its own penalty. Levees and embankments such as bunds, weirs, and barrages concentrate water that once spread slowly over shallow areas, sending flood peaks higher and floodwaters faster. During Hurricane Katrina, a levee breach in New Orleans killed several hundred people. Human-made embankments along the Yangtze have made its main channel prone to more frequent flooding, alongside a 30% loss of vegetation cover across the river's basin.

  Overfishing and aquaculture press from another direction. Aquaculture is expanding fast across the Asia-Pacific region, with 90% of the world's aquaculture farms in China, contributing 80% of global value. The shrimp farming industry has eliminated massive areas of mangrove, and burgeoning global demand for shrimp keeps the market ready. Invasive species add further harm: water hyacinth from South America overtook parts of Lake Victoria in East Africa.

• 8. Carbon, Climate, And Repair

  ›

  Peatlands cover only 3% of the world's land area, yet their degradation produces 7% of all carbon dioxide emissions. In Southeast Asia, peat swamp forests are drained, burnt, mined, and overgrazed. When peat that built up over thousands of years is exposed to air, it decomposes into carbon dioxide, and peat fires release enormous clouds of smoke that cross international borders almost yearly.

  Wetlands can swing both ways on carbon. They store roughly 44.6 million tonnes of carbon per year globally, an estimate from 2003, but they can also emit methane through anaerobic decomposition of soaked detritus, and some release nitrous oxide. Salt marshes and mangrove swamps sequester carbon at an average rate of 210 grams of carbon dioxide per square meter per year, while peatlands manage 20 to 30 grams. These coastal systems are sometimes called blue carbon ecosystems, and restoring them has been promoted for both climate mitigation and adaptation, though the cost-effectiveness of coastal blue carbon restoration as a pure mitigation action is questionable.

  Restoration runs along a spectrum. The least intrusive approach leaves the ecosystem to recover through succession alone, removing further disturbance and sometimes nudging it with prescribed burns or nucleation planting. Partial reconstruction mixes natural regeneration with engineering, ripping subsoil, laying mulch, or planting trees at scale. Complete reconstruction, the most expensive method, rebuilds the entire ecosystem from the ground up. Traditional ecological knowledge offers a holistic path, treating every part of a wetland as interconnected. Since 1971, work under the Ramsar Convention has sought to identify and protect wetlands of international importance, partly to combat the misconception that wetlands are wastelands.