UC Davis

Lakes and reservoirs provide a wide range of socioeconomic benefits, serve as critical biodiversity hotspots, and are recognized as key sentinels of climate change. The seasonal evolution of lake thermal structure plays a central role in regulating the ecological health and productivity of these systems. During much of the year, thermal stratification inhibits the vertical exchange of oxygen, carbon, and nutrients between surface and deep layers. Conversely, when stratification weakens, complete vertical mixing (i.e., turnover) can transport nutrients to the surface for biotic uptake and ventilate deep layers with oxygen. However, climate change is altering the frequency and intensity of mixing events, threatening the trophic status of lakes globally. While one-dimensional hydrodynamic models are often utilized to investigate long-term changes in lake mixing regimes, they lack the ability to resolve both deep water mixing mechanistically and horizontal variations in temperature that can contribute to deep water renewal. Therefore, in situ temperature measurements and three-dimensional modeling serve as important tools for a comprehensive understanding of vertical mixing dynamics.This dissertation examines the drivers and impacts of vertical mixing processes in three lakes varying in depth and mixing characteristics. The first study focuses on Clear Lake, a shallow, hypereutrophic, polymictic lake in Northern California beset by recurrent cyanobacteria blooms. Extensive field observations from 2019 – 2022 demonstrate how intermittent periods of stratification and mixing induce internal phosphorus loading from sediments and alter nutrient limitation in favor of nitrogen-fixing cyanobacteria. The second study investigates Lake Tahoe, a deep, oligomictic, oligotrophic lake straddling the California-Nevada state border. Analysis of a multi-decadal record of winter mixing depth and winter thermal structure reveals that the occurrence of sporadic turnover events is primarily regulated by the serendipitous passage of cold fronts during the late winter. Temperature observations during deep mixing events indicate that three-dimensional transport processes likely contribute to deep water renewal in winter. The third study focuses on Lago Llanquihue a deep monomictic, oligotrophic lake in northern Patagonia, Chile. Utilizing a three-dimensional hydrodynamic model, the study explores the long-term response of the lake to climate change and watershed development. Predictions under a worst-case greenhouse gas emissions scenario indicate that the lake will warm more gradually than comparative Northern Hemisphere systems and will likely remain monomictic for decades, suggesting that the climate impacts on Southern Hemisphere lakes will be less severe throughout the 21st century. Collectively, these investigations significantly contribute to the understanding of how vertical mixing influences water quality in lakes and establishes a framework for predicting how such dynamics may evolve in the future.