Green scum floats in the water at Silverwood Lake in San Bernardino County on Sunday, June 19, 2016.

An 'algae world'

The bacteria are some of the oldest microorganisms on the planet. While not all species produce dangerous toxins, experts say the numbers of toxin-producing bacteria are rising.

Drought and climate change intensify the problem — cyanobacteria grow thicker and faster when lake levels are low, temperatures are high and water is stagnant. At the same time, increasing agricultural runoff and sediment cycling provide more nutrients for the bacteria to feed on and grow into the large, paintlike blooms they’re known for.

“We now live in an algae world,” said Carolyn Jones, a spokeswoman for the East Bay Regional Park District, which began detecting cyanotoxins in 2014.

Last year, the park district closed five lakes after detecting high levels of microcystin, the most common cyanotoxin found in the United States.

• Photo Gallery: Toxic blue-green algae at Silverwood, Pyramid Lakes

While there were no reports of people getting sick from swimming in the lakes or from eating fish caught there, park officials believe at least three dogs died and others fell ill after being exposed to microcystin in the water.

Blooms typically appear in the U.S. during the warmer months from late May through October. But they also appear at other times of the year at toxin levels high enough to send swimmers to the hospital.

Algae-infested waters with enough toxins to give people gastrointestinal symptoms, neurological problems and other adverse health effects have been detected all over California, studies show.

“No one expects the algae to go away anytime soon,” Jones said. “We expect it to get worse.”