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Upper Klamath Lake (UKL) and Agency Lake in south-central Oregon are hypereutrophic due to phosphorus (P) loading from both geologic and agricultural sources in the watershed. Restoring historic lake-fringe wetlands to provide P sinks around the lakes has been accepted as a favorable means of reducing lake P levels and loading. Hydrologic management strategies differ in their timing of wetland filling and draining, and they may have significantly different outcomes on P forms and concentrations released to the lakes. To evaluate the effects of hydrologic management on P loading to the lakes, we investigated the biotic and abiotic mechanisms of P release related to timing and duration of inundation of wetland soils from four restoration sites through a laboratory and field study. More specifically, we evaluated four hypotheses related to hydrologic management and P release in the restored wetlands: 1) timing (temperature) of inundation affects the concentrations and forms of P released in study wetlands, 2) the nature of P dynamics in the study wetlands releases primarily soluble reactive phosphorus (SRP), as opposed to organic P, 3) abiotic factors including dissolved oxygen, pH, redox, organic matter, and bulk density levels influence P release, and 4) soil P fractions change over time with different flooding regimes. These hypotheses were investigated in a lab experiment in which dry wetland soil cores were flooded for 56 days and included sampling of total phosphorus (TP), SRP, dissolved oxygen (DO), pH, redox, and CO2. Measurements were also taken on soil cores when dry, flooded for one day, after experiment flooding, and after flooding in the field for soil pH, organic matter, bulk density, total P, microbial P, and inorganic P fractions. Higher release rates of TP were found in summer temperature treatments in all wetlands while release of SRP varied more with temperature and abiotic factors. Low DO and redox levels also influenced greater release of P from soil cores. Soil solution pH upon flooding resulted in dissolution of inorganic P fractions, leading to release of SRP to the water column. After dissolution, wetlands with mineral soils had greater capacities of adsorbing SRP into P fractions than the wetlands with organic soils. Microbial P was also a factor in SRP release; saturated biological demand resulted in higher mineralization than immobilization rates in two wetlands. Our data indicate that wetlands with hydrologic connectivity to the lakes and mineral soils released the lowest concentrations of TP, while SRP was variable. Further, our data provide evidence for determining best management strategies for wetlands to lower P loading to the lakes, which should be based upon soil type, how inorganic P is held in soil fractions, microbial activity, and the effect of abiotic factors such as temperature, DO, redox, and pH.
Upper Klamath Lake (UKL) and Agency Lake (AL) in southern Oregon are both hypereutrophic, in large part due to natural and anthropogenic loading of phosphorus (P), resulting in annual blooms of blue-green algae. Reduction of P loading to the lake is considered crucial to reduce the blue-green algae blooms, maintain water quality, and increase the fish populations within the lake. Restoration of fringe wetlands is one potential way to reduce external P loading to the lake. However, upon the initial period of flooding, restored wetlands have been found to also be a source of P into the lake, as a result of P resuspension due to years of soil disturbance. We adapted a mass balance model of the biological P uptake and release to examine how P wetland dynamics change over the course of a year in restored wetlands in the Upper Klamath Basin. Our analyses focused on 1) comparing the P release and sequestration processes over each season, 2) examining whether wetlands around the lakes act as a net source or sink of P to UKL, 3) investigating wetland management strategies to determine if there is any one that is most successful at sequestering P, and 4) if release and sequestration of P in restored wetlands contribute to the P dynamics of the broader ecosystem in the UKL. Results from this model indicate resuspension of P in the wetlands is high throughout the year, yet outflow only occurs during the first 16 days of the summer, and macrophyte uptake and sedimentation of P are most important in sequestering P. Additionally, our findings indicate that two of the modeled management strategies are successful at preventing P from reaching the lakes, and that wetlands around the lake act as a net sink of P to UKL over time. However, the reduction or termination of external loading is not likely to reduce the algal blooms in the lakes, as the amount of P recycled from the lake sediments each year far exceeds the capabilities of the current wetlands.
The Clean Water Act (CWA) requires that wetlands be protected from degradation because of their important ecological functions including maintenance of high water quality and provision of fish and wildlife habitat. However, this protection generally does not encompass riparian areasâ€"the lands bordering rivers and lakesâ€"even though they often provide the same functions as wetlands. Growing recognition of the similarities in wetland and riparian area functioning and the differences in their legal protection led the NRC in 1999 to undertake a study of riparian areas, which has culminated in Riparian Areas: Functioning and Strategies for Management. The report is intended to heighten awareness of riparian areas commensurate with their ecological and societal values. The primary conclusion is that, because riparian areas perform a disproportionate number of biological and physical functions on a unit area basis, restoration of riparian functions along America's waterbodies should be a national goal.