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Wetlands of the Prairie Pothole Region:
Invertebrate Species Composition, Ecology, and Management

Prairie Pothole Wetlands


The prairie pothole region (PPR) of North America covers approximately 715,000 km2, extending from north-central Iowa to central Alberta (Figure 21.1). The landscape of the PPR is largely the result of glaciation events during the Pleistocene Epoch. The last glaciers retreated from the PPR approximately 12,000 years ago, leaving behind a landscape dotted with many small depressional wetlands called potholes or sloughs. The present climate of the midcontinent PPR is dynamic, characterized by interannual variation between wet and dry periods in which abundant rainfall can be followed by drought (i.e., the wet/dry cycle). The association between prairie wetlands and groundwater tables of the region is complex and dynamic. Hydrologically, prairie wetlands can function as groundwater recharge sites, flow-through systems, or groundwater discharge sites. The hydrologic function a particular wetland performs is determined by variations in climate, its position in the landscape, the configuration of the associated water table, and the type of underlying geological substrate. The unique hydrology and climate of this region have a profound influence on the water chemistry, hydroperiod, and ultimately the biotic communities that inhabit prairie wetlands.

Prairie Pothole Region
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

Glaciation events during the Pleistocene Epoch were the dominant forces that shaped the landscape of the PPR (Winter 1989). When the glaciers retreated, a landscape dotted with numerous small, saucer-like depressions was exposed. These depressions, caused by the uneven deposition of glacial till, the scouring action of glaciers, and the melting of large, buried iceblocks are known today as prairie potholes or sloughs.

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

The PPR is in the midcontinent of North America and is subject to the climatic extremes of this region (Winter 1989). Temperatures can exceed 40° C in summer and -40° C in winter. Isolated summer thunderstorms may bring several inches of rain in small localized areas while leaving adjacent habitats entirely dry. Also, winds of 50 to 60 km hr-1 can quickly dry wetlands during the summer or create windchills below -60° C during winter.

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

Although PPR wetlands receive the majority of their water from snowmelt runoff in the spring and rarely as summer precipitation, the association between prairie wetlands and groundwater tables of the region is complex and dynamic (Winter and Rosenberry 1995). Hydrologically, prairie wetlands can function as groundwater recharge sites, flow-through systems, or groundwater discharge sites. Groundwater recharge wetlands receive their water primarily from the atmosphere and there is little or no groundwater inflow. As a result, the mineral content of water in recharge wetlands is extremely low. Wetlands that function as flow-through systems both receive and discharge water and solutes from and into the ground. Water in flow-through wetlands generally reflects the chemical composition of groundwater. Wetlands that function as groundwater discharge sites receive the bulk of their solutes from groundwater and their principal water loss is from evapotranspiration. As a result, the salinity of water in discharge wetlands can be highly variable and in some cases can exceed the salinity of seawater. The hydrologic function (recharge, discharge, flow-through) of a particular wetland is determined by variations in climate and by their position in the landscape, the configuration of the associated water table, and the type of underlying geologic substrate. The hydrologic function of individual wetlands defines their unique hydroperiod and chemical characteristics and ultimately the plant community they support. Hence, wetland classes based on vegetation (Stewart and Kantrud 1971) reflect the range of hydrologic function within any given wetland class. Temporary wetlands tend to recharge groundwater, seasonal wetlands can have either a recharge or flow-through function, semipermanent wetlands tend to have either a flow-through or discharge function, and saline tend to function mostly as discharge sites.

Chemistry

The chemical characteristics of prairie wetlands also vary in relation to fluctuations in climate and hydrology. Prairie wetlands have dissolved solid concentrations that span the gradient from fresh to extremely saline (LaBaugh 1989). Hydrologic processes, especially those that define how individual wetlands receive and lose water, largely determine the salt concentration of individual wetlands at any given point in time. Wetlands range in specific conductance from 42 (Petri and Larson 1973) to 472,000 µS cm-1 (Swanson et al. 1988; LaBaugh 1989), while fluctuations of individual wetlands can range from 1,160 to 43,600 µS cm-1 (LaBaugh et al. 1996 and unpublished data). Most wetlands in the PPR are alkaline (pH > 7.4)(LaBaugh 1989), with values as high as 10.4 in North Dakota marshes (Swanson et al. 1988).

Vegetation

Plant communities in prairie wetlands are dynamic and continually changing as a result of short- and long-term fluctuations in water levels, salinity, and anthropogenic disturbance. van der Valk and Davis (1978a) proposed a general model describing how wetland plant communities respond to water level fluctuations due to the wet/dry cycle. Four wetland stages are identified: dry marsh, regenerating marsh, degenerating marsh, and lake marsh.

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

A number of wetland classification systems are available (Stewart and Kantrud 1971, Millar 1976, Cowardin et al. 1979, Brinson 1993) but Stewart and Kantrud (1971) is specific to the glaciated prairies. Using Stewart and Kantrud (1971), there are seven wetland classes, based on the vegetational zone occupying the central, deepest part of the wetland basin and occupying at least 5 percent of the total wetland area. The seven vegetational zones identified by Stewart and Kantrud (1971) are the wetland-low-prairie, wet-meadow, shallow-marsh, deep-marsh, permanent-open-water, intermittent-alkali, and fen zones.

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.

Temporary (Class II) Semipermanent (Class IV)
(a)(c)
Seasonal (Class III)Alkali (Class VI)
(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

The agricultural value of the PPR has tremendously impacted prairie pothole wetlands. Wetland drainage (both surface and tile) to enhance agricultural production has been the primary factor resulting in the loss of wetlands in this region (Tiner 1984, Canada-United States Steering Committee 1985, Millar 1989, Dahl 1990, Dahl and Johnson 1991). Remaining wetlands are impacted by a number of agricultural practices that result in elevated sedimentation rates (Martin and Hartman 1986, Gleason and Euliss 1996a), drift of agricultural chemicals into wetlands (Grue et al. 1989), large inputs of nutrients (Neely and Baker 1989), unnatural variance in water-level fluctuation (Euliss and Mushet 1996), and altered vegetative communities (Kantrud and Newton 1996). Major nonagricultural impacts include alteration of hydrologic and chemical regimes due to road construction (Swanson et al. 1988) and urban development. The extent to which landuse has altered the ecology of aquatic invertebrates is poorly studied but must be understood to facilitate effective management of prairie wetlands.
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