Tilden Regional Park’s Lake Anza closed after reappearance of toxic algae

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A toxic blue-green algae bloom has reared its head again in Lake Anza, forcing East Bay Regional Park District officials to close the popular summer destination just as swim season comes to an end.

The Tilden Regional Park lake was shut down earlier this week, according to EBRPD spokesperson Carolyn Jones, after officials discovered unsafe levels of toxic microcystin during their regular testing. With swim season set to end next Friday, Jones said officials may simply leave the lake untreated and “let nature take its course.”

“Sometimes the algae clears up on own, sometimes it just sticks — we just have to keep a really close eye on it,” Jones said.

While the toxic algae is dangerous for most swimmers, causing skin irritation and gastrointestinal issues, it can prove fatal for dogs. According to Jones, a few dogs have died at Lake Chabot in Castro Valley, where toxic algae has also been blooming for the last couple years.

While the lake will remain off-limits, the greater Tilden Regional Park will stay open, Jones said. In addition, she said, several other Bay Area lakes are still open, including Lake Temescal in Oakland, Shadow Cliffs Lake in Pleasanton and Quarry Lakes in Fremont.

This is not the first time the district has had to close Lake Anza due to a toxic blue-green algae bloom. Last September, EBRPD closed the lake when excessive heat and dry weather fueled a blue-green algae bloom with more than twice the allowable toxin concentration. Lake Anza later reopened in February, after an increase in rainfall flushed out the toxic algae.

“Generally speaking, stagnant water and warmer water temperature cause algae to happen,” Jones said. “Algae is a natural normal phenomenon, but why it turns toxic suddenly is kind of a scientific mystery.”

The district experienced its first-ever toxic algae bloom in 2014, with officials noting then that California’s drought played a large role in the occurrence, which led to unusually low water levels and warm temperatures in the lake.

While EBRPD officials will be unable to investigate why the algae turns toxic in certain conditions, Jones said research is being conducted on the phenomenon. She added that the toxic algae has been appearing in lakes and rivers across the world.

“It’s really sad, obviously no one here likes to close a lake, especially when swim season is going on, and Lake Anza is a really beautiful popular place,” Jones said. “We’re really sorry for the inconvenience.”

Despite the inconvenience, Jones said, swimmers — and their pets — will be the safer for it.

Alexandra Yoon-Hendricks is the city news editor. Contact her at [email protected] and follow her on Twitter at @ayoonhendricks.