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Ecological Studies at the Woodworth Study Area

Impact of Agricultural Land-Use on Prairie Wetland Ecosystems: Experimental Design and Overview

Robert A. Gleason* and Ned H. Euliss, Jr.
National Biological Service
Northern Prairie Science Center
8711 37th Street Southeast
Jamestown, ND 58401

Most wetlands in the Prairie Pothole Region (PPR) are on private lands and are embedded within an agricultural landscape (1, 2). The influence of agricultural land-use practices on wetland ecosystems is largely unknown but potential impacts include the direct and indirect application of agrichemicals (2, 3) and increased sedimentation. Cultivation of wetland basins and associated watersheds also facilitates transport of chemicals adsorbed on soil particles into wetlands. Intuitively, increased sedimentation leads to the ultimate loss of wetland habitat as basins are filled with allochthonous materials and increased turbidity and covering of substrates decreases primary production. Although a few studies in the PPR have examined the impact of agricultural activities on wetland sedimentation rates (4, 5), the impacts of increased sedimentation on water quality, primary productivity, aquatic invertebrates, and bird use are poorly understood.

To address this information need, a 2-year field study was initiated in 1993 to examine the impacts of common agricultural land-use practices on prairie wetlands and to assess potential impacts on wetland biota in the PPR. This research was a collaborative effort involving the National Biological Service's Northern Prairie Science Center (NPSC), the U.S. Environmental Protection Agency's Mid-Continent Ecology Division (EPAMED), the U.S. Army Corps of Engineers Waterways Experiment Station (USAWES), the National Research Council (NRC), the University of Minnesota's Cooperative Fish and Wildlife Research Unit (MCFWRU), and Humboldt State University. The objectives of this collaborative work were broad, including a determination of the effects of sedimentation on water quality, vegetation, aquatic invertebrates, waterfowl use, and aquatic food webs. Additionally, the work was designed to evaluate management tools used to mitigate sedimentation impacts on wetlands. NPSC monitored macroinvertebrate communities and waterfowl use, and EPAMED monitored water quality and plant and algal communities (6, 7); sedimentation data were collected and processed jointly by NPSC and the EPAMED. Data collected jointly on weather, sedimentation, water balance, and basin morphometry by NPSC and EPAMED were used to develop a hydrologic sediment model for the PPR by the USAWES (8). In addition, investigations on secondary productivity and sediment impacts on food webs was initiated by the NRC with support from EPAMED (9). Lastly, in vitro experiments on the effects of sediment and agrichemicals on aquatic invertebrates was conducted by MCFWRU. In this paper, we provide the overall framework for this larger interdisciplinary effort, including the experimental design, methods, and evaluation, and provide an overview of the findings on sedimentation, waterfowl use, and macroinvertebrates. Funding for this study was provided by EPAMED and NPSC; access to study sites was provided by the U.S. Fish and Wildlife Service (USFWS).

Experimental Design

The experimental design consisted of assigning five wetlands to the following replicate land-use practices: 1) idled native grassland, 2) idled non-native grasslands (i.e., lands similar to Conservation Reserve Program lands; CRP), 3) summer-fallowed agricultural lands, and 4) summer-fallowed agricultural lands with wetlands protected by vegetative bufferstrips. All 20 study wetlands were on USFWS Waterfowl Production Areas (WPA's) located on the eastern edge of the Missouri Coteau, near Woodworth, North Dakota. Sixteen wetlands were located on the Woodworth Study Area (WSA), and the remaining 4 wetlands were located on WPA's within 10 km of Woodworth.

The 4 watershed treatments we selected for evaluation represent common land-use practices in the PPR. Summer fallow, bufferstrip, and CRP treatment wetlands had prior-cultivated cropland land-use history. Summer fallow is a common crop rotation practice in the PPR that consists of idling the land during the growing season, except for periodic cultivation to control weeds. Summer fallow fields have no protective covering and are highly vulnerable to soil loss. Our bufferstrip treatment was similar to summer fallow except that a standard U.S. Soil Conservation Service 7.62 m bufferstrip (10) of grassy vegetation was left around the perimeter of the wetland to reduce erosion of upland soil into wetland basins. Our CRP treatment was representative of lands currently enrolled in the CRP program in North Dakota except that they were located on WPA's and were managed by the USFWS as dense nesting cover for waterfowl. Our native prairie treatment had no prior tillage history and was still dominated by native mixed-grass prairie vegetation although some exotic plant species were present.

We selected seasonal wetlands (11) for study because they are commonly cultivated (12) and they are the most abundant class of wetlands in the PPR (1, 13, 14). We selected a population of potential wetland replicates that were similar with respect to size, soils, historically similar wetland class (1970-88), and land-use practices (1950's-present) based on an extensive database maintained by NPSC on the WSA (15). Wetlands also were visually inspected to ensure they had generally similar vegetation and had non-integrated watersheds of similar size and slope. Population size of wetlands available for random assignment to land-use treatments was constrained by our selection criteria and compatibility of certain experimental land-use treatments (i.e., those requiring tillage) with management goals of the USFWS. Although a completely random assignment of study wetlands to treatments was not possible, we believe the sites selected were representative of area wetlands given our logistical constraints. All land-use treatments requiring tillage were located on the WSA, an area set aside for manipulative research projects since the mid-1960s. All wetlands in our sample had similar land-use histories within treatment and were classified as seasonals (11) 75% or more of the time from 1970-1988 (Table 1). Wetlands ranged in size from 0.09 to 0.47 ha with shoreline lengths of 93 to 365 m and had watersheds from 0.5 to 1.8 ha (Table 1). Soils were of a Barnes-Buse-Parnell complex (Table 1) with slopes of 0 to 35%, and soil capability classification ranges of IIIe-VIIIe, indicating the main hazard was from soil erosion when nonvegetated (16). Major soils such as Barnes occurred on side and foot slopes and Buse soils on summits and shoulder slopes. Parnell was the major wetland soil. Although we attempted to select wetlands with non-integrated watersheds, surface outflow and/or inflow was observed in 8 wetlands during unusually high water levels in 1993 and 1994 (Table 1).

In 1993, we delineated wetland basins and watersheds. Wetland basins were delineated based on zonation by hydrophytic vegetation and consideration of change in soil type whereas watershed boundaries were delineated by topography; watersheds were defined as the catchment area that contributed surface runoff to a particular wetland.

Tilling of watersheds began in 1993 and continued as needed (usually monthly) during the 1993 and 1994 field season. Because only the watersheds were tilled, our treatments were not entirely representative of an agricultural landscape where wetlands are located in large tilled fields devoid of perennial cover. Also because of the proximate nature of study wetlands to nesting cover and other managed wildlife habitat, observations of wildlife use were likely inflated relative to a more representative agricultural landscape.

Elevations were determined in each wetland basin along five random transects that radiated from the center of the wetland (defined as the deepest point in the wetland basin) using a laser level. These elevations were used to define each wetland's basin and watershed morphometry and the mid-elevation of the wet-meadow and shallow emergent vegetative zones. A nest of 4 different sediment trap designs was installed along each transect (hereinafter called sediment transects) at the mid-elevation of the wet-meadow and shallow emergent vegetative zone, and the mid-elevation between the shallow emergent zone and center of the wetland. Traps installed included cylinder sediment traps (17, 18), filter sediment traps (19), wind deposited sediment traps designed for this study, and feldspar horizon markers (20). Cylinder sediment traps were used to estimate total downward flux of allochthonous and autochthonous sediment particles, feldspar horizon markers and filter sediment traps provided measures of net-sediment input, and wind sediment traps measured wind deposited sediment. Surface runoff was measured using modified surface flow traps (21) in areas of concentrated flow at the wetland edge and watershed boundary.

To reduce disturbance of substrates along sediment transects, invertebrate and vegetative monitoring was conducted on transects that radiated from the center of the wetland and bisected the angle between the sedimentation transects. Invertebrates were sampled on the left side (facing the wetland center) of the transect and vegetation work was conducted on the right. Other instrumentation installed include staff gauges which were installed in the center of each wetland and rain gauges which were installed in each watershed. In 1994, weather stations were also installed on all 4 WPA's. Aerial photographs of each wetland were geo-referenced using geographic positioning system equipment in 1994. Variables monitored for collaborative purposes are listed in Table 2.

Preliminary Results and Conclusions

Preliminary results indicate that soil loss was greatest in summer fallow, followed by bufferstrip, CRP, and native prairie. Sedimentation rates estimated from cylinder sediment traps in 1993 indicated that in all wetland zones, summer fallow had significantly greater (P>0.05) total sedimentation rates than native prairie, CRP, and bufferstrip wetlands. Further, the inorganic fraction of sediment entering wetlands was significantly greater (P>0.05) in summer fallow than in the other land-use treatments. A complete synthesis on the interactions of response variables with explanatory variables is not yet available but will be provided in subsequent reports. However, it was clear that overall invertebrate density and biomass was similar among all land-use treatments both years. Analysis of individual invertebrate taxa indicated that most differences observed were probably related more to dynamic hydrologic cycles rather than to land-use treatment. Differences in waterfowl use among treatments also were not detected. Although some impacts from increased sedimentation were observed, monitoring of cumulative impacts from periods exceeding 2 years may be required to fully document impacts on invertebrates and birds. The dynamic hydrology of prairie wetlands has a profound influence on the ecology of waterfowl and other biota. For example, the first year of this study was one of the driest years recorded in this past century and the second year was one of the wettest. Studies that capture the temporal periodicity of drought cycles (i.e., 10-30 years) will be required to elucidate all but the most catastrophic impacts. Regardless, our study identified significant transport of sediments from uplands into prairie wetlands with the greatest impacts observed in wetlands surrounded by summer fallow followed by bufferstrip wetlands, non-native grassland (CRP) wetlands, and relatively pristine native prairie wetlands, respectively. However, this is based on sediment data collected during the first year of tillage when much organic debris was still present in tilled soils to retard erosion. Although sedimentation data for 1994 are not yet available, field observations suggest that bufferstrip wetlands performed poorly during the second year of our study, perhaps as poorly as we observed on wetlands in summer fallow in 1993. Reasons for this reduced performance were likely related to the oxidation and loss of binding organic debris in tilled soils and the fact that unusually high water levels in 1994 flooded bufferstrips, thus reducing their effective width.
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