Statewide Intensification of Harmful Algal Blooms Across California Lakes and Reservoirs

Monitoring cyanobacteria is crucial for assessing water quality, safeguarding public health, and understanding ecosystem dynamics impacted by harmful algal blooms. This study explores the potential of satellite remote sensing (SRS) to assess risks of cyanotoxin exposure in California's recreational waters from 2002 to 2011 and 2016 to 2023. Utilizing SRS data, we compared cyanobacteria abundance across five lakes with cyanotoxin data and advisories from the California Department of Water Resources (DWR). SRS-based advisories were aligned with DWR/in situ based advisories 54%-100% of the time. Lake-specific assessments of agreement showed Lake Oroville with the highest overall accuracy (100%) and Pyramid Lake with the lowest (54%). SRS generally overpredicted DWR-based alerts in about 30% of instances and under-detected DWR-based alerts at a rate of 42%, likely due to differences in the way satellites sample across continuous spatial domains but at coarse resolutions versus in situ sampling at discrete locations. We extended our SRS monitoring capability to an additional 71 lakes to conduct a statewide assessment of toxin alerts over time. There were 10 lakes that experienced cyanobacteria alerts 12%-88% of the time across our study. When comparing 2002 to 2011 and 2016 to 2023, we observed higher rates of toxin alert frequency, duration, and a shift toward earlier starts of the year for high-risk blooms across all regions of California, with the greatest in southern California. Despite limitations in spatial resolution, SRS provides consistent, near-real-time data essential for timely cyanotoxin risk assessments and public health alerts, complementing traditional in situ sampling.