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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.
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.
Despite advances in modeling, such as graphical user interfaces, the use of GIS layers, and databases for developing input files, the approaches to modeling phosphorus (P) have not changed since their initial development in the 1980s. Current understanding of P processes has evolved and this new information needs to be incorporated into the current
Wetland restoration has numerous potential ecological and societal benefits, one of which is the retention of phosphorus (P) and consequent protection of downstream water bodies from eutrophication. Past studies focused on influents to and effluents from a variety of wetland types have documented net P retention. However, some wetland systems are less effective at P capture and wetland P retention capacity can change over time. Certain wetland types - especially riparian wetlands restored on former agricultural land - remain understudied. In Vermont, most of the over 4000 potential wetland restoration sites in the Lake Champlain Basin are located on current or former agricultural fields, and little information is available to inform estimates of net P retention (i.e., P balances) for such sites. In this dissertation, I examined various factors affecting P balances in riparian wetlands restored on historically farmed soils of Vermont. P balance in a riparian wetland is largely a function of particulate P capture (e.g., deposition of particle-attached P during floods) and soluble reactive P (SRP) loss (e.g., release of SRP from soils). In Chapter 1, I determined the threshold in P saturation ratio (PSR) for riparian soils in Vermont, enabling calculation of a soil P storage capacity (SPSC) metric. I then quantified soil SRP release using intact soil core incubations with simulated floods for sites ranging from active farms to mature wetlands and confirmed that PSR, SPSC, and other soil parameters were strong predictors of SRP loss during inundation. In Chapter 2, I monitored P dynamics in soil, water, and vegetation at three restored riparian wetlands on former agricultural land in the Lake Champlain Basin, focusing on factors that affect P deposition and SRP release. At wetland sampling plots, observed inorganic sediment gain and decreased water column total suspended solids concentrations relative to the river/inflow indicated that wetlands were effectively trapping particles. Accretion of inorganic P (i.e., best estimate for mineral P deposited during floods) ranged from 0.1 to 1 g P m-2 yr-1 depending on site and elevation. Elevated SRP concentrations in wetland water columns relative to the river sources indicated internal SRP release from soils, and high frequency data indicated that factors such as temperature, dissolved oxygen, and primary production likely influence SRP dynamics. In Chapter 3, I developed a wetland P dynamics model that can generate estimates of net P retention from a simple set of soil and hydrologic inputs, considering both P deposition and SRP release. For proof of concept, I simulated the wetlands monitored in Chapter 2 using two years of monitoring data and a set of model scenarios. I found that net total P balance was typically positive (-0.04 to 0.24 g P m-2 yr-1), with average P retention efficiency of ~40%, though there was substantial variability depending on site and scenario. P retention efficiency was especially sensitive to changes in influent P and total suspended solids concentrations, with the greatest net P retention predicted for systems receiving influent floodwater with high P concentrations. Reduction of influent SRP concentrations promoted SRP release from soils, suggesting that legacy soil P in the wetlands might cause a time lag between the adoption of upstream best management practices and reduction in downstream SRP concentrations. In the future, the model developed in Chapter 3 can be applied more broadly to investigate the potential P retention benefits of wetland restoration at candidate sites across Vermont. Together, the information put forth by this dissertation provides a suite of data and tools that researchers and managers can use to enhance the P retention benefits of riparian wetland restoration.