Wetlands

Wetlands are low-lying, depressional ecosystems that are permanently or periodically saturated with water at or close to the surface. The vegetation of wetlands must be adapted to the physical and chemical stresses associated with flooded substrates. The most common types of wetlands are swamps, marshes, shallow open waters, and mires, the latter consisting of peat-accumulating fens and bogs. Wetlands vary greatly in their productivity, mostly because of intrinsic differences in the rate of supply of nutrients. Wetlands provide important habitat for a wide variety of plants and animals. However, wetlands are rapidly disappearing because they are being drained and in-filled for agricultural, urbanization, and industrial purposes. Wetlands are also being degraded by nutrient loading, which causes eutrophication, and by pollution associated with inputs of toxic chemicals and organic materials. Losses of wetlands and the biodiversity that they support are an extremely important aspect of the environmental crisis.

Wetlands can be characterized on the basis of their hydrology, morphology, water chemistry, and vegetation. All of these factors can vary regionally and locally, depending on the climate, character of the surrounding watershed, and the species that are present (that is, the biogeographic region). The major kinds of wetlands are described below, with an emphasis on North American types.