Areal Extent of Sediment Overburden in Restored Prairie Wetlands and the Impact on Recolonization by Aquatic Invertebrates and Hydrophytes in the United States Prairie Pothole Region.
Background and Justification
The Prairie Pothole Region (PPR) is a major biogeographical region that encompasses 715,000 km2, and extends from north central United States to south central Canada (Kantrud et al. 1989). This region is pocketed with millions of shallow wetlands of glacial origin that support diverse and unique assemblages of wetland biota (Kantrud et al. 1989). Superimposed over this region is an intensively farmed landscape that is a major producer of cereal grains. As a consequence of agricultural intensification and cropland expansion, numerous wetlands have been drained and less than 50% of the estimated 8 million ha of wetlands remain in the PPR (Dahl 1990; Dahl and Johnson 1991). In response to past wetland losses and increased public awareness of wetland functions and their associated values (Brun et al. 1981; Ludden et al. 1983; Winter 1989; Stevens et al. 1995), numerous wetlands have been restored by international, federal, state, and private agencies to reverse the impacts of wetland drainage (Dornfeld and Warhurst 1988; Interagency Committee 1992). Although tens-of-thousands of drained wetlands have been restored throughout the PPR, the success of wetland restoration is rarely evaluated.
The goal of most restoration programs is to restore wetland functions and biological integrity to approximate predrainage conditions. Techniques to restore wetlands typically focus on restoring the hydrologic regime by plugging ditch and tile drains and relying on natural processes for recolonization of hydrophytes and aquatic invertebrates (e.g., reestablishment from relic seed and egg banks and dispersal of propagules from surrounding wetlands). However, most wetlands targeted for restoration have been drained and farmed for extensive periods of time which may impede or make untenable the assumed recolonization process of invertebrates and plants (Galatowitsch and van der Valk 1994). Earlier studies examining the response of plant and animal populations to wetland restoration have indicated that both rapidly recolonize after reflooding (Dornfeld and Warhurst 1988; LaGrange and Dinsmore 1989; Sewell 1989). Further, use of restored wetlands by waterfowl typically begins immediately once reflooded (LaGrange and Dinsmore 1989). However, interpretation of these earlier studies is limited because they did not compare the flora and fauna of restored wetlands to that of comparable natural wetlands.
More recent studies that have compared restored wetlands to natural wetlands have indicated that the success of restorations is highly variable. Galatowitsch and van der Valk (1996) found that in recently restored wetlands wet meadow and sedge meadow zones did not reestablish rapidly and that the number of wet meadow and sedge meadow species was lower in restored than in natural wetlands; whereas deep marsh and submersed aquatic zones were comparable between restored and natural wetlands. Likewise, Delphey and Dinsmore (1993) found that vegetative zones lacking in restored wetlands limited horizontal structure and concomitant breeding bird diversity. Reasons for varying success in recolonization of certain plant species has been related to impacts of past landuse. For example, relic seed and invertebrate egg banks may be lost because of prolonged drainage, cultivation and/or artificially shortened hydroperiods (Wienhold and van der Valk 1987; Galatowitsch and van der Valk 1994, 1996; Euliss and Mushet unpubl. data).
Another disturbance that may influence reestablishment of invertebrate and plant communities is the impact of anthropogenic sedimentation. Cultivation of wetland catchments has increased sedimentation of wetland basins (Martin and Hartman 1987; Dieter 1991; Gleason 1996; Gleason and Euliss 1996; Luo et al. 1997). Most restored wetlands are designed to achieve historic pool levels, but accelerated sedimentation from cultivation may actually decrease original water depths. Hence, wetlands may be shallower than their natural analogues; such conditions may favor development of monodominant stands of emergent vegetation (e.g., cattails) that contribute little to vegetative diversity and exacerbate problems with farmers because they provide roost sites for depredating blackbirds (Linz et al. 1996). Further, loss of wetland volume from sedimentation may influence water permanence and hydrologic regimes that ultimately determine the hydrophyte and aquatic invertebrate communities that characterize specific wetland classes (Kantrud et al. 1989). Low plant diversity also influences the diversity and composition of invertebrate communities (Krull 1970; Voights 1976). Sediment overburdens (i.e., the layer of upland soil from agricultural fields deposited on wetland sediments) may also cover viable seed and egg banks and render them unavailable for recolonization of restored wetlands. In vitro experiments have shown that sediment depths as little as 0.25 cm can significantly reduce species richness, emergence, and germination of hydrophytes (Jurik et al. 1994; Wang et al. 1994). Consequently, research is needed to evaluate the influence of sediment overburdens on recolonization of hydrophytes and aquatic invertebrates to identify alternate management options to restore wetlands.
This study was developed to address this research need and to complement a larger multiagency research effort being coordinated by Northern Prairie Wildlife Research Center (NPWRC) on restored and natural wetlands in the PPR (NPWRC 1997a). From June-September 1997, NPWRC conducted an extensive survey on natural and restored wetlands in the PPR of Montana, North Dakota, South Dakota, Minnesota, and Iowa. The goal of this research effort was to measure variables with intrinsic value for testing regional-level hypotheses and/or those needed by collaborators to link their studies to the larger sampling universe of the extensive survey. The extensive survey focused on seasonal and semipermanent wetlands restored on Conservation Reserve Program (CRP) land or similar grassland habitats in three physiographic regions in the PPR: Missouri Coteau, Prairie Coteau, and Glaciated Plain (Bluemle 1977). In each landform, the extensive survey sampling design consisted of selecting seasonal and semipermanent wetlands in each of the following wetland categories: 1) wetlands in CRP and similar grasslands restored for 1-5 yrs, 2) wetlands in CRP or similar grasslands restored for >5-10 yrs, 3) drained wetlands in CRP or similar grasslands (hereafter called drained wetlands), 4) nondrained wetlands in CRP or similar grasslands (hereafter called CRP wetlands), and 5) pristine wetlands with no history of tillage in their catchments. NPWRC will provide sediment overburden and land-use information on extensive survey study wetlands as a special linkage variable for this study.
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