Wetlands

Introduction

Wetlands are areas in which the soil is completely saturated with water. The plants growing in a wetland have adapted to be able to survive and grow in water-soaked soil. Swamps, bogs, peat lands, and marshes are examples of wetlands.

A wetland is located between two very different environments, one located more inland (a terrestrial ecosystem) and the other being the waters of the lake or ocean (an aquatic ecosystem). The wetland is an interface or ecotone between the terrestrial ecosystem and the aquatic ecosystem. The area where one ecosystem blends in with another is referred to as an ecotone.

Although once regarded as wasteland, wetlands have vital environmental functions and, when located on a flyway of migratory birds, are rest and refueling stops for the birds during their seasonal journey. Today, many wetlands are nurtured and, where necessary, restored.

Historical Background and Scientific Foundations

In wetlands, the water table (the underground zone where the underlying soil is completely saturated with water) is very near or at the surface. The saturated soil of a wetland lacks oxygen. Wetlands may not always be saturated with water, however. Depending on the season, tide cycle, presence of a drought, the type or amount of precipitation, and its location, a wetland can be a flooded zone sometimes, and a grassy area another time.

There are additional stresses in wetlands located on the ocean coast (intertidal wetlands). At high tide, the wetland is inundated with saltwater, while at low tide freshwater predominates. A type of tree called the mangrove has adapted to the high salt level by the structure of the roots and leaves, which makes it more difficult for salty water to enter and for the moisture to escape.

Generally, wetlands become wet in three ways: precipitation in the form of rain or snow, the emergence of groundwater, and the flow of freshwater into the wetland from surrounding areas via watercourses or flow of marine water from the nearby ocean. The location and topography of the area is influential in determining the movement of water into the wetland and the speed at which the water moves through the wetland. For example, an area that lies lower than the surrounding land will tend to receive water flowing downhill.

The periodically saturated and oxygen-free soil represents an environment that is too harsh for many varieties of plant to grow. Those that have become adapted to growth in oxygen-free soil are referred to as hydrophytes or wetland plants. Examples include cattails, bulrushes, water lilies, and duckweed. When trees are present, they can have roots raised above the waterline, which increases the exchange of gas between the roots and the air. In tropical regions, mangrove trees contribute sulfur, making a wetland more acidic and increasing the rate at which organic material is decomposed. This helps provide more food for other life in the wetland.

The plants in wetlands can be used as a food source for animals and, if uneaten, also provide nutrients when they decompose. The resulting flow of energy creates what is termed a food web. Although other environments such as prairie land and temperate forests also create food webs, wetland associated food webs are especially productive and support a large and varied population.

Wetlands are also an area where waterfowl can hide. This is important for breeding. Wetlands that are located near watercourses can also be a form of flood control. If the watercourse is breached during spring runoff or during a heavy rain, the excess water can gather in the wetland, to be gradually returned to the surrounding region. Near the coast, wetlands can buffer the effects of storms, providing a place where surging water can collect and dissipate. However, these benefits no longer exist in many regions, where wetlands were filled in to create land for agriculture or housing.

Wetlands can be located in an area where water is abundant. Intertidal wetlands are one example. Wetlands can also be located in a region where surface water is otherwise sparse. Prairie wetlands are an example. Such wetlands are important for wildlife, commercial livestock, and migrating species especially during dry periods.

A well-known wetland is the Florida Everglades, which is actually a very slow moving river almost 62 mi(100 km) wide and over 100 mi (160 km) in length.

Impacts and Issues

Wetlands have been described as environmental kidneys, because they function to filter out sediment and pollution as the water moves slowly through the wetland vegetation. Usually the water emerging from a wetland is cleaner than the water that entered. This principle has been applied to sewage treatment that utilizes wetland-like regions as a step in the treatment process. Nutrients such as phosphorus and nitrogen, which are commonly present in fertilizer and which can enter the water in

WORDS TO KNOW

BOG: Area of wet, spongy ground consisting of decayed plant matter.

PEAT: Partially carbonized vegetable matter that can be cut and dried for use as fuel.

TIDE CYCLE: A period that includes a complete set of tide conditions or characteristics, such as a tidal day or a lunar month.

runoff, can be retained by wetland vegetation, reducing the nutrient level in the water and so reducing the likelihood of nutrient-fueled explosive growth of microorganisms that can deplete oxygen for the water.

Furthermore, wetlands can retain nutrients, which makes them a source of great biodiversity and a haven for migrating species. Indeed, estimates are that North American wetlands located along migratory pathways are used by upward of 500,000 migrating birds each spring and fall, representing 30% of the continent’s migrating bird population. Internationally, wetlands are used by at least 100,000 birds each year.

Wetlands also benefit humans as a source of food and fuel. For example, in Thailand, the rice that is grown

IN CONTEXT: WETLANDS FILTER WATER

Natural wetlands also contribute to the purification of water. Wetlands can serve as a depositional sump and provide biological filtering.

and harvested in the rich wetland soil is one of the staple foods. In northeastern regions of the United States and maritime Canada, wetland regions are used to grow cranberries. In Russia and some regions of Europe and the United Kingdom, peat has provided a fuel source for millennia.

Although in many places emphasis is on the protection and restoration of wetlands, this was not always the case. The Environmental Protection Agency (EPA) estimates that in the 1600s there were more than 22 million acres of wetlands in the continental United States. However, by the mid-1950s, more than half of these had been drained and the land converted to other uses, with wetlands tending to be viewed as wasteland with little natural value or function. In North America, many wetlands were drained to provide more land for development. Coastal wetlands were particularly vulnerable because their drainage provides land for development of oceanfront housing. According to the EPA, about 60,000 acres of U.S. wetlands are still lost every year.

Everglades National Park, formed in 1947 by President Harry Truman, exists to protect the remaining portion of the Florida Everglades. Over half of the original area of the everglades has been lost to agricultural and urban development.

In the everglades alone restoration efforts by the U.S. Army Corps of Engineers as part of the Comprehensive Everglades Restoration Plan have already cost billions of dollars. For increasing water-strapped Florida, however, the efforts are worthwhile, since the everglades are of crucial influence to the availability of freshwater.

The EPA and Army Corps of Engineers jointly establish environmental standards for wetlands and monitor discharges into wetlands. If necessary, the EPA has the legal authority to intervene to stop unacceptable discharges.

Wetlands are also threatened by air and water pollution. For example, insecticides and herbicides carried into a wetland in the inflowing water can be toxic to plants and wildlife.

In the United States, some wetlands are federally regulated as part of the Clean Water Act. Wetlands protection is also important internationally. One example is the Ramsar Convention on Wetlands, which was signed in Ramsar, Iran, in 1971. As of 2008, the convention had 158 parties and included almost 400 million acres (161 million hectares) of wetland. The convention also addresses the influence of climate change on migratory species.

See Also Clean Water Act; Coastal Zones; Conservation; Wildlife Population Management; Wildlife Protection Policies and Legislation

BIBLIOGRAPHY

Books

Grandin, Temple, Terry Williams, and Rosalie Winard. Wild Birds of the American Wetlands. New York: Welcome Books, 2008.

Lockwood, C., and Rhea Gary. Marsh Mission: Capturing the Vanishing Wetlands. Baton Rouge: Louisiana State University Press, 2005.

Mitsch, William, and James Gosselink. Wetlands. New York: Wiley, 2007.

Web Sites

Sierra Club. “Wetlands Overview.” http://www.sierraclub.org/wetlands/ (accessed May 2, 2008).

Brian D. Hoyle