For centuries, wetlands were mistreated and misunderstood. Generally called swamps or marshes, they were considered useless, waterlogged, even dangerous land better off filled, farmed, and developed. That's one reason why over half the original wetlands in the lower 48 states have been lost, and much of the rest degraded.
But over the past few decades, wetlands have come to be better understood and valued as a key piece of nature's water purification system, as well as wildlife habitat. Wetlands are so important that today, scientists are spending a lot of effort on the difficult task of trying to recreate lost or damaged wetlands, restoring some of their crucial ecosystem functions.
Nakivubo Swamp is a wetland on the northern shores of Lake Victoria near Kampala, Uganda. Most Kampala residents lack city water and sewage treatment, so wastewater is allowed to flow into the swamp. Although not a perfect solution, this wetland helps keep some pollutants from getting into the lake, source of the city's water.
Wetlands are like sponges: They can soak up surges of stormwater and help recharge groundwater. But we often deal wetlands a double whammy by filling them in so they can't absorb water, and by paving the surrounding landscape, enormously increasing surface runoff. Today, scientists are learning to construct new wetlands that can hold and treat stormwater. In some areas, constructed wetlands can be a cost-efficient alternative to building both wastewater and drinking-water treatment plants.
There are many kinds of wetlands, from coastal salt marshes to forest bogs. You might see a wetland like one fringing the shores of Lake Michigan, one of the five Great Lakes of the United States and Canada. Those lakes hold about 20 percent of all the accessible fresh water on Earth. Fringing wetlands, constantly threatened by development, are an important natural water purifier.
Wetland plant communities--reeds, grasses, and other plants--are among the most productive ecosystems on Earth. The amount of living matter--or biomass--in a wetland may be 10 to 1,000 times that of dry land nearby. Some of these plants, especially cattails and certain grasses, absorb pollutants.
The abundance of plants means there are huge numbers of roots holding wetland soils in place. That's one reason wetlands on shorelines prevent erosion.
The microbes that cluster on the roots of many wetlands plants do a lot of the wetland's water cleansing. For example, some use nitrate--a fertilizer ingredient dangerous to aquatic life--as a substitute for oxygen, removing more than 10 times the amount taken up by the plants themselves. Microbes also help change some heavy metals to less toxic forms.
When water runs off the land--in a heavy rainstorm, for example--it picks up soil. When that runoff is caught in a wetland, the soil has a chance to settle to the bottom, where it becomes sediment. As a result, the water leaving a wetland is cleaner than the water entering.
Because the water in a wetland is shallow, the Sun's ultraviolet rays sometimes penetrate to the bottom. These UV rays can kill bacteria and inactivate some viruses.