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A toxic blue-green algae bloom is spreading in Spring Lake, where officials are warning dog owners to protect their pets by preventing them from swimming in the popular Santa Rosa lake.

The algae bloom was clearly evident Monday as a thick green scum on the top of stagnant water in a cove next to a popular trail that circles the lake, leaving a turquoise stain where it dried on the side of rocks.

"I hate to do it. I know how important this lake is to people, but their dogs are at risk," said Clayton Creager, a senior scientist with the North Coast Regional Water Quality Control Board, who recommended the lake be posted with warnings.

The levels of toxin could be potentially lethal to a dog that swam anywhere in the lake and then licked the water off its coat, Creager said.

The Sonoma County Parks Department on Monday afternoon was posting the lake as off-limits to dogs and to all wading, swimming and any water contact at the request of the water board.

"It is a health advisory to avoid contact with the water and we are doing it right now," said parks Operations Director Bert Whitaker.

The lake is already off-limits to swimmers, who could suffer rashes, eye irritation and even nausea, Creager said.

Boating is still allowed, and fishermen are warned to wash their fish thoroughly and not eat the innards.

The algae bloom does not affect the popular swim lagoon, which is chlorinated and has a liner that keeps its water separate from the lake.

Whitaker said this is the fourth year the algae has appeared.

"It comes in blooms and dissipates," Whitaker said. "It seems to be a three-week cycle."

Although it is commonly called blue-green alga, it is actually a cynobacteria that at high levels can be a health hazard.

It thrives in stagnant water and hot weather.

Creager said a water sample would be taken and tested by the Environmental Protection Agency, which could take 10 days.

Just looking at the algae, however, Creager said, he expected that bacteria levels would be at least double the allowable limit.

"It is on the toxicity level of sarin nerve gas. This is nasty stuff," Creager said.

Creager confirmed the fears of Dave and Judy Burson of Santa Rosa on Monday, who saw the green algae and were keeping their dog Scout out of the water.

"Normally she would already be wet," said Dave Burson.

The algae bloom was also seen in a cove where the Sonoma County Water Agency is working on a lake overflow structure and repairing a valve that also drains the lake.

Spring Lake was created by the Water Agency in the mid 1960s as part of the Central Sonoma County Water Project, which was meant to prevent flooding in Santa Rosa by creating a string of reservoirs as retention ponds.

The Water Agency last Friday pumped the polluted water from the cove onto an adjacent field to keep it from getting into Santa Rosa Creek. The algae cause a lack of oxygen in the water that is a problem for steelhead.

The agency has also put devices in the channel between the lake and Santa Rosa Creek to add oxygen to the water flowing into the creek, Assistant General Manager Mike Thompson said.