Physical Settlement and Accretion Further Reading

Wetlands are defined variously to include a wide spectrum of habitats where the land is inundated by shallow water or is saturated by surface or ground water at a frequency and duration every year that water becomes the primary factor controlling the environment and the associated plant and animal life. The soils that develop under the prolonged influence of waterlogging or submergence are known as hydric soils, and the plants that are adapted to or require such hydrological conditions are called 'hydrophytes'. The vast majority of wetlands are characterized by the presence of vegetation comprising of macroalgae, mosses, or herbaceous or woody vascular plants. However, habitats such as gravel beaches and rocky shores without hydric soils and vegetation are also wetlands as defined by the Ramsar Convention as well as the United States Fish and Wildlife Service.

Wetlands occur in all climatic zones on the Earth from tundras to the tropics, and include bogs, fens, marshes, swamps, springs, lagoons, mangroves, shallow lakes, and temporary water bodies such as playas, potholes, sebkhas, dambos, chotts, chaurs, and billabongs. Riverine flood-plains generally comprise a mosaic of habitats varying from swamps, marshes, and shallow water bodies (oxbows) to water-saturated lands that lie along the river channels and are periodically flooded by the river. The beds of marine algae (kelps) and sea grasses in shallow coastal seas as well as the coral reefs are also considered as wetlands. Besides these natural wetlands, there are a large variety of man-made wetlands such as fish ponds, paddy fields, and shallow reservoirs. For simplicity, wetlands can be grouped into four major types which are distinguished by, and lie along the gradients of, hydrological regimes, nutrient status, and salinity that in turn also determine the dominant vegetation type (Figure 1).

Bogs are typically ombrotrophic, acidic, and dominated by Sphagnum mosses. They are deficient in nutrients because of their dependence entirely upon the precipitation. Low-temperature and acidic conditions result in extremely slow decomposition, and hence the accumulation of partly decomposed organic matter - the peat. Fens are also peat-accumulating wetlands in similar environments but receive water and nutrients from the surrounding areas. Fens reflect the chemistry of the geological formations through which these waters flow. In limestone areas the water is high in calcium carbonate resulting in fens that are typically buffered to a near neutral pH of 7. However, the level of calcium or magnesium bicarbonate varies widely in fens. At low levels of bicarbonate the pH may be closer to pH 4.6 resulting in an acid fen. At very high levels of bicarbonate, the water may reach a pH of 9. Accordingly, fens differ also in their plant and animal communities. Bogs and fens occur in similar climatic and physiographic regions, and often side by side, one grading into the other. Marshes (including salt marshes) are herbaceous wetlands, mostly on mineral soils with variable supply of nutrients, and develop under a wide range of hydrological regimes. Swamps are dominated by woody vegetation (trees and shrubs), and are often referred to as forested wetlands or wetland forests. Common examples are the vast floodplain forests of the Amazon river basin, the bottomland forests and Taxodium swamps of southern and southeastern USA, and the riverine swamps in Asia and Australia. Some tropical swamps, such as those in Southeast Asia, also accumulate peat. The papyrus swamps in Africa are in fact marshes dominated by the giant sedge, Cyperus m e g ra E

Nutrient poor (oligotrophia)

Tide flooded

Tide flooded ra E

Nutrient poor (oligotrophia)

Nutrient rich (eutrophia)

Figure 1 Diagrammatic ordination of major types of wetlands in relation to the hydrological and nutrient gradients. Modified from Gopal B, Kvet J, Loffler H, Masing v, and Patten BC (1990) Definition and classification. In: Patten BC, Jorgensen SE, Dumont HJ etal. (eds.) Wetlands and Shallow continental water Bodies. vol. 1: Natural and Human Relationships. pp. 9-16. The Hague: SPB Academic Publishing.

Nutrient rich (eutrophia)

Figure 1 Diagrammatic ordination of major types of wetlands in relation to the hydrological and nutrient gradients. Modified from Gopal B, Kvet J, Loffler H, Masing v, and Patten BC (1990) Definition and classification. In: Patten BC, Jorgensen SE, Dumont HJ etal. (eds.) Wetlands and Shallow continental water Bodies. vol. 1: Natural and Human Relationships. pp. 9-16. The Hague: SPB Academic Publishing.

papyrus. Mangroves are forested wetlands restricted to tropical and subtropical regions in estuarine deltas. Shallow water bodies and lagoons and also the shallow water coastal wetlands are generally dominated by submerged herbaceous vegetation.

Natural wetlands are estimated to cover about 5.7 million km2, that is, roughly 6% of the Earth's land surface, of which 30% are bogs. Majority of wetlands lies at the interface between deep open water and the uplands, and is therefore, transitional (or ecotonal) in nature. These interfaces include the littoral zones (areas between the highest and lowest water level) of large lakes and reservoirs, the riverine floodplains, and the coastal areas that are regularly flooded by the tides.

Early human civilizations such as the Egyptian and Mesopotamian developed around the marshes, whereas others depended greatly on wetlands for a variety of resources (for food, fuel, and fiber). In many regions of the world, wetlands formed an integral part of the socio-cultural ethos of the human communities. Papyrus was used to make yachts and paper, reeds were very widely used for housing and thatch, and many plants and animals served as food. Today, half the human population depends for subsistence on two major wetland produce -rice and fish. Yet, the wetlands were for long treated as wastelands (typically by the western world) that were drained, filled, and reclaimed or converted to other land uses. However, during the past 50 years, human perceptions have changed gradually: first recognizing their importance as habitats for numerous kinds of waterfowl and later as 'liquid assets' or 'waterlogged wealth'.

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