WHAT ARE WETLANDS?
Wetlands are transition zones between land and aquatic systems where the water table is usually near or at the surface, or the land is covered by shallow water. Wetlands range in size from less than one acre to thousands of acres and can take many forms, some of which are immediately recognizable as “wet.” Other wetlands appear more like dry land, and are wet during only certain seasons of the year, or at several year intervals. The U.S. Army Corps of Engineers reports that most U.S. wetlands lack surface water and waterlogged soils during at least part of each growing season.
Some of the more commonly recognized types of wetlands are marshes, bogs, and swamps. Marshes are low-lying wetlands with grassy vegetation. Bogs are wetlands that accumulate wet, spongy, acidic, dead plant material called peat. Shrubs, mosses, and stunted trees may also grow in bogs. Swamps are low-lying wetlands that are seasonally flooded; they have more woody plants than marshes and better drainage than bogs.
Hydrology and Wetland Formation
In wetlands, when the soil is flooded or saturated, the oxygen used by the microbes and other decomposers in the water is slowly replaced by oxygen in the air, because oxygen moves through water about ten thousand times slower than through air. Thus, all wetlands have one common trait: hydric (oxygen-poor) soils. As a result, plants that live in wetlands have genetic adaptations in which they are able to survive temporarily without oxygen in their roots, or they are able to transfer oxygen from the leaves or stem to the roots. This anaerobic (without oxygen) condition causes wetland soils to have the sulfurous odor of rotten eggs.
Local hydrology (the pattern of water flow through an area) is the primary determinant of wetlands. Wetlands can receive groundwater in-flow, recharge ground-water, or experience both inflow and outflow at different locations.
HOW WETLANDS FUNCTION
Wetlands provide essential ecological functions that benefit people and the ecological systems surrounding the wetlands, as well as the wetland itself. The plants, microbes, and animals in wetlands are all key players in the water, nitrogen, carbon, and sulfur cycles.
Wetland functions fit into several broad categories:
- High plant productivity
- Temporary water storage
- Trapping of nutrients and sediments
- Soil anchoring
Not all wetlands perform all functions, nor do they perform all functions equally. The location of the wetland in the watershed and its size determine how it functions. (A watershed is the land area that drains to a stream, river, or lake.) Other factors that affect wetland function are weather conditions, quality and quantity of water entering the wetland, and human alteration of the wetland or the land surrounding it. The values of wetland functions to human communities depend on the complex relationships between the wetland and the other ecosystems in the watershed. An ecosystem consists of all the organisms in a particular area or region and the environment in which they live. The elements of an ecosystem all interact with each other in some way and depend on each other either directly or indirectly.
Wetlands—Nursery, Pantry, and Way Station
Wetlands are diverse and rich ecosystems, which provide food and shelter to many different plants and animals. The combination of shallow water, high nutrient levels, and primary productivity (plant growth and reproduction) is perfect for the development of organisms that form the base of the food chain. The water, dense plants, their root mats, and decaying vegetation are food and shelter for the eggs, larvae, and juveniles of many species. Smaller animals avoid predators by hiding among the vegetation while they wait to prey on still smaller organisms. Fish of all sizes seek the warmer, shallow waters to mate and spawn, leaving their young to grow on the rich diet provided by the wetlands. Food and organic material that is flushed out of wetlands and into streams and rivers during periods of high water flow feed downstream aquatic systems, including commercial and sport fisheries.
Estuarine marshes, for example, are among the most productive natural ecosystems in the world. They produce huge amounts of plant leaves and stems that make up the base of the food chain. When the plants die, decomposers such as bacteria in the water break them down to detritus (small particles of organic material).
Wetlands’ Role in Biodiversity
Wetlands are the source of many natural products, including furs, fish and shellfish, timber, wildlife, and wild rice. A wide variety of species of microbes, plants, insects, amphibians, reptiles, fish, birds, and other animals make their homes in or around wetlands because of the availability of water. For others, wetlands provide important temporary seasonal habitats. Physical and chemical features such as landscape shape (topology), climates, and abundance of water help determine which species live in which wetland.
In “Wetlands and People” (February 22, 2006, http://www.epa.gov/owow/wetlands/vital/people.html), the EPA notes that “more than one-third of the United States’ threatened and endangered species live only in wetlands.” When wetlands are removed from a watershed or are damaged by human activity, the biological health of the watershed declines. Wetland health has a commercial impact as well. Dahl indicates that 75% of the fish and shellfish commercially harvested in the United States and up to 90% of the recreational fish rely directly or indirectly on wetlands for their survival. Dahl also notes that in 2004, 72% of freshwater mussels were imperiled and 39% of native freshwater fish species were at risk of extinction.
Wetlands absorb water, much like sponges. By temporarily storing runoff and flood waters, wetlands help protect adjacent and downstream property owners from flood damage. Wetland plants slow the flow of water, which contributes to the wetland’s ability to store it. The combined effects of storing and slowing the flow of water permit it to percolate through the soil into groundwater, which recharges aquifers, and to move through the watershed with less speed and force.
Wetlands are particularly valuable in urban areas because paved and other impermeable surfaces shed water, increasing the rate, velocity, and volume of runoff so that the risk of flood damage increases. Loss or degradation of wetlands indirectly intensifies flooding by eliminating absorption of the peak flows and gradual release of floodwaters.
Nutrient and Sediment Control
When water is stored or slowed down in a wetland by the plants and root masses that grow there, sediment settles out and remains in the wetland so that the water leaving the area is much less cloudy than the water that entered. The loss of cloudiness or turbidity has important consequences for both human health and the ecological health of the watershed. Turbidity has been implicated in disease outbreaks in drinking water. Furthermore, turbid water bearing silt has been responsible for smothering plants and animals in rivers, streams, estuaries, and lakes.
Wetlands can also trap nutrients (phosphorous and nitrogen) that are dissolved in the water or attached to the sediment. Nutrients are either stored in the wetland soil or used by the plants to enhance growth. If too much nutrient material reaches rivers, streams, lakes, and reservoirs, it can cause eutrophication, resulting ultimately in the death of many aquatic organisms. (See Figure 6.12 in Chapter 6 and the related discussion.)
Wetlands also play an important role in soil anchoring. The thick mesh of wetland vegetation and roots acts like a net and helps hold soil in place even during periods of relatively high water flow. Removing wetland vegetation the lines a stream or river leads to poorly anchored soil and an increased water flow, which carries away the soil. The result can be severe erosion and changes to the contours of channels, making them deeper and flatter. As a result, aquatic communities at the erosion location are disrupted or eliminated, and downstream aquatic systems are damaged by silt.
Marsh plant fringes in lakes, estuaries, and oceans protect shorelines from erosion in a similar fashion. The plants reduce soil erosion by binding the soil in their root masses. At the same time, the plants and root masses cushion the force of wave action, retarding scouring of shorelines.