California’s Algae Bloom Kills Tens of Thousands of Fish as it Expands North

by Lauren Boisvert
(Image Credit: Artem Hvozdkov/Getty Images)

A deadly algal bloom cropped up in San Francisco Bay in July of this year, and it’s been wreaking havoc on California’s marine ecosystem since then. San Francisco Bay is 60 miles from north to south, and these algae took over. In mid-September, the bloom expanded into San Pablo Bay to the north and Napa County shores and continues to persist into October. According to Emily Harwitz for High Country News, it is the “largest and longest-lasting algae bloom in the bay’s recorded history.”

More than growing rampant in the bay, the algae are killing off tens of thousands of fish as it moves north. Much more than just your average fish, though. The species that are dying off are varied and vulnerable: “anchovies, bat rays, striped bass, leopard sharks, bottom-dwelling worms and mollusks,” says Harwitz, including white sturgeon that have been living in the bay for decades. These many species are washing up on the shores of the bay, yes, but countless others are sinking to the bottom, never to be counted.

The bloom is a strain of Heterosigma akashiwo, microscopic, mobile algae that create toxic blooms. “Akashiwo” is Japanese for “red tide.” Some algae are helpful, like this strain in Seattle. But blue-green algae are always toxic. When the toxins in the bloom kill the fish, the algae then decompose the bodies, essentially sucking the oxygen out of the water. Then, more fish die from a lack of oxygen.

“It’s like a wildfire in the water,” said senior scientist at the San Francisco Baykeeper Jon Rosenfield. “Once this got to a certain stage, there was really nowhere for [the fish] to swim to.”

Fish Dying By the Tens of Thousands In San Francisco Bay as Algae Bloom Expands Unrelenting

The bloom is also affecting Lake Merritt, a lagoon in San Francisco Bay, that was once plentiful and crucial to Indigenous life. At the start of the 19th century and the turn of the 20th it was used as a dump, but by the 1980s the city began to clean it up, rerouting sewage and wastewater out of the lagoon. The water has slowly started to heal, but now this algal bloom is causing chaos in Lake Merritt again.

A biologist with the U.S. Geological Survey’s California Water Science Center, Keith Bouma-Gregson, told High Country News that he was taking water samples from Lake Merritt when he spotted several dead sturgeon, one of the oldest species of fish dating back 200 million years. “That’s like going out to the forest and seeing your old-growth long-lived species [like redwoods] getting hit pretty hard,” said Bouma-Gregson. “That was a real sobering moment of recognizing that this bloom truly was harmful.”

‘Wildfire in the Water’: How Algae Blooms Start, and What to Do About Them

Jon Rosenfield continued his analogy of the wildfire, saying, “The spark is analogous to the cigarette that starts the big wildfire. The conditions were always here to start this algal bloom.”

The NOAA reports several different causes of algal blooms: “runoff from agriculture, dissolved chemicals introduced into water supplies via rainfall or irrigation, and effluent from sewage treatment plants.” These incidents, coupled with humid and hot conditions, create the excess nutrients in the waterways that toxic algal blooms feed on. As for what people are doing about it, well, there’s not much to do in the short-term other than clean up what the algae bloom leaves behind.

Restoring water quality and marshes in the area could help keep excess nutrients out of the water, though. The NOAA also has resources for communities to create research and monitoring systems for harmful algae blooms.