Wetlands Ecosystems For example, various classification systems segment the great delta of the Mississippi and Atchafalaya rivers into many types of floodplain forest, freshwater marsh, salt marsh, streams, and lakes They assign each prairie pothole in the prairie region of the United States and Canada to one of about 20 classes of prairie wetlands The mangrove forests of Central America are made to seem distinct from the adjacent beds of sea grasses Functionally, however, these diverse wetlands are linked and might be better viewed as major regional wetland ecosystems Water, sediments, plant propagules, fish, birds, other animals, genetic material, and pollutants move among them One type changes through time to become another Diversity emerges in both space and time from the components as well as their interconnectedness In fact, prior to human modification of them, most wetlands existed as complexes of several different types functioning as integrated systems linked by movements of water, sediment, nutrients, plants, and animals The vast rain-fed and riverine floodplains that form the Sudd in southern Sudan, for example, consist of a complex and dynamic mix of different types of grassland (e.g., Echinochloa pyramidalis or Hyparrhenia rufa dominated), wild rice marshes (Oryza longistaminata), woodlands (Acacia seyal or Balanites aegyptiaca dominated), and open water that cover more than 45,000 km2 The great floodplain of the Orinoco River and its tributaries form a nearly 10-million wetland complex of streams, ponds, shallow oxbow lakes, riverine forest, marshes, and seasonallyflooded palm (Copernicia tectorum) savannas and grassland in Venezuela and eastern Columbia The eastern shore of Lake Ontario, New York, is bordered by a 17-mile long interconnected system of streams, shallow ponds, marshes, fens, and shrub swamps Most temperate zone estuaries include freshwater, brackish, and saltwater marshes along with tidal creeks and mudflats Mangrove swamps, saline lagoons, sea grass beds, and coral reefs form interconnected systems along shallow coasts in subtropical and tropical regions These complexes, or assemblages of wetlands within regions, represent another level of wetland diversity that only partially remains today Current classification systems not capture such regional ecosystems, though clearly they exist Many people recognize wetland complexes such as the Everglades and the Prairie Pothole Region as regional ecosystems Classification systems, however, only recognize hierarchy in types, not in assemblages of wetlands within regions Current efforts by conservation organizations and government agencies to move to ecoregional mapping attempt to overcome this limitation Continental and Regional Diversity Climate and geomorphology, along with biogeographic factors, control the diversity of wetlands on different continents and in different regions Wetland development is promoted where precipitation annually exceeds evapotranspiration on a long-term basis In such climates, wetlands develop in a number of landscape settings, including depressions in the land surface, areas where minimal land surface slope slows surface water runoff, along breaks in the slope of the land surface, and even on slopes if precipitation greatly exceeds 387 evapotranspiration In semiarid, arid, and desert climates, wetlands occur far less frequently, but they are found (a) in areas with depressions or lowlying land along rivers or coasts where runoff converges from extensive uplands or mountains, (b) along coasts regularly inundated by tides, (c) in areas where permafrost or other impermeable layers below ground prevent or slow infiltration of surface water, and (d) in springfed oases In addition to topographic variation, variation in the composition and stratigraphy of surficial geological deposits promotes diversity in the types of wetland that occur within a region The composition of subsurface materials plays a major role in determining the chemistry of water entering wetlands, and thereby the plant species composition of wetlands For example, wetlands occurring in regions dominated by limestone deposits tend to have high-pH waters rich in base cations that promote the growth of distinctive rich fen vegetation In contrast, waters draining areas with granitic bedrock tend to be low in pH and base cations; such areas more frequently contain bog-like vegetation Areas with thick deposits of glacial till can contain a mix of rock types that, in turn, may support a variety of wetland types A diversity of types in a given region also is promoted where surface topography and subsurface stratigraphy of geological deposits produce groundwater flow paths of varying lengths Wetland water chemistry is strongly influenced not only by water source but also by the length of groundwater flow paths that enter the wetland and the number of different groundwater flow systems that feed the wetland Different flow systems may contribute water with different chemistry because the water originates from different geological deposits If the water originates from the same geological materials, longer flow paths may yield higher concentrations of solutes because longer flow paths generally correlate with longer groundwater residence time (i.e., longer contact time with geological materials) Biogeographic factors also influence the number of wetland types occurring in different regions and continents Aquatic plants generally are known for their remarkably wide geographic range, but many species have smaller ranges determined in part by biogeographic factors (i.e., the climatological, geological, glacial, and floristic history of a region) The distributions of these more restricted species help define distinctive biogeographic regions that contain wetlands with species compositions unlike those from other biogeographic regions For example, shallow ponds that support many species of the Atlantic Coastal Plain flora might be classed broadly as emergent marshes, but this would miss their unique contribution to the diversity of North American wetlands The flora of these coastal plain ponds, which has an unusual abundance of rare and uncommon species, bears little resemblance to the emergent marshes of other biogeographic regions While no systematic comparison has been made of wetland ecosystem diversity in different regions, the factors controlling regional wetland diversity are understood All other things being equal, regions with the greatest climatic and geomorphologic variation are likely also to contain the highest diversity of wetland types If wetland types are defined to the level of plant species composition, then biogeography enters into the equation Thus, continental diversity will vary as a