UC Merced

The increasing frequency of cyanobacterial harmful algal blooms (cyanoHABs) in California’s inland waters poses significant risks to public health and recreational water use. Wildfires, by increasing nutrient runoff and altering water temperature and light conditions, may exacerbate the occurrence of cyanoHABs. As wildfires become more frequent and severe in California, understanding their effects on water quality is essential. This dissertation leverages satellite remote sensing (SRS) to monitor cyanoHABs, quantify their temporal and spatial trends on a large scale, and explore the role of wildfire as a driver of cyanoHABs across the entire state of California. SRS provides temporally dense and spatially explicit data, which is crucial for timely cyanotoxin risk assessments and could enhance traditional in-situ sampling methods. Chapter 1 provides a framework for evaluating the public health utility of SRS for enhancing global cyanotoxin monitoring, using San Luis Reservoir as a case study. The findings indicate that public health alerts derived from SRS and World Health Organization guidelines correspond highly with public health advisories issued by state authorities based on laboratory toxin analyses. Chapter 2 expands on these findings with four additional lakes, demonstrating that point-based data outperforms lake-wide summaries. Lake-wide approaches offer a broader perspective but often underestimate the true variability and potential hotspots within a lake. This chapter also identified lakes with persistent high advisory levels of cyanoHABs across the state (74 lakes in total). Chapter 3 investigates the relationship between wildfires and cyanoHABs across California, finding a reduction in differences between burned and unburned lakes from 2016 to 2022, along with increased bloom occurrences. Although most sites didn't show a significant post-fire increase in cyanobacteria alerts, those that did often saw increases, with some recovering by the second year. The findings from this dissertation underscore the need for more extensive studies and long-term monitoring to address the impacts of changing climate and wildfires on water quality and public health.