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Types of Wetlands
More about wetlands:
- Types of Wetlands (from EPA's Wetlands Division
- Importance of Wetlands
General Characteristics of Wetlands
Wetlands come in many forms. Although half of North America's original 200 million acres of wetlands have been lost or destroyed, the remaining aquatic wetland ecosystems may be grouped as:
Most of the soil in a wetlands environment is saturated with water long enough during a growing season that it no longer contains enough oxygen for most land-based plants to grow. Some types of plants thrive, however, these are the wetlands plants, the water-loving plants. Scientists generally classify a wetland according to the type of plants it contains. In fact, they divide wetlands into very specific habitats, such as mangrove swamps and peat bogs. Here you'll find an account of how wetlands form, their general characteristics, and brief descriptions of the most prominent kinds of wetlands – marshes, swamps, and bogs (and fens).
What Makes a Wetland?
Here are the most common ways that wetlands form – and remain waterlogged:
- Rain and runoff that regularly saturates a low-lying area
- Groundwater very close to the earth that bubbles up through porous soil, providing a constant "flow from below"
- Floodwaters spreading out from large, periodically flooding waterbodies such as rivers and lakes
- Coastal waters cyclically immersing the land, such as along Lake Erie
Not all wetlands originate completely on their own. Here are some ways that animal and human behavior may encourage wetland formation: Beavers dam rivers and streams, making those waters more likely to flood large surrounding land areas. State fish and wildlife agencies may intentionally flood dry areas to provide more breeding and nesting grounds for waterfowl and other species dependent on wetlands for reproduction. Construction may unintentionally block the natural flow of surface water, causing it to overflow its banks. Not every stretch of land under shallow water is a wetlands, though.
Marshes
Marshes predominantly contain plants without strong woody stems and branches, called herbaceous plants. Plants that flourish in marshes can grow in the varying water levels of the marsh, with their stems partly immersed in the water and partly above it. Marshes may be either freshwater or saltwater, depending on the type of water they contain.
Texas Environmental Center (TEC) documents that freshwater marshes constitute 90 percent of America's wetlands. Common plants in freshwater marshes include cattails, water lilies, and arrowheads. Clumps of soft-stemmed plants are also plentiful and include sedges and grasses. Freshwater marshes are home to ducks, geese, waterfowl including snowy egrets and great blue herons, raptors such as hawks and bald eagles, and mammals including raccoons, minks, and beavers.
Saltwater marshes are found near the coastlines in cove and bay areas protected from the destructive force of the surf. Herbaceous plants and grasses of saltwater marshes differ from those in freshwater marshes only in the sense that they are adapted to growing in saltwater. Animals of saltwater marshes include great blue herons, clapper rails, ducks, raccoons, blue and fiddler crabs, clam worms, killifish, and snails.
Swamps
Swamps are wetlands whose dominant vegetation is comprised of woody plants such as trees and shrubs. The water may be anywhere from a few inches to over a foot deep. Though swamps are usually saturated with water during the growing season, they may dry out in the heat of a long summer.
A freshwater swamp may be either a forested swamp or a shrub swamp. Typical vegetation in northern freshwater swamps include red maple, black willow, northern white cedar, alder, and cottonwood. Water oak, tupelo and bald cypress are common to southern swamps.
In the hotter Southern United States, places like southern Florida and regions of Texas and Louisiana, saltwater swamps called mangrove swamps are also common.
The soil saturation and diversity of vegetation in swamps lures a wide variety of animal life into their murky environments. Alligators and cottonmouths are the more famous swamp inhabitants. Other animals not adapted to the shadowy, mucky area such as panthers, foxes, raccoons, and bears wander through swamps in search of food, water, and shelter.
Bogs
Bogs are freshwater wetlands found in cold regions. In North America, many bogs were formed over 10,000 years ago when the last of the glaciers departed, leaving lakes called kettle holes. (Kettle holes formed when blocks of glacial ice became buried beneath the ground, then melted). In some instances, the retreating ice left shallow basins where rain collected and formed the wetland.
Not all bogs were formed by glaciers. Any poorly drained lakebed or depression in the earth has the potential to fill with rainwater and become a bog.
The bottom layer of a bog consists of peat. Peat is an organically rich material that forms when plants die, fall into the water, and are compressed over time, forming a thick layer. This layer of peat can be up to forty feet deep. Bogs are sometimes called "peatlands."
Why doesn't that plant material just decompose into a normal layer of soil? The acidic nature of existing peat, cold temperatures, and low oxygen levels resulting from poor water circulation prevent bacteria and other decomposers from settling and feeding on the plant material. So, the rate of decomposition in a bog is very slow.
The surface of a bog is often covered by a thick concealing mat of vegetation. The "mat" forms when sphagnum moss moves into open water in which aquatic plants have become entangled. Over time, the sphagnum may grow into a mat that carpets the surface of the entire bog. Other plants may then take root in the sphagnum, including trees like the tamarack.
Stepping on the mossy carpet of a bog gives the sensation of "bouncing," because the moss is really a sheet of intertwined sphagnum, plant roots, and peat floating on and soaking in water.
Bog plants are well adapted to the high acidity, low oxygen, low nutrient supplies, and moist growing surface found in bogs. Large plants like the black spruce develop complex root systems to help absorb oxygen and nutrients. Smaller plants like orchids develop symbiotic relationships with other creatures which can supply them with the nutrients they need such as fungi. Among the more interesting and unique plants of bogs are bladderworts, pitcher plants, and sundews that trap and digest insects and tiny animals.
Although many large animals do not live exclusively within bogs, a variety of wildlife including moose, bear, and deer often find food, water, and shelter throughout the year.
Fens are similar to bogs since both have moist, sponge-like ground. Fens, however, have a higher pH and more nutrients. These wetlands form in poorly drained areas. They frequently surround open bodies of water usually located in regions of the northeast, the northern plains and Canada.
Although fens have diverse and interesting flora, including some rare species of orchid, they are commonly dominated by grass-like species such as sedges.