Oakland's Lake Temescal will be closed to swimmers, waders and cannonball divers for the foreseeable future after a scuzzy layer of toxic blue-green algae has enveloped the lake, park officials said.

While blooms of algae are common at the popular North Oakland swimming hole, the decomposing algae in this particular bloom are producing a toxin called microcystin that can be harmful to humans, said Hal MacLean, water management supervisor for the East Bay Regional Park District.

"This one is different," MacLean said. "This one actually has a measurable level of toxin, so we had to close the lake. But is kind of weird, because they don't always produce it, so you can't know when it is going to happen."

Scientists aren't exactly sure what caused the algae to become toxic, but MacLean speculated that the drought may have had an impact.

MacLean said park authorities closed the lake to swimmers last week after discovering worrisome amounts of microcystin in the water.

The lake, popular in the kind of warm temperatures expected in Oakland over the next few days, will stay closed while scientists decide how to remove the algae, MacLean said.

"The blue-green algae, when it dies, creates a scum, and the scum can release the toxin as well," MacLean said.

Meanwhile the park system has had to move the summer Junior Lifeguard classes to other locations, or teach first aid on dry land, MacLean said.

"It is a big impact because these kids come for the water, and we can't provide that," MacLean said. But "it is not very good if you cool off and then get sick."
