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Water-Level Fluctuation in Wetlands As a Function of Landscape Condition in the Prairie Pothole Region


Wetlands in the Prairie Pothole Region (PPR) of North America are severely altered by agricultural practices, including wetland drainage, physical manipulation of upland and wetland soils, sedimentation, and the application of agricultural pesticides and fertilizers. Sixty-five percent of the original wetland area in the PPR has been drained (National Research Council 1982, Tiner 1984, Dahl 1990). Remaining wetlands receive sediments (Martin and Hartman 1987) and chemical drift from adjacent fields (Grue et al. 1989). Moreover, wetland drainage and intensive management of agricultural fields alter the natural hydrologic cycles of many remaining wetlands. Removal of grasses and other native vegetation from catchment areas alters surface runoff dynamics and hence exacerbates impacts associated with sedimentation and agricultural chemicals absorbed on soil particles. Nonvegetated catchments have less capacity to mitigate excessive surface runoff, resulting in wetland water levels that are more variable than those in landscapes dominated by grasses and forbs. In addition to direct changes in hydrology, excessive runoff from major precipitation events can dilute salts in wetlands (LaBaugh et al. 1996). Due in part to the many problems associated with osmoregulation in fresh water (Potts and Parry 1964, Hutchinson 1967, Wetzel 1983), many organisms have developed specific tolerances to concentrations and species of salts present in wetlands (Moyle 1945, Macan 1961). Thus, excessive fluctuation in salinity resulting from unnatural dilution due to altered catchments may affect the natural composition of wetland flora and fauna. In this study, we tested the hypothesis that water levels fluctuate more in wetlands within landscapes dominated by tilled agriculture than in wetlands within landscapes dominated by grasslands. Despite the obvious and critical role of hydrology to the structure and function of wetlands, few ecological studies in the PPR have considered hydrology. One reason for the lack of hydrologic data in many studies of prairie wetlands is the high cost associated with required equipment. To obtain hydrology data for this study, we designed an inexpensive device that records maximum and minimum water levels in wetlands.

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