UC Riverside

ABSTRACT OF THE THESIS

Stable Isotopic Composition and Phosphorus Concentration in Sediment From Two Southern California Lakes

by

Simone Claire Boudreau

Master of Science, Graduate Program in Environmental Sciences

University of California, Riverside, December 2015

Dr. Michael A. Anderson, Chairperson

Excessive nutrient loading to lakes reduces water clarity, degrades biological habitat, limits light penetration, and often results in anoxia. Increased watershed urbanization is a major contributing factor of externally loaded nutrients. But even in lakes that have successfully reduced external nutrient loading rates, high nutrient concentrations may persist due to internal nutrient recycling within lake sediments. Internal phosphorus (P) cycling dynamics are a balance between sedimentation, which depends on the mineralization rate of settling organic matter; diagenetic processes within the sediment after deposition; and remobilization of P after deposition, which depends on mobile P content and elemental composition of the sediment. The interplay between these processes determines whether phosphorus is recycled or sequestered in the sediment, as well as how long it remains bioavailable. Measurement of the stable isotopes of carbon and nitrogen in sediment have enabled assessment of the sources and degree of cycling of organic matter, which is important in controlling P

burial and release. In this study, stable isotopic composition, elemental composition, and phosphorus forms in the sediment from two southern California lakes that are impaired due to nutrients are assessed to determine how dynamic hydrologic conditions and management actions have influenced water and sediment quality in order to gain insight into how sediment properties and biogeochemical processes respond to varying conditions in each lake. The results of the study provide a comprehensive understanding of the sedimentological processes occurring in the lakes, which is imperative to accurately predict eutrophication trajectories and the impact that future events will have on water quality in order to develop the most effective nutrient mitigation strategies.