Marshes are produced by flooding, and, as a consequence, have distinctive soils, microorganisms, plants, and animals. The soils are usually anoxic or hypoxic, allowing vast numbers of microorganisms, particularly bacteria, to transform elements including nitrogen, phosphorus, and sulfur among different chemical states. Marsh plants often have hollow stems to permit movement of atmospheric oxygen downward into their rhizomes and roots. Marshes are some of the most biologically productive habitats in the world, and therefore support large numbers of animals, from shrimps and fish through to birds and mammals. Marshes are one of six types of wetlands, the others being swamp, fen, bog, wet meadow, and shallow water. Humans can affect marshes by changing water levels with drainage ditches, canals, dams, or levees. Other human impacts can arise from pollution by added nutrients, overhar-vesting of selected species, or building road networks in landscapes.
See also: Floodplains; Mangrove Wetlands; Peatlands; Riparian Wetlands; Swamps; Temporary Waters.
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