UC Davis

Around the world, oligotrophic lakes are changing. Once pristine shores are increasingly covered in dense mats of periphyton, known as filamentous algae blooms (FABs). Periphyton blooms occur in lakes' infrequently monitored littoral zones and thus are likely underreported and poorly understood. While some blooms have known causes, such as increased groundwater nutrient levels or an influx of invasive species, many FABs occur for unknown reasons. This dissertation examines the drivers contributing to periphytic FABs. In three chapters, Lake Tahoe, California-Nevada, USA, is selected as a case study because, unlike many lakes, Tahoe has a long history of periphyton monitoring. Overall, eulittoral, splash zone, periphyton at Lake Tahoe did not show a biomass increase from 1982 through 2019. Periphyton was measured consistently at 0.5 meters depth at multiple locations. Thus, as lake level changed over a range of 2.5 meters, monitoring recorded the biomass of either the diatom and green algae dominated community that inhabited the near-surface substrate, or the cyanobacteria dominated community that thrived at greater depths. Trend analysis showed that the stalked diatom and green algae communities did not show a statistically significant change in biomass over time. However, the cyanobacteria measurements showed a significant decline in biomass over time. The effects of temperature, lake surface elevation, bed-shear stress, mid-lake nutrients, days since the measured substrate was last exposed to air, and photosynthetically active radiation on periphyton biomass were evaluated using Bayesian modeling. The monitored and derived drivers of algae explained 32% of the variability in Lake Tahoe periphyton biomass. Temperature, nitrate, days since last exposed, and photosynthetically active radiation had significant correlations with periphyton biomass. The model could be improved by adding additional relevant data, such as more localized nutrient information. Looking toward the future, climate change model results were analyzed to project periphyton community and biomass shifts over time in Lake Tahoe. Local climate change conditions will affect many of the drivers of periphyton, including warmer lake temperatures, more nutrient inflows, and greater variability in lake levels. Combining the many changes, two potential outcomes are offered. In one scenario, diatoms continue to dominate the euphotic zone of periphyton, but biomass increases. In the other scenario, an increase in nutrients and temperature shifts the lake towards a green algae dominated euphotic zone. In both scenarios, cyanobacteria are likely to remain the dominant periphyton in the sublittoral zone. These findings suggest that in addition to periphyton drivers, periphyton biomass and community composition should be continually monitored to understand potential change. Despite the need for more ecohydrological information, few studies analyze groundwater and periphyton. Even fewer programs consistently monitor groundwater and periphyton together. Barriers to studying this transition zone area are created by underlying geology, intra-lake and cross-lake landscape processes, benthic biological systems, and data gaps. Trusted and new periphyton and groundwater monitoring methods are reviewed, and recommendations are proposed for integrating these methods. Using these combined methods and the combined knowledge of interdisciplinary experties, we can further hydrology and ecology. The chapters of this dissertation provide rationale and methodologies for increased understanding of FABs. Ideas for further future research are provided in the conclusion section. This dissertation has been formatted so that the chapters standalone as individual manuscripts. Each chapter has its abstract and references within the chapter.