From the USGS:
A new study by several scientists examines the environmental conditions that fueled a massive algal bloom in the San Francisco Bay. By analyzing water quality, circulation, and microscopic communities, the research helps explain how rare combinations of factors can drive major ecological events, including blooms of Heterosigma akashiwo, a species first identified in Japan.
Tracking the Bloom Across the Bay
Algal blooms are not unusual in the San Francisco Bay, where nutrient levels are high and seasonal conditions regularly support biological growth. Most blooms, however, remain relatively small. To understand why one particular bloom became so extreme, scientists from the U.S. Geological Survey California Water Science Center examined conditions from South Bay through Central Bay and into San Pablo Bay.
Using years of monitoring data, researchers looked at how water movement, temperature, and water quality aligned during the event, as well as how microscopic communities shifted as the bloom developed. During this bloom, measured chlorophyll levels, a common indicator of algal abundance, were more than 40 times higher than what is typically seen in the Bay, leading to low-oxygen conditions and widespread fish die-offs.
A similar algal bloom occurred again the following summer, but at lower intensity and with fewer ecological impacts. By closely examining the extreme 2022 event, the study shows how routine blooms can escalate when multiple environmental factors line up at the same time. Understanding those rare combinations helps scientists better anticipate when future blooms may pose greater risks to Bay ecosystems and nearby communities. The research was published in the scientific journal Estuaries and Coasts in December of 2025.
RESEARCH ARTICLE: Assessing Environmental Drivers and Protist Community Dynamics that Shaped the Historic August 2022 Heterosigma akashiwo Bloom in San Francisco Bay, California
Abstract
San Francisco Bay, California, typically has chlorophyll a (chl-a) concentrations below 10 µg L−1, despite nutrient loadings exceeding those in many estuaries with recurring harmful algal blooms (HABs). However, in August 2022 there was a Heterosigma akashiwo (raphidophyte) bloom with chl-a concentrations exceeding 450 µg L−1, resulting in widespread hypoxia and fish die-off. We used protist community (18S ribosomal ribonucleic acid gene metabarcoding) and environmental data collected 2–3 times per month from 2015–2023 to assess differences between 2022 and other years, and capture dynamics pre-, during, and post-bloom. The 2022 protist community clustered separately from other years in non-metric multidimensional scaling and hierarchical cluster analyses. A diatom bloom in June generated the highest median June chl-a concentration (15 µg L−1) recorded between 1977–2023 and drew down nutrients (nitrogen, silicate), leading to the establishment of a heterotrophic community in July with low dinoflagellate abundance (0.9% of protists versus 5.9% average in other years). The absence of dinoflagellates, potentially including H. akashiwo predators, plus suitable habitat (13–14 daylight hours, temperatures 17–21˚C, nitrate concentrations 17–24 µmol L−1, neap tides) likely provided opportunity for H. akashiwo to proliferate from 0.03% to 48% of protists between July 20 and August 25. As the bloom collapsed, the genus Gyrodinium (dinoflagellate predator of H. akashiwo) increased from 3.0% to 52% of protists between August 25 and August 31, likely exerting grazing pressure. Results show that antecedent protist community dynamics play important roles in shaping HABs, and interactions between biotic and abiotic factors govern the development of blooms.
Click here to read this open access article at Springer Link.
