Mass mortality of aquatic organisms in the San Francisco Estuary – using community science to document the impacts of a severe harmful algal bloom

Abstract

In July 2022, a harmful algal bloom of Heterosigma akashiwo originated in a man-made lagoon and expanded to more than 4,500 km² of open-water habitat in the San Francisco Estuary, triggering a widespread mass mortality event involving bony fishes, sharks, rays, crustaceans, mollusks, polychaetes, and jellyfish. Notably, the event affected the southernmost spawning populations of White Sturgeon ( Acipenser transmontanus ) and federally threatened Green Sturgeon ( Acipenser medirostris ). Because the bloom arose rapidly and without precedent, coordinated response efforts by resource managers began only after the event was well underway, limiting documentation of its early impacts. However, members of the public and local conservation groups submitted numerous observations through platforms such as iNaturalist, non-profit reporting portals, and agency hotlines. To reconstruct the progression and consequences of the HAB, we synthesized information from these community-science platforms together with available agency observations. In total, 401 distinct community reports and 14 scientific reports documented mortality spanning 25 families, including 18 identified fish species. A total of 691 sturgeon carcasses were reported, many of which were mature White Sturgeon. Although severe hypoxia developed for five days near the end of the bloom, most mortality occurred prior to this period, indicating that toxic effects of H. akashiwo were likely the primary driver of sturgeon deaths. Our findings underscore the value of community engagement, community science, and social-media reporting for documenting stochastic disturbances such as harmful algal-bloom-induced mass mortality events, and highlight the need for improved planning, monitoring, and rapid response capacity among local, state, and federal agencies to address future harmful algal-bloom-induced mortality events.

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