- A region of vast swamps and marshes, the Pantanal in southwestern Brazil is one of the world's …
Wetlands are usually classified as swamps, marshes, or bogs, according to the type of soil and plant life they contain. Swamps and marshes both occur in low-lying areas near rivers or on flat areas along coasts between the high and low watermarks. Both swamps and marshes have mineral soils because they have access to mineral-rich groundwater. The difference between the two types of wetland is that swamp plant life consists largely of trees, while marsh plant life is dominated by grasses. A bog has spongy, peaty soil. It contains almost no minerals because the primary source of water in a bog is rainwater, which contains few minerals. Consequently, bog plant life consists of simple plants that can subsist on less nourishing soil—mosses, sedges, and reeds, for example. SwampsSwamps are often found in low-lying regions around rivers, which supply the swamp's water. Some swamps develop from marshes that slowly fill in, allowing trees and woody shrubs to grow. The shade from these plants eventually kills many of the marsh plants. Plants that began to decay in swamps some 250 million years ago formed some of today's most valuable deposits of coal (see Coal). Freshwater swamps.Riverbanks, floodplains elevated only a few feet above river level, abandoned river channels, and oxbow lakes may have standing or sluggishly flowing water for much of the year and thus support swamps and marshes. Many trees can survive and even thrive in freshwater swamps as long as their roots are not submerged in water for long periods of time. The bald cypress is an example of a tree particularly adapted to growth in swamps, but gums, willows, maples, and alders are also common. Wildlife may be abundant in a swamp, depending mostly on the diversity of the swamp's plant life. There are many swamps in the United States, including extensive swamps along the Mississippi River and its lower tributaries; large cypress swamps northwest of the Florida Everglades; and the great Dismal Swamp of North Carolina and Virginia (actually a mixture of waterways, swamps, and marshes). The Florida Everglades is a marsh-swamp combination, as is the the Okefenokee Swamp in Georgia. Saltwater swamps.The flooding and draining of seawater forms saltwater swamps along flat coastal areas. In tropical and subtropical regions, regularly flooded, protected areas develop mangrove swamps. The high salt content makes such swamps a forbidding habitat for most plants, but the hardy mangrove will grow even in pure sand at the edge of the sea (see Mangrove). The deltas of the Mekong, Amazon, Congo, and Ganges rivers, and the north coasts of Australia and Sumatra have extensive mangrove swamps. The tops of the mangrove trees shelter a diverse group of animals, and such creatures as snails, crabs, and worms may live among the mangroves' roots. MarshesLike swamps, marshes often form at the mouths of rivers, especially in extensive delta regions. The river brings a steady supply of slow-flowing water and rich sediment to the marsh, creating a perfect environment for marsh grasses. These grasses have shallow roots that spread and bind the muds together, slowing the flow of water and encouraging the spread of the marsh. Freshwater marshesinclude the common river-mouth marshes and the small marshes, called prairie potholes, that have developed in countless little depressions in the Midwestern United States. Grasses, sedges, reeds or rushes, and rice are common marsh plants. As in a swamp, wildlife is often abundant. Well-known river-mouth marshes include those of the Camargue in the Rhône Delta and the Danube in Romania. The deltas of the Nile, Tigris-Euphrates, Mekong, and Amazon also have marshes. The Okavango Marshes east of the Kalahari desert in Botswana are perhaps the best example of marshes formed in an interior, closed basin. Saltwater marshesare formed by the same process as are saltwater swamps and are among the most productive natural systems. Species of the genus of grasses known as cordgrass, or marsh grass, are common. Various other grasses may also take root. Most of the animal inhabitants of saltwater marshes are insects, invertebrates, and crustaceans. Saltwater marshes are common along the East coast of the United States and in the Arctic, Northern Europe, Australia, and New Zealand. BogsBogs generally form in areas where glaciers once scoured out the rock, leaving depressions that filled with water and eventually formed bogs. Bogs can be divided into three types: typical bogs of cool regions, dominated by the growth of mosses and heaths; fens, dominated by grasslike plants; and tropical tree bogs. Typical bogs.Typical, or sphagnum, bogs depend almost entirely on rain for water and minerals. Sphagnum moss, the dominant plant, absorbs many of the already scarce minerals from the water and replaces them with acid. It also slows the passage of air into the water. This combination of lack of oxygen, high acidity, and lack of minerals slows the natural decomposition of plant materials. As a result, layers of dead moss accumulate and gradually form peat—a source of fuel and the first stage in the development of coal (see Peat). Bogs generally begin with a small body of water with a floating mat of vegetation next to the shore. This mat develops into a floating bog mat of mosses and heaths, which thickens and grows toward the center of the water. Bits from the bottom of the mat sink to the lake bottom so that the lake is filled from both the bottom up and the top down. Bogs are called quaking bogs when the vegetation is still floating. Then the surface will sink or quake when a person or animal walks on it. Both people and animals have drowned when they broke through the surface. Nonfloating bogs may also quake if the peat is thick and spongy. Plant life in a typical bog is limited because the environment is so inhospitable. Other than bog mosses, heaths, and a few grasses and sedges, there may be some insect-eating plants and orchids. Wildlife is not common. Bogs cover vast areas in Canada, Northern Europe, and Russia. Fens.Fens receive groundwater that has some dissolved minerals and is only moderately acidic. As a result, they contain a wider variety of plants than do bogs—mainly grasslike plants, grasses, sedges, and reeds. Fens usually occur near sphagnum bogs. Tropical tree bogsoccur only in areas where the water is very low in minerals. The plants are mainly broad-leaved evergreen trees, and the peat that forms is made almost entirely from tree remains. Tropical bogs cover extensive areas in Malaysia, Indonesia, tropical South America, and Africa. |