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Sedimentation of Prairie Wetlands

Effects on Water Quality Functions

The water quality functions wetlands provide are dependent upon interactions between vegetation, substrates, and microbial populations (Hemond and Benoit 1988; Hammer 1992). Wetland soils are the primary media wherein microbial mediated transformation of nutrients and storage of pollutants occur. The most active sites of chemical transformations are the thin aerated zones at the soil-water interface, and the thin aerobic zone surrounding the roots of vascular plants (Hammer 1992; Mitsch and Gosselink 1993). For agrichemicals that require biological or chemical transformation for solubilization and subsequent removal, there is some potential for sediment burial to decrease the release of bound nutrients (Neely and Baker 1989). However, we generally have a poor understanding of the impact of sedimentation on specific processes involved with improving water quality in prairie wetlands (Hemond and Benoit 1988; Adamus and Brandt 1990).

Although incomplete, we have a better understanding of the indirect impact sediment exerts on water quality through its influence on hydrophytes, organic exchange substrates, and microbial populations. Reduction of light available for photosynthesis due to turbidity and the burial of macrophyte seed banks are obviously negative impacts of excessive sediment entering wetlands from adjacent fields. Aquatic macrophytes and algae are important in the uptake, short-term storage, and cycling of nutrients in wetlands; negative impacts on plants from sediments may alter water quality functions. Increased input of allochthonous inorganic matter to wetlands (Martin and Hartman 1987; Gleason 1996) would reduce the availablity of organic exchange surfaces important for sorption of contaminants, especially on the thin aerobic zone at the soil-water interface. While the impact of sedimentation on microbes has not been studied (Adamus and Brandt 1990), sediment fallout may cover microbes, or organic matter needed for microbial processes, or alter redox profiles important in the performance of water quality processes. Finally, the ability of wetlands to remove and retain sediments is a basic concept of improved water quality, but many PPR wetlands are closed systems that can totally fill with sediments and hence lose their capacity to function properly. The trade off between the importance of sediment removal as a water quality benefit and maintaining the topographic life of wetland basins clearly needs to be integrated into management strategies of wetlands.

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