Wetlands

Wetlands are areas of land where water covers the surface for at least part of the year and controls the development of soil. Plants and animals that live in wetlands are adapted to living in conditions where the soil is waterlogged. There are many different types of wetlands, but they fall into five general classifications: freshwater marshes, freshwater swamps, salt marshes, mangrove swamps, and bogs (also called fens). In general, swamps have trees, while marshes have plants that have soft stems like grasses, reeds, and sedges (grass-like plants). Bogs are characterized by thick mats of peat, which is made of mosses growing on decayed plants and animals.

Freshwater marshes

Freshwater marshes are found on the edges of lakes, ponds, and rivers. They are particularly common near the floodplains of rivers. About one-quarter of all the wetlands in the United States are freshwater marshes.

Plants that grow in freshwater marshes have adapted to survive in waterlogged conditions. They do not need stiff stems to hold them up because the water provides buoyancy (the ability to float). Water lilies are typical plants that grow in freshwater marshes and they have flexible stems that bend and straighten as the water level changes. The leaves of the water lily float on the surface of the marsh. Because the roots of freshwater marsh plants are often underwater or in mud, they are frequently exposed to low oxygen concentrations. They have developed spongy stems full of air spaces that can transport oxygen to the plant roots. Some marsh plants, like duckweed, don't have roots at all. They float and absorb minerals from the water.

Animals that live in freshwater marshes have also adapted to use their wet environment to their advantage. Many of the insects that live in freshwater marshes have aquatic stages that take up a majority of their life. For example, dragonflies spend up to five years as larvae that live underwater. The adult fly stage only lasts for one season. Many birds make the marsh their home. Often, birds like the American bittern are striped and well-camouflaged by the marsh grasses. Other birds, like the herons, have extremely long legs and wade through the marsh hunting for fish. Ducks are especially common marsh birds. They have special organs that produce oils to waterproof their feathers. Nearly three-quarters of all ducks in North America breed in freshwater marshes in prairies.

Freshwater swamps It is estimated that about two-thirds of all wetlands in the United States are swamps. Trees are the dominant plant in this environment and the trees species found in swamps vary depending on location. In the northern part of North America, evergreens such as spruce, fir, and cedar are commonly found in swamps. In New England, red maple dominates. Black willows and cottonwoods dominate swamps along the Mississippi and Connecticut rivers. In the southern part of the United States, bald maple, tupelo, and sweetgum trees are commonly found in swamps. The bald maple produces knobs that look like knees that are probably used to acquire oxygen for the roots of the trees, which are buried in mud that has no oxygen. Bald maples, tupelos, and sweetgum trees also grow buttresses (support limbs) that help hold up the trees because their roots are close to the surface of the mud where oxygen is a little more accessible.

Trees are crucial to the ecology (relationships between living organisms and their environment) of the freshwater swamp. Because many of the trees that live in swamps lose their leaves each fall, swamp water is highly enriched from decomposing plant material. Many worms, newborn insects, and crustaceans (aquatic animals with no backbone and a hard shell) live by cutting apart fallen tree leaves for food. Bacteria break down the leftovers and produce nutrients (substances necessary for plant and animal growth) for other animals. These animals attract birds, snakes, and mammals to swamps. In addition, trees provide important hiding places for birds. Woodpeckers drill holes in trees and use them for nests. After the woodpeckers abandon their holes, other birds like owls, titmice, and wrens take over. As trees die and fall over into the swamp waters, ducks and snakes use rotted holes for their homes.

Salt marshes Salt marshes are marshes that are found along ocean coasts. One of the most important features influencing salt marshes is the tide that goes in and out each day. When the tide is high, marsh grasses, algae, mussels, crabs, snails, and worms are covered in cool, salty water. When the tide is low, these same organisms may bake in the sun. The animals that live in salt marshes have developed different adaptations to overcome these extreme changes in environment. Land snails crawl up and down the stems of marsh grasses keeping away from the water as the tide goes up and down. Land crabs dig burrows (tunnel or hole) in the mud, which they seal up to protect themselves from the waters of the high tide. Marine (ocean) snails scurry into pools in the sand and close their shells tightly as protection from drying out during low tide.

Salt marshes are places of great biodiversity (variety of life) and productivity. Nutrients circulate into salt marshes from the ocean and rivers. In addition, as plants and animals die and are buried in the marsh, bacteria digest and convert them into nutrients. All of these nutrients support an enormous amount of plant growth. One acre of salt marsh can produce 4.8 tons (4.3 metric tons) of plant material each year, almost twice that of a corn field given fertilizers. This plant growth, in turn, supports many species of invertebrates (animal without a backbone), fish, and birds.

Mangrove swamps

Mangrove swamps are swamps found on the edge of oceans in tropical (hot) regions. Mangrove swamp water is salty and the plants and animals that live in mangrove swamps have unique characteristics that allow them to handle these salty conditions. Many of the plants that live in mangrove swamps have special organs that remove salt from their tissues. In addition, many of the sediments (particles of sand, gravel and silt) in swamps are low in oxygen, so plants must be able to adapt to low oxygen levels.

Mangroves are trees that can tolerate saltwater. Many mangroves have special adaptations to gather oxygen from the air. There are a variety of different species of mangroves and they each grow best in slightly different environments. In North America, red mangroves are found closest to the water. They have tough roots called prop roots that help anchor them to unstable sandy soils in shallow water. These roots trap sediments and also provide habitats for juvenile fish and invertebrates. In particular, oysters, anemones, snails, and snakes all live attached to red mangrove roots. Red mangroves have special pores called lenticels in their stems that they use to take in oxygen. Black mangroves are found behind the red mangroves, closer to land. They have special roots that grow like spikes out of the swamp water to gather oxygen from the air. White mangroves are found closer to the land behind black mangroves. On the edge of the swamp that is closest to land, a mangrove called a buttonwood is most common.

Many birds and mammals make their homes in mangrove swamps. Roseate spoonbills, ibis, and pelicans are all commonly found digging their bills into the swamp sand hunting for fish and invertebrates. Egrets, herons, and spoonbills also use the mangrove canopy for nesting, in order to avoid predators such as mammals and fish. Rabbits and raccoons are land mammals that can be found in mangrove swamps. The manatee is a marine mammal that makes its home in the swamps of Florida. It is herbivorous (plant-eating), and grows to enormous sizes (3,500 pounds or 1,600 kilograms). Manatees are classified as an endangered species; the greatest threat to their existence is boat propellers.

Bogs

Bogs are unusual environments. They occur near springs, slow-moving streams or small ponds. Water in bogs is usually very acidic, and because bacteria generally do not grow well in acidic environments, bog water has few bacteria. As a result, dead plant material is not decomposed; instead it piles up, pressing down on layers of plant material below it. This produces peat, which is so thick and solid that it can be used like coal for fuel. As bogs are waterlogged, they do not become solid and walking across a bog feels like walking across a waterbed.

Some species of moss and sedge (a grass-like plant) can grow on top of the peat. Most of these plants have adapted to take in oxygen from the air because the soil in a bog is usually low in oxygen. One of the most important bog plants is called sphagnum moss. Sphagnum moss is acidic and thus, few bacteria grow on it. Besides its antiseptic qualities, sphagnum moss. is highly absorbent and was used extensively for bandages during World War I (1914–18). It is soft and spongy; dry sphagnum moss can absorb water until it weighs as much as twenty times its original weight. Native Americans frequently used sphagnum moss for baby diapers.

With few bacteria living in acidic bog waters, decomposition occurs slowly and nutrients remain locked inside dead plant material. Plants that do grow in bogs must be creative in order to get nutrients. Many species of carnivorous (meat-eating) plants live in bogs. They receive nutrients from the prey that they capture, mostly small insects, but still use photosynthesis (process of producing food using sunlight) to generate sugars. Some examples of carnivorous bog plants are Venus flytraps, bladderworts, and pitcher plants.

Importance of wetlands

Wetlands are places of great biological diversity. In swamps and marshes, the water is shallow enough that light can penetrate to the bottom, meaning that a great deal of photosynthesis can take place. This, in turn, leads to many different types of herbivorous animals, considered the primary consumers, and animals that in turn eat the herbivores, considered the secondary consumers. Wetlands are very important breeding grounds for both fish and invertebrates. Nearly two-thirds of all shellfish and bony fish in the ocean rely on wetlands for some part of their lives. About one-third of all endangered species spend time in wetlands, according to the U.S. Fish and Wildlife Service.

Wetlands have several important ecological roles. Because of their ability to trap water, they act as sponges during heavy rains, slowing the flow of water into rivers and minimizing floods. This decreases erosion (wearing away of land) while protecting homes and other structures that could otherwise be flooded with water. In addition, this slowing of rainwater allows some of it to seep into aquifers (underground basins containing fresh water), which replenishes the source of much drinking water. Wetlands also greatly reduce pollution and a process called eutrophication. Eutrophication is the process of water becoming enriched with nutrients, causing a population explosion of aquatic plants and reducing the oxygen in the water. This results in the loss of animal and plant life. Wetlands absorb many of these nutrients, allowing plants—which take in pollutants—to grow.

Wetlands are not permanent features. The natural cycle of a wetland is to convert to a land habitat as sediments accumulate. The rate at which this occurs depends on the sediment accumulation rate, the rate of precipitation (rain and other water), and changes in sea level. Since humans began using wetlands for agriculture in ancient Mesopotamia (a region in modern-day southwest Asia), they have influenced how wetlands have changed. Agriculture uses fertilizers, which add enormous concentrations of nutrients to the soil. When it rains, these fertilizers run into rivers and oceans and initiate eutrophication. Building pipelines used to transport water and dams, and draining wetlands have accelerated the destruction of wetland habitats throughout the world. This causes a loss of habitats for many animals and plants, and is a loss to the environment and people.

Juli Berwald, Ph.D.