San Diego gets a Climate Change Enhanced "The Birds" Algae Bloom with Diatoms making a Toxin
In 1961, residents of Capitola, California, woke to a surreal and terrifying sight: seabirds slamming into homes, convulsing, and dying en masse. The event inspired Alfred Hitchcock’s classic film The Birds. Decades later, researchers uncovered the likely cause: domoic acid poisoning triggered by a bloom of toxic algae.
Today, the same phenomenon is reappearing with increasing frequency, most recently in San Diego County, where high levels of domoic acid are affecting marine life, prompting shellfish warnings, and alarming coastal scientists.
At the center of this issue is a group of algae known as diatoms, specifically the genus Pseudo-nitzschia.
What Is Pseudo-nitzschia?
Diatoms are single-celled algae encased in intricate silica shells. They are photosynthetic and form the base of many marine food webs. Pseudo-nitzschia is a genus of diatoms that, under certain environmental conditions, produces domoic acid — a potent neurotoxin that bioaccumulates in fish and shellfish and causes amnesic shellfish poisoning in humans and neurological damage in marine mammals and seabirds.
Unlike most diatoms, certain species of Pseudo-nitzschia respond to nutrient and temperature shifts by ramping up domoic acid production. Their blooms are often invisible to the naked eye but devastating in ecological effect.
Recent San Diego Bloom Events
In March 2025, SeaWorld San Diego reported rescuing over 20 sea lions and multiple seabirds suffering from domoic acid poisoning. Around the same time, eight dolphins were found dead along the coast, and the California Department of Public Health issued a shellfish consumption advisory for San Diego County due to elevated domoic acid levels.
Marine mammals displayed signs of severe neurological distress: disorientation, seizures, and beaching behavior. According to NOAA and UC researchers, these symptoms match those historically tied to Pseudo-nitzschia blooms.
“This is a classic domoic acid outbreak,” said one NOAA spokesperson. “We’re seeing it more often, and it’s arriving earlier in the year.”
The Climate Connection
The rise in toxic Pseudo-nitzschia blooms is not coincidental. It’s consistent with climate-driven changes in ocean conditions. Warmer sea surface temperatures, ocean stratification, and shifts in upwelling patterns all contribute to bloom frequency and toxicity.
Here’s how:
-
Warming oceans favor Pseudo-nitzschia growth over other plankton.
-
Stratified water columns allow surface diatoms to stay in nutrient-rich layers longer.
-
Nutrient input from runoff adds nitrogen that some species convert into toxins.
-
Ocean acidification may also increase domoic acid toxicity (still under study).
Multiple models suggest that Pseudo-nitzschia outbreaks will continue to increase in frequency and expand in geographic range as ocean conditions become more unstable.
Why This Matters
Domoic acid doesn’t just affect marine animals — it poses a threat to human health and fisheries. Shellfish closures disrupt coastal economies. Marine mammal strandings overwhelm rescue groups. Ecologically, these events alter predator-prey relationships and destabilize food webs.
And unlike “red tides” caused by dinoflagellates, Pseudo-nitzschia blooms are often colorless and difficult to detect until symptoms emerge in top predators.
Conclusion
The algae likely behind the 1961 bird incident — and many more events since — are no longer a mystery. Pseudo-nitzschia, once an obscure diatom genus, is now a marker of how climate change is reshaping marine ecosystems.
For scientists, educators, and the public, understanding and monitoring these algae is critical. At Algae Research and Supply, we remain committed to advancing knowledge and supporting solutions that connect algae science to global sustainability.
