Effects of timing and duration of flooding on phosphorus release in four restored wetlands around Agency and Upper Klamath Lakes, Oregon

Abstract

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.