Summer 2022 Algal Bloom in SF Bay (Updated 10/5/22)

A harmful algae bloom event occurred in San Francisco Bay in late July. Discolored waters in the Oakland estuary served as an early indication of the event, and the algae species was identified as Heterosigma akashiwo. The bloom subsequently spread throughout South Bay (between Bay Bridge and Dumbarton Bridge), and also extended into other regions of the Bay. The bloom peaked in late August, and by mid-September, the bloom had dissipated.  

Heterosigma akashiwo has been observed intermittently in various locations around the Bay over the past 20 years, after first being detected here in 2002. Heterosigma akashiwo can be toxic to fish, although the nature of that toxicity is not well understood.  While toxicity to humans or pets has not been documented in the scientific literature, some local jurisdictions are warning residents to avoid water contact on a precautionary basis.

Local scientists from the San Francisco Estuary Institute, USGS, and UC Santa Cruz have actively monitored water quality related to the bloom. Within the bloom, very high concentrations of the algae were detected. By August 31, the algae levels in South Bay had decreased substantially (compared to the prior week), and were accompanied by substantial decreases in dissolved oxygen levels. As algae stop growing, they begin to biodegrade, a process which consumes oxygen.  A significant fish die-off was also observed in multiple locations around the Bay. The science teams are investigating the causes of the fish mortality.  Monitoring work is continuing, and the science teams will provide periodic updates to regulators and other stakeholders. 

Nutrients are one contributor to algal blooms, but the trigger for this specific event is not known. Although we don’t fully understand the causes of blooms like this, public wastewater utilities in the Bay Area are funding ongoing scientific studies to better characterize how specific conditions, such as weather and hydrologic patterns, climate change, nutrient concentrations, and other factors, can initiate and sustain algal blooms. We are also planning and implementing projects to reduce nutrients discharged to the San Francisco Bay, which may lessen the extent of these events in the future.