Wetlands are the most threatened ecosystems on Earth, and among the richest
Swamps and marshes were long seen as wasteland to drain. In fact they filter water, soften storms, store carbon and teem with life. A 2005 global assessment found wetlands under more pressure from environmental degradation than any other ecosystem on the planet, from dredging and draining to pollution and overgrazing.
A wetland is land soaked or flooded often enough, whether permanently, for years at a time or just seasonally, that its soils run short of oxygen. Plants there have roots adapted to waterlogged ground, which is what sets wetlands apart from both dry land and open water. One compact definition calls them communities of hydric soils and water-loving plants. A lawn that puddles after a storm does not qualify.
Types are sorted by vegetation and water source. Marshes are dominated by reeds, cattails and sedges; swamps by trees and shrubs, though Europe's reed swamps break that rule; and mangrove forests by salt-tolerant woody plants. Bogs and fens, both kinds of peatland, complete the four main categories. Water may arrive from tides, rivers overflowing, rainfall or meltwater, and it can be fresh, brackish, salty or alkaline. The giants include the Amazon basin, the West Siberian Plain, the Pantanal and the Sundarbans in the Ganges-Brahmaputra delta.
Wetlands do a great deal of free work. They purify water as plants absorb excess nitrates and phosphorus, stabilise shorelines, buffer storms and absorb floods. Many lock away carbon and help slow climate change, but the picture is mixed: soggy, oxygen-starved decay also makes some wetlands significant sources of methane, and a few release nitrous oxide.
Lawmakers need sharper definitions. The Ramsar treaty, working since 1971 to protect wetlands of international importance, counts marsh, fen, peatland or water, natural or artificial, fresh or salt, including sea areas no deeper than six metres at low tide. The United States regulates areas saturated often enough to support plants adapted to soggy soil. Whatever the wording, every definition hinges on hydrology, with soils and living things following from it.
Source: Wetland
