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Figure 21.1. The prairie pothole region (PPR) of North America.

Aquatic invertebrates inhabiting prairie pothole wetlands are well suited to cope with the highly dynamic and harsh environmental conditions of the PPR. Because of midcontinent temperature and precipitation extremes, wetlands of the PPR periodically go dry, freeze solidly in winter, and exhibit steep salinity gradients. These salinity gradients are due to the interrelation among precipitation, evapotranspiration, interaction with groundwater, and variation in the composition of soils. Due to the harsh environmental conditions of the PPR, the overall diversity of aquatic invertebrates within each wetland is low because taxa are mostly restricted to a few ecological generalists. Those that do occur possess the necessary adaptations that allow them to exploit the naturally high productivity of prairie wetlands.

Despite the harsh climate, the PPR is an extremely productive area for both agricultural products and wildlife. The landscape has been substantially altered since settlement of the PPR in the late 1800s. Economic incentives to convert natural landscapes to agriculture are great and have resulted in the loss of over half of the original 8 million hectares of wetlands (Tiner 1984, Dahl 1990, Dahl and Johnson 1991). Land-use impacts on wetland biota include enhanced siltation rates, contamination from agricultural chemicals, altered hydrology, the spread of exotic plants, and habitat fragmentation due to wetland drainage and conversion of native prairie grasslands into agricultural fields.

The highly dynamic PPR is a unique area that is of critical importance to migratory birds in North America and to the aquatic invertebrates that supply them with dietary nutrients. Despite the value of the knowledge generated thus far, a critical need still exists to expand our knowledge of wetland invertebrates to better our understanding of the PPR ecosystem and its susceptibility to anthropogenic influence. Given the highly dynamic nature of PPR wetlands and the extreme variation in chemical characteristics and hydroperiod, it is essential that invertebrate studies be placed in the proper conceptual framework to maximize the application of study results. Herein we describe the highly dynamic nature of prairie wetlands and suggest that invertebrate studies be evaluated within the context of hydrologic, chemical, and climatic events that characterize the region.

Geology

The deposition of glacial till was unevenly distributed throughout the PPR. Large moraines accumulated along the terminal ends of glaciers and formed ridges of low, rolling hills in a northwest to southeast orientation, such as the Missouri Coteau. Where glaciers retreated quickly, large, gently rolling areas of glaciated plains were formed, and extremely flat lake beds developed where glaciers dammed meltwater. Due to the geologically young nature of the landscape and moderate rainfall, there are few natural surface drainage systems. Consequently, most wetlands in the PPR are not connected by overland flow.

Climate

Besides the normal seasonal climatic extremes, the semiarid PPR also undergoes long periods of drought followed by long periods of abundant rainfall. These wet/dry cycles can persist for 10 to 20 years (Duvick and Blasing 1981, Karl and Koscielny 1982, Karl and Riebsame 1984, Diaz 1983, 1986). During periods of severe drought, most wetlands go dry during summer, and many remain completely dry throughout the drought years. Exposure of mud flats upon dewatering is necessary for the germination of many emergent macrophytes, and it facilitates the oxidation of organic sediments and nutrient releases that maintain high productivity. When abundant precipitation returns, wetlands fill with water and much of the emergent vegetation is drowned. Changes in water permanence and hydroperiod by normal seasonal drawdown and long interannual wet/dry cycles has a profound influence on all PPR biota, but is most easily observed in the hydrophytic community (van der Valk and Davis 1978a).

The PPR has a north-to-south and a west-to-east precipitation gradient, with areas to the north and west receiving less precipitation than those to the south and east. However, even in the wetter southeastern portion of the region, wetlands have a negative water balance. Evaporation exceeds precipitation by 60 cm in southwestern Saskatchewan and eastern Montana and by 10 cm in Iowa (Winter 1989). Despite this negative water balance, many wetlands contain water throughout the year and go dry during periods of extended drought.

Hydrology

Chemistry

Vegetation

During drought periods marsh sediments and seed banks are exposed. During this dry marsh phase seeds of many mudflat annual and emergent plant species germinate on exposed mudflats, with annual species usually forming the dominant component (van der Valk and Davis 1976, 1978a, Davis and van der Valk 1978a, Galinato and van der Valk 1986, Welling et al. 1988a,b). When water returns, the annuals are lost but the emergent macrophytes survive and expand by vegetative propagation (i.e., regenerating marsh). Depth and duration of the flooding period, combined with the tolerances of the individual species of macrophytes, will determine how these wetland communities develop. If the wetland experiences only shallow flooding, the emergent macrophytes will eventually dominate the entire wetland. However, prolonged deep-water flooding results in the elimination of emergent macrophytes (i.e., degenerating marsh) due to the direct effects of extended inundation and the expansion of muskrats and their consumption of macrophytes. If water levels remain high, the lake marsh stage is eventually reached. Submersed macrophytes become established and dominate in the open water areas. A drawdown of the wetland will be necessary for reestablishment of emergent macrophytes.

Salinity modifies vegetation responses to water level fluctuations. Increasing salinity results in a loss of diversity, with the most saline wetlands having the fewest plant species (Kantrud et al. 1989). Soil salinity is also very important during the dry marsh phase, regulating the germination of emergent macrophytes on exposed mudflats (Galinato and van der Valk 1986). Kantrud et al. (1989) present information describing the salinity tolerances of many prairie wetland plant species, as well as the predicted changes that may occur as salinity changes over the course of the wet/dry cycle.

Until recently little was known about the algal communities of prairie wetlands or their responses to changes in wetland hydrology (Crumpton 1989, Goldsborough and Robinson 1996, Robinson et al. 1997a,b). Algal biomass may be lower than macrophyte biomass, but their overall productivity may be similar due to high turnover rates (Murkin 1989). Four algal communities are recognized within wetland habitats: epipelon (motile algae inhabiting soft sediments), epiphyton (algae growing on submersed surfaces such as macrophytes), metaphyton (floating or subsurface mats of filamentous algae), and phytoplankton (algae of the water column) (Goldsborough and Robinson 1996). A conceptual model describing wetland stages where each of these four communities is dominant has been developed by Goldsborough and Robinson (1996).

Wetland Classes

Stewart and Kantrud's (1971) wetland classification, while based on vegetational characteristics, reflects differences in water permanence and can be related to the water regime modifiers used by Cowardin et al. (1979). Wet-meadow vegetation (e.g., Poa palustris, Hordeum jubatum) dominates areas that typically contain water for several weeks after spring snowmelt. Shallow-marsh vegetation (e.g., Eleocharis palustris, Carex antherodes) dominates areas where water typically persists for a few months each spring, and deep-marsh vegetation (e.g., Typha latifolia, Scirpus acutus) occupies areas where water persists throughout the year. The permanent-open-water zone, characterized by the lack of vascular plants, dominates the central part of wetlands that rarely dry, even during periods of extended drought.

In terms of total area, wetlands of the temporary (Class II), seasonal (Class III), and semipermanent (Class IV) classes comprise the majority of the wetlands in the PPR (Figure 21.2). Ephemeral (Class I) wetlands, while numerous, are small and are not considered wetlands by Cowardin et al. (1979). Permanent (Class V) and alkali (Class VI) wetlands, although usually large in size, are few in number (Stewart and Kantrud 1971). Fens (Class VII) are not common in the PPR, but their unique biota has raised some concern to preserve existing sites as areas of special ecological significance.

(a)	(c)
(b)	(d)
Figure 21.2. Common wetland classes in the prairie pothole region: (a) temporary (Class II); (b) seasonal (Class III); (c) semipermanent (Class IV); (d) alkali (Class VI).

Within wetlands, vegetational zones frequently alternate between two or more distinct phases. These phases are identified by changes in the plant communities brought about by fluctuations in water levels or by changes in land-use practices. The six phases identified in Stewart and Kantrud's (1971) classification are the normal emergent, open-water, drawdown bare-soil, natural drawdown emergent, cropland drawdown, and cropland tillage phases. Although phase changes do not effect wetland classification, they do alter the vegetative structure available to invertebrate communities.

In addition to normal shifts between phases, vegetational zones also shift from one type to another in response to extended drought or above-normal precipitation. If this change occurs in the central, deepest part of a wetland, it can change its classification. During extended drought, wet-meadow vegetation often expands and dominates the central, shallow-marsh zone of seasonal wetlands. Conversely, during extended periods of above normal precipitation, shallow-marsh vegetation frequently expands into the wet-meadow zone. Thus, a seasonal (class III) wetland may shift to a temporary (Class II) wetland; the converse may occur during extended periods of above normal precipitation. As wetlands shift among phases and classes, the characteristic shift in vegetation affects a complimentary shift in the invertebrate community; in general, enhanced vegetative diversity results in an increase in invertebrate richness (Driver 1977).

Landuse Influences