California’s mega algae bloom is like a ‘wildfire in the water’

Lake Merritt, in the center of Oakland, California, is a tidal estuary connected to the Pacific Ocean. It usually teems with life, both human and marine. In early September, its 3-mile shoreline was bustling with joggers. But in the sunset-blackened waters, the gleaming white corpses of thousands of decaying fish bobbed along in the gentle tide and piled up in mounds along the lagoon’s edges.

In late July, an algae bloom began spreading in San Francisco Bay, which stretches 60 miles north to south. The bloom has since exploded, expanding north into San Pablo Bay, including the shores of Napa County, and conditions in mid-September were still dire, with the fish kill reaching into the tens of thousands. It is the largest and longest-lasting algae bloom in the bay’s recorded history.

Though the conditions—a combination of warm summer waters, sunlight and heavy nutrients—have been ripe for an algae bloom for decades, the scale of the resulting fish die-off exceeded scientists’ most dire models. Tens of thousands of anchovies, bat rays, striped bass, leopard sharks, bottom-dwelling worms and mollusks — even humongous, decades-old white sturgeon—are washing ashore dead while countless more are sinking to the bottom.

The devastating casualties are linked to an algae bloom of Heterosigma akashiwo. In the first stage of the bloom, H. akashiwo kills fish through some kind of toxic effect, the specifics of which are still unknown. Once the algae start to die, bacteria in the seawater get busy decomposing them, 

a process which sucks up oxygen, as confirmed by the dissolved oxygen readings taken by scientists throughout Lake Merritt and the bay in early September. And fish can’t survive in oxygen-depleted water. “It’s like a wildfire in the water,” said Jon Rosenfield, a senior scientist at the San Francisco Baykeeper, an environmental advocacy group. “Once this got to a certain stage, there was really nowhere for (the fish) to swim to.”

a process which sucks up oxygen, as confirmed by the dissolved oxygen readings taken by scientists throughout Lake Merritt and the bay in early September. And fish can’t survive in oxygen-depleted water. “It’s like a wildfire in the water,” said Jon Rosenfield, a senior scientist at the San Francisco Baykeeper, an environmental advocacy group. “Once this got to a certain stage, there was really nowhere for (the fish) to swim to.”

Lake Merritt was once a healthy estuary that provided water, food and a way of life to the Ohlone people who lived near its shores. But in the 19th century, after pushing Indigenous people out of the area, European colonists used Lake Merritt’s waters as a dumping ground for sewage and wastewater.

For decades the city still routed sewage pipes into its waters. As one historian noted, by the turn of the 20th century, Lake Merritt had become “a cesspool and a menace to public health.” It wasn’t until the 1980s that the city began infrastructure projects to clean up the lake, including rerouting the sewage pipes to wastewater treatment plants. Since then, Lake Merritt has seen a steady increase in water quality.

For decades the city still routed sewage pipes into its waters. As one historian noted, by the turn of the 20th century, Lake Merritt had become “a cesspool and a menace to public health.” It wasn’t until the 1980s that the city began infrastructure projects to clean up the lake, including rerouting the sewage pipes to wastewater treatment plants. Since then, Lake Merritt has seen a steady increase in water quality.