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. According to Oregon Department of Environmental Quality’s (ODEQ) Total Maximum Daily Load (TMDL) analysis, elevated P levels drive severe algal blooms causing pH and dissolved oxygen to often reach toxic levels for fish in the lakes. Restoring historic lake-fringe wetlands to provide P sinks around the lakes is accepted as a favorable means of reducing lake P levels and loading, and substantial portions of the lake margins have been returned to wetlands in an effort to recreate endangered fish habitat and improve water quality of the lakes. However, it is unclear how phosphorus is stored and released in these wetlands, with differences in hydrologic management strategies creating varied wetting and drying cycles in the soil. The role of decomposition, activated by these wetting and drying cycles, can be important in the release of P into surface water. Understanding how hydrologic management of restored wetlands affects P release or sequestration may minimize external P loading to the lakes.
Wetland restoration sites around UKL and Agency Lake are characterized by one of three different hydrologic management methods:
- Active management through mechanical pumping of an unconnected wetland,
- Passive management with direct hydrologic connection, and
- Passive management without direct hydrologic connection.
These three 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. Through the analysis of P and numerous other relevant properties of the soil prior to, immediately following, and long after flooding, patterns and processes associated with P release in four restored wetlands can be evaluated.
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:
- Timing (temperature) of inundation affects the concentrations and forms of P released in study wetlands,
- The nature of P dynamics in study wetlands releases primarily orthophosphorus, as opposed to organic P,
- Abiotic factors including dissolved oxygen, pH, redox, organic matter, and bulk density levels influence P release among temperature treatments, and
- Soil P fractions change over time with temperature and flooding regimes.
In addition, we are currently investigating the overall phosphorus budget of the Upper Klamath Lake. In particular, understanding the link between external phosphorus loading and internal loading from lake sediment is crucial in order to determine the best management strategies to reduce the phosphorus contribution to the lake.
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