Northern Prairie Wildlife Research Center
Ephemeral and temporary are the names given in one classification for the palustrine wetlands which lose water during the first few weeks after the spring snowmelt period. These wetlands are generally less than a half acre in area. The water is usually less than a foot deep, and is normally fresh, with specific conductance less than 500 micromhos. Dominant plants are short to fairly tall junciform species of low prairies and wet meadows. Typical species include switchgrass, prairie cordgrass, and Canada reedgrass. Because of their shallow depth, these wetlands are first in the spring to develop an invertebrate fauna. In wet years, this fauna is heavily used by breeding birds as a source of protein during the egg laying period and also provides food for spring migrant waterbirds. Vegetation in these wetlands is used for nesting by such species as sedge wren, LeConte's sparrow, and Savannah sparrow.
Seasonal wetlands are another class of palustrine wetland that is abundant in the Prairie Pothole Region. Except during dry years, these wetlands usually contain water well into the growing season. They average less than 2 feet deep in the early spring and the average size is about an acre. Most seasonal wetlands spill regularly and maintain a specific conductivity under 2,000 micromhos. Seasonal wetlands are dominated by fairly tall junciform emergents and a few subdominant submergent or floating plants. Dominant species include whitetop, slough sedge, and tall mannagrass. Except during drought years, the populations of freshwater anostracans that develop in these wetlands are important protein sources for early-nesting and migrant waterfowl, and other marsh and aquatic birds. Vegetation in seasonal wetlands provides the principal nesting substrate for the horned grebe, Wilson's phalarope, and red-winged blackbird in North Dakota.
Palustrine wetlands that usually maintain water throughout spring, summer, and fall, and often into the following spring as well, are termed semipermanent wetlands. These average about 5 acres in area and about 2 to 4 feet in depth. Spill is much less frequent than in temporary and seasonal wetlands, and some semipermanent wetlands never spill. In consequence, the specific conductance of semipermanent wetlands ranges from less than 500 to over 70,000 micromhos. The dominant plants in the central or deepest zones of semipermanent wetlands are usually tall, robust emergent bulrushes or cattails or, in the open water phase, pondweeds, milfoil, and other submerged aquatics. In addition to their great vegetative diversity, the great value of semipermanent wetlands to birds stems from their production of amphipods, snails, and other invertebrate animal life during the summer and fall when other wetlands are often dry. Semipermanent wetlands form the principal nesting areas for all of North Dakota's overwater nesting waterfowl and most of our other marsh and aquatic birds, including the black-crowned night heron, American coot, and black tern. They are also the principal habitat for the mink and muskrat and provide wintering areas for deer and upland game birds.
Fens are palustrine wetlands characterized by the surficial exposure of alkaline groundwater. These wetlands average about 10 acres in area. Water in most fens ranges in specific conductance from 1,000 to 3,000 micromhos. Coarse emergents such as phragmites and common cattail, and several shrubby species of willow usually dominate the central zones of fen wetlands. Muskgrass, a large alga, can usually be found in the small open water seepage areas of fens. Fens have boggy or quaking bottom soils. Thus, they receive heavy use by several species of marsh birds whose nests may be easily damaged by large grazing animals. Fens also are important watering areas for sandhill cranes during migration and are much used by mink, wintering deer, and upland game.
Lacustrine wetlands are generally large areas of open water with active, wave-formed shorelines and no persistent emergent vegetation in the central or deepest zones. Permanent fresh wetlands are numerous in a small area in northern North Dakota called the Turtle Mountains, but uncommon throughout the rest of the Prairie Pothole Region. Many of these wetlands contain water of sufficient depth to maintain fish populations. The bottoms of these wetlands may be unvegetated or support stands of various deepwater pondweeds. Freshwater amphipods, decapods, reptiles, and amphibians are common. These wetlands are heavily used by migrant waterfowl and breeding gulls, cormorants, pelicans, and grebes. Subsaline and saline permanent wetlands cannot support fish populations. The principal salts in these wetlands are sulfates and chlorides of sodium and magnesium. Conductivities may be in excess of 100,000 micromhos. Permanent saline wetlands have no overflow outlets. The bottoms may be unvegetated or support stands of widgeongrass attractive to migrant waterfowl. Certain copepods, ostracods, and anostracans adapted to the highly saline water may also be abundant, which makes these wetlands highly attractive to migrant waterbirds.
Alkali wetlands are lacustrine wetlands characterized by the intermittent occurrence of shallow saline water. These wetlands average over 100 acres in area and, like permanent saline wetlands, may contain water with specific conductance in excess of 100,000 micromhos. No emergent plants grow in the central zone of these wetlands, but salt water widgeongrass and certain algae are often found in abundance. Principal invertebrates are anostracans and ostracods. These wetlands provide the primary habitat for several species of migrant shorebirds and are also heavily used by migrant waterfowl. Common breeding species on these wetlands are the avocet, piping plover, and Wilson's phalarope.
Riverine wetlands include those with periodically or continuously moving water contained within a channel. Only three rivers in North Dakota, the Missouri, Yellowstone, and Red, are considered permanent riverine wetlands. Permanent riverine wetlands support species such as sturgeon and paddlefish which are not found in intermittent riverine wetlands. The intermittent types are most productive of small fish such as minnows and suckers, and are heavily used by mink, muskrat, and beaver.
Precipitation and groundwater flow patterns are the principal factors affecting the hydrology and limnology of prairie wetlands. Secondary influences are grazing and fire. All prairie wetlands except the permanently-flooded lacustrine and riverine types undergo irregular drying and oxidation of their bottom soils. This release of nutrients is the main reason why prairie wetlands are so biologically productive. Shallow wetlands at high elevations in coarse soils tend to dry out quickly because they rapidly lose water through bottom seepage as well as through evapotranspiration. These wetlands spill regularly and surface water, when present, is relatively fresh. Wetlands at intermediate elevations in less porous soils experience throughflow or slow seepage outflow. Water in these wetlands tends to be slightly to moderately saline, or it can be quite fresh if spillage is frequent. Concentrations of dissolved solids is greatest in wetlands at low elevations that experience only net seepage inflow from groundwater. Wind action is the only mechanism which removes salts from these wetlands and such action only occurs during drought conditions. Differences in salinity result in great differences in the plant and animal associations that are found in prairie wetlands. Grazing by large ungulates and muskrats also markedly influences the natural plant communities found in these wetlands. Because many prairie wetlands frequently go dry, fire probably was another important factor influencing wetland vegetation and nutrient cycling in presettlement times.