Wetlands may range in size from wet hollows a few meters square to the vast peatlands of the west Siberian Plain (Neishstadt, 1977; Walter, 1977) and the Hudson/James Bay Lowland (Wickware et al., 1980; Pala and Weischet, 1982), which between them cover more than 10⁶ km². They occur in a great diversity of types, which can be grouped into the seven major categories listed in Table 1 (Mitsch and Gosselink, 1993). Each category can be divided into two or more subcategories. Northern peatlands, for instance, are usually divided into circumneutral fens and strongly acid bogs (Gorham and Janssens, 1992a). In fens, the bicarbonate buffer system regulates pH, whereas in bogs, colored organic acids buffer the aqueous system at a pH of about 4.

Despite the significance of wetland ecosystems, which cover 26 percent more area in the conterminous United States than do fresh and saline lakes, reservoirs, rivers, and bays (Frayer, 1991), they seldom receive much attention in curricula designed for limnologists. In six familiar limnology textbooks (Table 2), wetlands account for a little more than 1 percent of the textual material.

This paper summarizes material that might be included in a limnology course encompassing wetland ecosystems (see also Gore, 1983; Mitsch and Gosselink, 1993).

Wetlands are waterlogged landscape features and often develop at the margins of rivers and lakes. In the latter case the lakes may eventually—through the deposition of silt and peat—become converted wholly to wetland. The largest wetlands, however, generally form on very flat terrain in which damp mineral soils are invaded in their wettest parts by peat-forming vegetation. Peat impedes and dams up the natural drainage and brings about an expansion of the waterlogged area, eventually swamping large areas of upland forest. Very large peatlands often develop intricate and beautiful landscape patterns, which represent perhaps the most delicate mutual interaction between hydrology and vegetation on the surface of the earth (Sjörs, 1961; Heinselman, 1963; Wright et al., 1992). Ground water upwelling is a major factor in the development of these