Wetlands are terrestrial ecosystems characterized by poor drainage and the consequent presence most or all of the time of sluggishly moving or standing water saturating the soil. They are usually classified, according to soil and plant life, as bog, marsh, or swamp. Wetlands occur along marine coasts and in areas far removed from the oceans and their influence. Because wetlands occur at the interface of a body of water and the land, they are examples of boundary ecosystems.
Wetlands in coastal areas can be classified into three basic types: mangroves, salt marshes, and freshwater tidal marshes. Other important coastal systems not formally considered wetlands but found at the boundary between land and water are seaweed-based systems, sea-grass beds, and coastal mudflats.
The fundamental characteristics of shoreline ecosystems are determined by the amount of energy in the water available to move sediments. This energy is supplied by wind-driven currents, tidal currents, and wave action. In high-energy areas the fine sediment is carried away, leaving bedrock, boulders, or cobbles. This creates a prime habitat for seaweeds. As the energy level of water movement progressively lessens, sediments ranging from pebbles to sand, silt, and mud can settle and remain in place. Soft sediments provide a suitable habitat for salt marshes or mangrove forests between tide marks and for sea grasses below the low-tide mark. On a coastline consisting of alternating headlands and embayments, the headlands are most likely to be exposed to strong wave action and to be inhabited by seaweed communities, while the sheltered embayments are more likely to have soft sediments with rooted plant communities. The characteristics of shoreline communities are discussed according to the type of plant production on which they are based.
Mangrove swamps are found along tropical and subtropical coastlines throughout the world, usually between 25° N and 25° S latitude. The mangrove swamp is an association of halophytic trees, shrubs, and other plants growing in brackish to saline tidal waters of tropical and subtropical coastlines. This coastal forested wetland (called a “mangal” by some researchers) is infamous for its impenetrable maze of woody vegetation, unconsolidated peat, and many adaptations to the double stresses of flooding and salinity. Approximately 68 species of mangrove trees exist in the world. Their uneven distribution is thought to be related to continental drift and possibly to transport by primitive humans. Mangrove swamps are dominant particularly in the Indo-West Pacific region, where they have the greatest diversity of species—30 to 40 species of mangroves, compared with about 10 species in the Americas.
Black mangroves (Avicennia germinans), a species native to Florida. Thomas Eisner
In the tropics and subtropics the intertidal areas of soft sediment are usually colonized by mangrove trees. Beneath them lies a waterlogged mixture of mud and decaying mangrove leaves that has very little oxygen; an aboveground root system allows the trees to take in air. This network of aerial roots forms a tangled mass that traps sediment but makes a mangrove forest very difficult for large animals (or humans) to penetrate. Small seaweeds and microscopic algae grow on the trunks and roots of the mangroves, and microscopic algae grow on the surface of the mud. This substrate, along with the decaying mangrove leaves, supports a rich and diverse animal community. Crabs and shrimps are often abundant, and clams and snails of many kinds abound. Mudskippers (family Periophthalmidae), which are fish that have developed the capability of leaving the water and moving over the mud surface in pursuit of prey, are found in mangrove systems, as is the mud lobster (Thalassina anomala), which lives in burrows. Because the plankton of adjacent coastal waters is often relatively unproductive, the productivity of the mangrove forests is an important element of the productivity of the whole coastal zone.
Along intertidal shores in middle and high latitudes throughout the world, salt marshes replace the mangrove swamps of tropical and subtropical coastlines. These marshes flourish wherever the accumulation of sediments is equal to or greater than the rate of land subsidence and where there is adequate protection from destructive waves and storms. Dominated by rooted vegetation—primarily salt-tolerant grasses—that is periodically inundated with the rise and fall of the tide, salt marshes have a complex zonation and structure of plants, animals, and microbes. This biota is tuned to the stresses of salinity fluctuations, alternate drying and submergence, and extreme daily and seasonal temperature variations. Salt marshes are among the most productive ecosystems of the world. A maze of tidal creeks that exhibit fluctuating water levels and carry plankton, fish, and nutrients crisscross the marsh, forming conduits for energy and material exchange with the adjacent estuary. The salt marsh forms an important interface between terrestrial and marine habitats.
Sea lavender (Limonium vulgare) growing with glasswort (Salicornia europaea). © Jan van de Kam/Bruce Coleman Ltd.
The most common site for a salt marsh, after estuaries and lagoons, is on the sheltered side of a sand or shingle spit. Alongshore currents deposit coarser material on beaches but carry the fine material until it reaches the quieter water behind the barrier. As plants colonize the area, they slow down the flow of water and cause even more silt to accumulate. The Atlantic coast of North America has over 600,000 hectares (2,300 square miles) of salt marshes dominated by the marsh grass Spartina.
On the European side of the North Atlantic the flora includes other important components such as the sea pink (Armeria), sea lavender (Limonium), and sea plantain (Plantago maritima). In the course of history large areas of salt marsh in Europe have been used for grazing cattle and sheep, and these areas subsequently have been dominated by the grasses Puccinella and Festuca. Early colonists in North America often erected dikes around the marshes to keep out the sea; the reclaimed land was used for agriculture in much the same way that it had been in Holland and Belgium.
Only a very small proportion of salt marsh vegetation is eaten directly by animals. The remainder dies, decays, and becomes suspended as fine particles (detritus) in the water. It was thought at one time that the export of this detritus on every ebbing tide supplied large amounts of nutritious food material to the animals in nearby estuarine or coastal waters. Detailed field studies have failed to support this view, and it is now thought that most of the production of salt marsh plants is decomposed by bacteria and fungi and that the plant nutrients are recycled within the marsh. Salt marshes are important habitats for oysters, shrimps, crabs, flatfish, and mullet. They also support large numbers of birds that stop over in the course of migration.
This category includes freshwater marshes close enough to coasts to experience significant tides but far enough upriver in the estuary to be beyond the reach of oceanic salt water. This set of circumstances usually occurs where fresh river water runs to the coast and where the morphology of the coast amplifies the tide as it moves inland. Freshwater tidal marshes are interesting because they receive the same “tidal subsidy” as coastal salt marshes but without the stress of salinity. They act in many ways like salt marshes, but the biota reflect the increased diversity made possible by the reduction of the salt stress found in salt marshes. Plant diversity is high, and more birds use these marshes than any other marsh type. In most parts of the world, the location of freshwater tidal marshes corresponds to sites determined by humans as optimal for habitation and eventual development of cities—i.e., those areas that provide a reliable source of fresh water as well as a physical connection to the sea for ships. Thus freshwater tidal marshes are among the wetland types that have been most altered or destroyed by urban development around the world. Examples of the impact human development has had on wetlands are found in Chesapeake Bay and the lower Delaware River in the eastern United States.
In seaweed-based systems seaweeds vary in size from giant kelps 40 metres (130 feet) or more in length, through the common rockweeds that are 1 or 2 metres (3.28 or 6.56 feet) long, to species that are so small as to be barely visible. They are algae and differ from flowering plants in having a holdfast instead of roots, a stipe instead of a stem, and a blade or thallus instead of leaves. They depend on water movement to continuously provide nutrients, which they take up through the surface of the blade. Kelp is a general term for large brown algae of the order Laminariales. They live predominantly just below low-tide mark and form dense beds reminiscent of underwater forests. They absorb a great deal of wave action, helping to defend shorelines against storms.
The giant kelps that occur along the Pacific coast of the United States and South America have been studied extensively because they are harvested for the extraction of alginates and other substances used in food processing. Typically growing in about 10 metres (32.8 feet) of water, they have large holdfasts from which several stipes originate. A young stipe grows as much as 45 cm (18 inches) per day, reaches the surface of the water, and then trails downstream. A large number of relatively small blades grow from the stipe and form a surface canopy with which they intercept light and nutrients. Giant kelp beds are home to a rich variety of invertebrates and fish, and, in many regions, to the sea otter (Enhydra lutis). Sea otters were once abundant around the North Pacific rim from Japan to California, but their range was greatly reduced by hunting. They have recently been reintroduced and populations are growing in many parts of British Columbia in Canada and Washington, Oregon, and California in the United States. Sometimes the sea urchin Strongylocentrotus becomes extremely abundant; in the course of feeding on the stipes of the kelps it may destroy kelp beds over large areas. The sea otter is a predator of sea urchins, and where it is abundant it has been shown to control sea urchin numbers. Abalone, a favourite food of sea otters as well as humans, are often abundant in kelp beds.
The characteristic kelps of the North Atlantic are species of Laminaria that grow in dense beds but extend only one or two metres (3.28 or 6.56 feet) above the bottom. A characteristic inhabitant of these kelp beds is the Atlantic lobster, Homarus americanus, which includes sea urchins in its diet. In the 1970s in Nova Scotia, Can., there was a major outbreak of destructive grazing by sea urchins. This outbreak was accompanied by a sharp decline in lobster populations, suggesting that when lobsters are scarce sea urchin numbers proliferate. However, the question of whether lobsters control sea urchin numbers is still undecided. In the Southern Hemisphere Macrocystis and Laminaria also occur, but the giant kelp Lessonia is important in South America, as is Ecklonia in South Africa and Australia.
Rockweed is a general term for the familiar brown seaweeds of the order Fucales, which grow between high-and low-tide marks (the intertidal zone) on rocky shores. In the Northern Hemisphere Fucus and Ascophyllum are common genera. The latter may be recognized by possession of small air-filled bladders on the fronds. It usually grows in more sheltered locations than Fucus. The intertidal zone is of interest because of the zonation of organisms that occurs there. It is in many ways an ideal laboratory in which to study the factors controlling the population size of seaweeds and invertebrates. A high proportion of the animals and algae in this zone are firmly attached to the rocks in order to withstand the force of waves breaking on the shore. Attached fauna include barnacles, limpets, periwinkles, and mussels. Barnacles are crustaceans that are attached to rocks along their backs, with upward-pointing legs that are surrounded by a row of protective hard plates. Limpets are mollusks that live under a very strong conical shell and cling to the rock by an adhesive “foot.” Barnacles filter fine particles of seaweed and plankton from the water, while limpets graze on the very small algae growing on the rock surface. Periwinkles are marine snails with hard shells that find shelter among the rockweeds on which they browse. Clamlike mussels are able to anchor themselves firmly to the rocks by means of strong threads; they feed by filtering water. Characteristic predators of these animals are large snails known as whelks, as well as crabs and starfishes. Several kinds of fish enter the rockweed zone at high tide and feed on the invertebrates.
Zonation of seaweeds and animals in the intertidal zone results partly from adaptation to a gradient of physical conditions and partly from competitive interactions between the organisms. The upper part of the intertidal zone is exposed to the air for a longer period and thus is at greater risk of drying out, baking, freezing, or being exposed to rainwater. Algal zonation occurs according to the ability of a species to tolerate these environmental factors, and this in turn influences the type of animal that will inhabit each zone of seaweed. The reverse effect also operates, because by their feeding activity, grazers exclude some seaweeds from zones to which they are otherwise suited.
At the next level in the food web (that of consumers), predators such as starfish control the abundance of grazing animals. In classic experiments on the coast of Washington state, the ecologist Robert Paine demonstrated that removal of the starfish Pisaster ochraceus from a section of shoreline caused the community to change from one containing 30 species to one totally dominated by the mussel Mytilus californianus. Mussels in this location have the ability to outcompete all other organisms for space on the rocks. Only when the mussel population is controlled by the starfish is a diverse community able to develop. Since these pioneering studies were carried out, many comparable effects have been demonstrated elsewhere. For example, in some places, barnacles are competitive dominants, but their abundance is controlled by limpets and whelks.
Sea-grass beds are found just below low-tide mark in all latitudes. In north temperate waters Zostera is the most common genus, while in tropical climates Thalassia, known as turtle grass, is an important element. As with marsh grasses, it seems that most of the plant material produced is decomposed by fungi and bacteria while the nutrients are recycled. The sea-grass beds slow the flow of water, causing deposition of silt in which worms and clams may burrow. The plants present a large surface area on which small algae grow, providing a nutritious source of food for browsing animals. The sea-grass beds also shelter many small organisms from their predators, and various species of fish lay their eggs close to sea-grass beds so that the young fish can take advantage of this shelter. Manatees and dugongs, often known as sea cows, are marine mammals that specialize in feeding on sea grasses. This was once a diverse and abundant group, but there are now only three species of manatee (genus Trichecus) and one dugong species (Dugong dugon). The manatees inhabit the eastern and western shores of the Atlantic, while dugongs are found from East Africa to Southeast Asia and Australia. They reach two to three metres (6.56 or 9.84 feet) in length and feed by ploughing along the bottom, ingesting rhizomes, stems, and leaves of sea grass. Dugongs in northern Australia can occur in herds of 100 to 200 and need very large areas of sea-grass beds to support them. Green turtles (Chelonia midas), which compete with dugongs for sea grass as food, occur throughout the tropics and are much more abundant than dugongs. In the area of the Great Barrier Reef, nesting colonies of green turtles have been observed that contain between 11,000 and 12,000 individuals.
Large areas of coastal habitat have sediments that are too unstable to support communities of large plants. They often have populations of microscopic algae growing at the surface, and they receive particles of decomposing organic matter derived from nearby seaweed or sea-grass beds. A beach near the high-tide level may be so unstable that few animals are able to live in it, but a little farther out to sea the mudflats or sand flats support a rich community of burrowing animals such as polychaete worms, clams, and burrowing shrimps. Many of the worms ingest the sediment and derive nourishment from the organic matter contained in it. Others have tubes that reach to the surface so that they can filter food particles from the water when they are covered by the tide. Clams usually feed in the same way. Crustaceans, starfish, and various kinds of finfish, especially flatfish, move over the mudflats at high tide in search of prey. Mudflats and sand flats are important feeding grounds for wading birds such as sandpipers, oystercatchers, and plovers. In temperate climates such birds may remain year-round, but many hundreds of thousands of birds make seasonal migrations between high-latitude summer habitats and low-latitude wintering grounds. Large flocks rely on intertidal flats for feeding along the way. For example, it has been shown that about 70,000 semipalmated sandpipers stop on the mudflats of the upper Bay of Fundy, in eastern Canada, in July and August of each year. Feeding predominantly on the burrowing amphipod shrimp Corophium volutator, each bird takes 10,000 to 20,000 shrimps and accumulates 13 to 18 grams (0.46 to 0.63 ounce) of fat, comprising one-third to one-half of the body weight, before taking off on a nonstop journey to the Lesser Antilles or the north coast of South America. At one time there was a plan to build a dam for tidal power that would have permanently flooded these tidal flats, and this would have been a disastrous loss of habitat for these migratory birds.
Inland wetland systems span freshwater marshes, bogs, forested swamps, and riparian ecosystems. Inland systems are typically characterized by the periodic influx of freshwater from precipitation or runoff from upstream areas, and they support biological communities that are largely dependent on freshwater environments.
The wetlands in this diverse group are unified primarily by the fact that they are all nontidal freshwater systems dominated by grasses, sedges, and other freshwater hydrophytes. However, they differ in their geologic origins and their driving hydrologic forces, and they vary in size from small pothole marshes less than a hectare in size to the immense saw grass monocultures of the Florida Everglades. Vegetation is dominated by graminoids and sedges such as the tall reeds Typha (cattails) and Phragmites, the grasses Panicum and Cladium, the sedges Cyperus and Carex, and floating aquatic plants such as Nymphaea and Nelumbo in temperate regions and Eichhornia crassipes in tropical and subtropical climes. Some inland marshes, such as the prairie glacial marshes of North America, follow a 5- to 20-year cycle of drought. During this period the marsh dries out and exposes large areas of mudflat upon which dense seedling stands germinate. When the rains return, flooding drowns the annual seedlings while allowing the perennials to spread rapidly and vigorously. Deterioration of the marsh follows and is sometimes associated with concentrated muskrat activity. The cycle then repeats.
The substrate of inland marshes has a higher pH and a greater availability of minerals than does the substrate of bogs. Freshwater marshes are often very productive ecosystems, and most of that productivity is routed through detrital pathways. Herbivory can be important, particularly by muskrats and geese, and consumers can have very significant effects on ecosystem development.
Bogs and fens belong to a major class of wetlands called peatlands, moors, or mires, which occur throughout much of the boreal zone of the world. Bogs and fens are distributed in cold temperate climates, mostly in the Northern Hemisphere. There, ample precipitation and high humidity from maritime influences, combined with low evapotranspiration, lead to moisture accumulation. Bogs are acid peat deposits that generally have a high water table (the upper surface of groundwater) but no significant inflow or outflow of streams. Because of their low pH, they support acidophilic (acid-loving) vegetation, particularly mosses. Fens are open wetland systems that generally receive some drainage from surrounding mineral soils and are often covered by grasses, sedges, or reeds. Extensive areas of bogs and fens occur in Finland, eastern Europe, western Siberia, Alaska, Canada (especially Labrador), and the north-central United States. Canada has approximately 1.3 million square km (502,000 square miles) of peatlands, making it the largest resource for peat in the world. In the United States, bogs and fens usually develop in basins scoured out by the Pleistocene glaciers and are clustered primarily around the Great Lakes region and in Maine.
Compression and chemical breakdown of dead plants and other vegetable debris cause formation of the organic substance known as peat, which is harvested and dried for use as fuel.
An organic fuel consisting of spongy material formed by the partial decomposition of organic matter, primarily plant material, in wetlands such as swamps, muskegs, bogs, fens, and moors. The development of peat is favoured by warm, moist climatic conditions; however, peat can develop even in cold regions such as Siberia, Canada, and Scandinavia. Peat is only a minor contributor to the world energy supply, but large deposits occur in Canada, China, Indonesia, Russia, Scandinavia, and the United States. Major users include Finland, Ireland, Russia, and Sweden.
Peat moss (Sphagnum flexuosum). K.G. Preston-Mafham/The Natural History Photographic Agency
The formation of peat is the first step in the formation of coal. With increasing depth of burial and increasing temperature, peat deposits are gradually changed to lignite. With increased time and higher temperatures, these low-rank coals are gradually converted to subbituminous and bituminous coal and under certain conditions to anthracite.
Dried peat burns readily with a smoky flame and a characteristic odour. The ash is powdery and light, except for varieties that have a high content of inorganic matter. Peat is used for domestic heating purposes and forms a fuel suitable for boiler firing in either briquetted or pulverized form. It also has been used to produce electricity.
Peat moss, which is also called bog moss or sphagnum moss, includes any of more than 150–300 species of plants in the subclass Sphagnidae, of the division Bryophyta, comprising the family Sphagnaceae, which contains one genus, Sphagnum. The taxonomy of Sphagnum species remains controversial, with various botanists accepting quite different numbers of species. The pale green to deep red plants, up to 30 cm (about 12 inches) tall, form dense clumps around ponds, in swamps and bogs, on moist, acid cliffs, and on lakeshores from tropical to subpolar regions. The veinless leaves and stem cortex contain many interconnected, enlarged dead cells, with external openings through which water can enter; the plants hold up to 20 times their weight in water.
Each spherical brown sporangium, or spore case, shrinks as it dries, creating internal pressure that casts off the lid (operculum) and shoots the spores as far as 10 cm (4 inches) from the plant. The metabolic processes of growing peat moss cause an increase in the acidity of the surrounding water, thus reducing bacterial action and preventing decay.
Peat moss forms several types of bogs in northern areas.
Dried peat moss has been used for surgical dressings, diapers, lamp wicks, bedding, and stable litter. It is commonly employed as a packing material by florists and shippers of live aquatic animals and as a seedbed cover and soil additive by gardeners, who value its ability to increase soil moisture, porosity, and acidity. Peat mosses are valuable in erosion control, and properly drained peat bogs provide useful agricultural land.
The term “swamp” usually refers to a wetlands system dominated by trees or other woody vegetation. A wide variety of such systems are found throughout the world. In the tropics vast swamps (which are also called riparian systems) are found along the great rivers, by which they are often inundated for many months. In temperate regions forested swamps can be dominated by trees that tolerate permanent to semipermanent flooding such as the bald cypress (Taxodium) or swamp tupelo (Nyssa) in the southern United States or the alder (Alnus) or maple (Acer) in more temperate climes.
Riparian systems occur along rivers and streams that periodically crest their channel confines, causing flooding. They are also in evidence in places in which a meandering channel creates new sites for plant life to take root and grow. The soils and amount of moisture they contain are influenced by the adjacent stream or river. These systems are distinguished by their linear form and by large fluxes of energy and materials delivered by upstream systems. In arid regions riparian systems can exist along or in ephemeral streams and on the floodplains of perennial streams. In most nonarid regions riparian zones usually develop first along the region of the stream where water flow is constant—i.e., the point at which sufficient groundwater enters the channel to sustain flow through dry periods. Riparian ecosystems exist as broad, alluvial valleys several tens of kilometres wide, as in the Amazon Basin in South America and in Bangladesh, or they can be narrow strips of vegetation along the bank of a stream, as is often seen in the arid western United States. The riparian zone is valuable to animals as a refuge, as an abundant source of water, and as a corridor for migration. This is particularly true in arid regions, where riparian zones may support the only significant vegetation in many kilometres.
Among ecosystems that are easiest to destroy permanently are these boundary ecosystems. Wetlands can be isolated from their hydrologic source and essentially destroyed if drainage areas are altered or impoundments are built. Major cities in the United States such as Chicago and Washington, D.C., stand on sites that were, in part, wetlands. The amount of wetlands lost worldwide is almost impossible to determine. However, it is known that in the lower 48 states of the United States, a relatively newly developed region of the world, more than half of the original wetlands have been lost, primarily to agricultural development.
Humans have been utilizing wetlands for centuries. Early civilizations, such as the ancient Babylonians, the Egyptians, and the Aztec, developed unique systems of water delivery that involved wetlands. Among the peoples currently living in proximity to wetlands (known as “wetlanders”) whose culture is linked to these systems are the Camarguais of southern France, the Cajuns of Louisiana, and the Ma‘dan, or Marsh Arabs, of southern Iraq; after hundreds of years, all still live in harmony with wetlands. Countless plant and animal products are harvested from wetlands in countries such as China. A thriving modern industry continues to depend on the harvest of cranberries from bogs in the United States. The Russians and the Irish have mined their peatlands for several centuries as a source of energy. Many countries throughout South and Southeast Asia, East Africa, and Central and South America depend on mangrove wetlands for timber, food, and tannin. For centuries salt marshes in northern Europe and the British Isles, and later in New England, have been used to graze animals and raise crops of hay. Thatch roofs and fences have been built from materials retrieved from these areas. Reeds from the wetlands of Romania, Iraq, Japan, and China are used for similar purposes. Techniques to produce fish in systems integrated into rice paddies or shallow ponds were developed several thousand years ago in China and Southeast Asia; crayfish harvesting is still practiced in the wetlands of Louisiana and the Philippines.
Recognition of the importance of wetlands is growing, with the result that many are being protected by local and national policies (particularly in the United States) as well as by international projects. Examples of these efforts include the Ramsar Convention, which is an international agreement set up for the protection of habitat for migratory waterfowl and other avian life, and the North American Waterfowl Management Plan. Wetland recognition and protection is becoming one of the most important facets of global natural resource protection.
Combining the attributes of both aquatic and terrestrial ecosystems, but falling outside each category, wetlands inhabit a space betwixt and between the disciplines of terrestrial and aquatic ecology. Consequently, their unique properties are not adequately addressed by present ecological paradigms. With their unique characteristics of standing water or waterlogged soils, anoxic conditions, and plant and animal adaptations, wetlands serve as testing grounds for “universal” ecological theories and principles such as succession and energy flow, concepts developed primarily with aquatic or terrestrial ecosystems in mind. These boundary ecosystems also provide an excellent laboratory for the study of principles related to transition zones, ecological interfaces, and ecotones. In order for wetlands to be protected or restored in the best possible manner, a multidisciplinary approach to their study is required.
