The San Luis Reservoir: A Vital Resource with a Hidden Threat
The San Luis Reservoir, located in Merced County, stands as one of California’s most significant artificial lakes. It serves multiple critical functions, including irrigation for the Central Valley’s farmland, providing drinking water to regions like Silicon Valley and the South Bay, and offering recreational opportunities such as swimming, boating, and fishing throughout the year.
Despite its importance, the reservoir has faced a recurring challenge: harmful algal blooms (HABs). These algae form dense, forest-green mats that can cover large sections of the lake’s surface. While they may appear harmless at first glance, these blooms can have serious consequences. They can contaminate water supplies, harm aquatic ecosystems, and pose health risks to humans and animals alike. In recent years, HABs have even been linked to the deaths of several pet dogs in California.
Monitoring Challenges and New Approaches
Traditionally, monitoring HABs has relied on collecting water samples from the shoreline. However, this method only provides limited insights into the broader patterns of algal growth across the entire lake. Brittany Barreto Martinez, who earned her graduate degree in environmental systems at the University of California, Merced, recognized the need for a more comprehensive approach.
During a sampling trip on Labor Day weekend, she observed how people used the lake for recreation despite the visible presence of algae. This experience inspired her to explore alternative methods for tracking algal blooms beyond the shoreline.
Tracking Blooms from Above
Algal blooms typically occur when water bodies become enriched with nutrients such as phosphorus and nitrogen. These conditions can lead to the growth of harmful species like cyanobacteria, which produce toxins that can cause illness when ingested. Runoff from agricultural practices, sewage, and wildfires can contribute to these nutrient imbalances, but the exact impact of human activities on HAB formation remains difficult to measure due to the limitations of traditional sampling techniques.
Barreto Martinez turned to satellite remote sensing as a potential solution. Working with her doctoral advisor, Erin Hestir, and other experts, she explored the use of satellites to detect HABs across the entire surface of the San Luis Reservoir.
Sensor Sensitivity and Data Accuracy
To determine if existing satellite technology could be effective, Barreto Martinez analyzed the spectral signatures of different materials. Satellites capture data based on how they absorb and reflect electromagnetic radiation, particularly visible light. However, the sensitivity and resolution of these sensors vary depending on their design and purpose.
Land-observing satellites offer high-resolution images but lack the spectral sensitivity needed to detect water-based features effectively. Ocean-observing satellites, while better suited for detecting large bodies of water, provide lower-resolution images. Barreto Martinez found that two specific satellites—Sentinel 2 and Sentinel 3—could offer a balanced solution.
Bridging the Data Gap
Sentinel 3 was able to identify the presence of cyanobacteria and chlorophyll, which serve as reliable indicators of harmful algal blooms. Sentinel 2, with its higher resolution, provided detailed information about the density of the blooms across different parts of the lake. By comparing satellite data with ground-based samples collected by the California Department of Water Resources (DWR), the team found strong agreement between the two methods.
In a 2024 study published in the journal GeoHealth, the research showed that Sentinel 2 matched ground samples in over 79% of cases, while Sentinel 3 achieved an 83% match. This demonstrated the potential of satellite remote sensing to accurately monitor HABs, helping agencies like the DWR issue timely public health alerts.
Expanding the Research
Barreto Martinez’s work also revealed an alarming trend: an increase in toxic HAB alert days across California’s lakes over the past 15 years, especially in Southern California. This rise coincided with a surge in severe wildfires, raising questions about the connection between fire activity and algal growth.
Erin Hestir, part of a NASA-funded project called KelpFire, is investigating how wildfire runoff affects coastal ecosystems. Her research suggests that burned areas can lead to increased sediment and nutrient runoff, potentially contributing to algal blooms in nearby water bodies.
Combining Technology and Human Expertise
While satellite data offers valuable insights, on-site sampling will continue to play a crucial role in monitoring water quality parameters that cannot be detected remotely. Barreto Martinez emphasizes that remote sensing complements, rather than replaces, traditional methods.
By integrating accessible and reliable satellite data with ground-based observations, researchers can enhance their understanding of HABs and improve public safety. For Barreto Martinez, the opportunity to connect advanced technology with real-world applications has been deeply rewarding.
Her work highlights the growing collaboration between academic institutions, government agencies, and space organizations like NASA. As new tools and techniques emerge, the ability to monitor and manage harmful algal blooms will become increasingly precise, ensuring safer water resources for communities across California.