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Assessing the Quality and Quantity of Depressional Wetlands in the Redwood River Watershed Utilizing a Probabilistic Survey Design

By John Genet 1 and Anthony Olsen 2

1 Minnesota Pollution Control Agency, Saint Paul, Minnesota
2 U.S. Environmental Protection Agency, WED, Corvallis, Oregon

Located in southwestern Minnesota, the Redwood River watershed lies within an area of the Prairie Pothole Region where land use is predominantly a mixture of row crop and livestock agriculture. In 2003 the condition of emergent depressional wetlands in this watershed was assessed using aquatic plant and macroinvertebrate indices of biological integrity (IBIs). The survey design followed a Generalized Random Tessellation Stratified procedure for discrete objects and used a modified National Wetland Inventory (NWI) digital coverage as the sample frame. Approximately 150 sites were evaluated to determine if the wetland still existed and belonged to the target population, 40 of which were assessed with IBIs. This evaluation served a dual purpose, allowing estimation of depressional wetland losses in the watershed subsequent to the generation of NWI maps in this region (ca. 1980) and condition assessments of those that remain. Cumulative distribution functions of IBI results estimated that 69% of the wetland basins in the watershed, representing 91% of the depressional wetland area, were biologically impaired. The number of wetland basins in the watershed decreased by 56%, representing a 21% decrease in depressional wetland area, since the early 1980s. This assessment indicated that emergent depressional wetlands in the Redwood River watershed have been impacted by various stressors that have either resulted in their elimination or degradation.

Keywords: wetlands, watershed, index of biological integrity, wetland plants, aquatic macroinvertebrates, and Prairie Pothole Region

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