What is a Wetland?

Simply stated, wetlands are a part of our landscape that is defined by the presence of water. More specifically, wetlands are areas where the presence of water determines or influences most, if not all, of an area's biogeochemistry – that is, the biological, physical, and chemical characteristics of a particular site.

Wetlands typically represent transitional zones between upland and aquatic ecosystems, although not always. Some wetlands may be scattered across the landscape in depressions that collect water or zones where groundwater surfaces.

Different types of wetlands can be characterized by how much water is found and when it occurs on a site, and the chemical nature of the water, soils, and/or underlying bedrock of the wetland ecosystem. Different plant communities may be found in different types of wetlands, with each member species adapted to the local hydrology, including the spatial and temporal distribution of water and its underlying chemistry. Many animal, fungal, and microbial species are completely dependent upon wetlands for critical stages in their lifecycles, while still other species choose to make use of wetlands for many of their life's activities.

Defining Wetlands

Although we can readily describe wetland characteristics and what they do, in practice there has been a great deal of difficulty in defining specifically what constitutes a wetland. A workable definition became critical in classifying habitat for legal purposes, especially in regard to determining what lands are protected by state and federal legislation. The U.S. Fish and Wildlife Service defined wetlands as follows in Classification of wetlands and deepwater habitats of the United States (Cowardin et al., 1979):

Wetlands are lands transitional between terrestrial and aquatic systems where the water table is usually at or near the surface, or the land is covered by shallow water... Wetlands must have one or more of the following three attributes: 1) at least periodically, the land supports predominantly hydrophytes; 2) the substrate is predominantly undrained hydric soil; and 3) the substrate is saturated with water or covered by shallow water at some time during the growing season of each year.

The U.S. Army Corps of Engineers, the federal agency responsible for enforcing federal laws protecting wetlands, has more recently determined that in order for an area to be considered a wetland, it must have all three of the attributes referenced above, i.e., the area must be predominantly characterized by wetland vegetation, soils, and hydrology.

Types of Wetlands

There are many different types of wetland, each determined by its hydrology, water chemistry, soils, and the predominant plant species found therein. Some wetlands are permanently flooded, while others are only seasonally flooded, but retain saturated soils throughout the unflooded periods. Still other wetlands may or may not ever be flooded, but maintain saturated soil conditions long enough for hydric soil characteristics to develop, i.e., chemical changes in the soil resulting from the low oxygen conditions associated with prolonged saturation. Wetlands may be characterized as dominated by trees, shrubs, or herbaceous vegetation. They may be fed by precipitation, runoff, or groundwater, with water chemistry ranging from very acidic to alkaline.

Marshes are wetlands that are permanently flooded or flooded during high water periods at the edges of rivers, streams, lakes, or ponds. Marshes may be dominated by submersed, floating-leaved, or emergent vegetation, including cattails, pondweeds, water lilies, and various sedges, rushes, spike rushes, grasses, and other forbs. Marshes can be subcategorized into emergent marsh and hemi marsh.

• Emergent marsh is the marsh found around shorelines out to relatively shallow water, and is generally characterized by up to 100% cover with emergent plant species. In the Midwest, these may consist of various graminoids such as river bulrush and rice cut grass, and characteristic forbs such as purple false foxglove, nodding bur marigold, pickerel-weed, and duck potato. These marshes are ideal habitat for a wide range of animals, including raccoons, Great Blue Herons, and a multitude of dragonflies, butterflies, and other insects. Emergent marshes also provide critical habitat for rare amphibians and reptiles, such as the plains leopard frog and Blanding's turtle. In the marsh habitat around Hennepin and Hopper lakes, one can observe breeding populations of rare bird species such as the Yellow-headed Blackbird and the Least Bittern, both of which are state-threatened.

• Hemi marsh is found in deeper water, and is characterized by an open mix of emergent and/or floating-leaved vegetation interspersed with a submersed plant community. The submersed community may consist of species like sago pondweed, coontail, and wild celery, while the emergent or floating-leaved group may include deeper water species like broad-leaved cattail, American lotus, mosquito fern, white water lily, and common bur reed. The combination of emergents and floating-leaved species with open water creates an ideal combination of food and cover for many aquatic-dependent birds and amphibians. American Bitterns and Great Egrets comb these areas in hunting, while Common Moorhens and Pied-billed Grebes use them as areas to nest and rear their young. The rich vegetation also provides exceptional habitat for fish and is a great production area for the zooplankton and insects that are a critical part of the site's intricate food web.

Sedge meadows (or wet meadows) are wetlands with permanently or near-permanently saturated soils. They may form a transitional zone between marshes and other wetlands with less saturated soils, or occur in wet depressions and swales, or around groundwater discharge zones. The meadows are wet grasslands often dominated by sedges and grasses with relatively few forbs. They may be low in species diversity (with as few as a single dominant species), but relatively rich in some of the rarer species adapted to saturated soil conditions. There are very many sedge species, with characteristic dominants including the lake sedge, tussock sedge, or brown fox sedge. At the Dixon Waterfowl Refuge, various rare and conservative sedges may be found, such as the brown bog sedge, fringed sedge, common yellow lake sedge, and Bebb's oval sedge, along with fringed brome and oval-stem spike rush. Birds frequenting this habitat include the King Rail, Sandhill Crane, Northern Harrier, and Sedge Wren. Reptiles such as the northern water snake and amphibians like the pickerel frog and cricket frog are also common.

Wet prairie is an ecosystem found across much of the Dixon Waterfowl Refuge at Hennepin & Hopper Lakes between sedge meadows and mesic prairies. Wet prairies are herbaceous wetlands dominated by a mixture of graminoids (grasses and sedges) and forbs, such as little bluestem, northern dropseed, prairie indian plantain, marsh phlox, and foxglove beardtongue. Wetland areas that are intermediate between wet prairie and mesic (dry) prairie can be characterized as wet-mesic prairie, the driest type of wetland in the Midwest. Animals that may be found in wet prairies include Henslow's Sparrows, Short-eared Owls, eastern hog-nosed snakes, and coyotes.

Fens and seeps are wetlands that are fed by surfacing groundwater. The type of vegetation found within these wetlands is dependent upon the water chemistry and pH.

• Fens are typically alkaline from groundwater emerging from calcareous or dolomitic soils or bedrock zones, and many of the species found there can only grow under those conditions. Fens are dominated by herbaceous vegetation such as grass of Parnassus, bog lobelia, or beaked spikerush, but may also include trees or shrubs, such as various shrubby cinquefoils and/or willows.

• Seeps are typically found along the base of slopes or glacial moraines where water emerges from saturated soils or a spring. These usually small areas consist of plant such as clearweed, jewelweed, low nutrush, and marsh marigold.

Bogs are basin wetlands for which precipitation is the only source of water, i.e., they are typically not fed by surfacing groundwater or streams. Bogs are generally dominated by sphagnum mosses, which may form a floating mat over deeper water that supports a rich assortment of other species adapted to acidic water conditions. Sphagnum mosses acidify the water down to pH levels as low as 3.0, comparable to that of acid rain. Some of the unique plants adapted to these acidic conditions include some of the carnivorous plants such as the sundews and pitcher plants, as well as such economically important species as blueberry and cranberry.

Sources:

Chicago Region Biodiversity Council. 1999. Chicago Wilderness Biodiversity Recovery Plan. Chicago Region Biodiversity Council, Chicago, IL.

Cowardin, L.M., V. Carter, F. C. Golet, E.T. LaRoe. 1979. Classification of wetlands and deepwater habitats of the United States. U.S. Dept. of Interior, Fish and Wildlife Service, Washington D.C.

Mitsch, W. J., J.G. Gooselink. 2000. Wetlands. John Wiley and Sons, Inc.