Park officials shut down swimming access to Oakland's popular Lake Temescal Tuesday and warned dog walkers not to allow their pets to drink the algae-filled water. Visitors to the popular park were greeted with warning signs and yellow caution tape draped at several access points around the lake. A naturally occurring bloom of blue-green algae is growing in the lake. Officials said the algae turns toxic in the water and can cause nausea, vomiting and gastrointestinal problems in humans. In dogs, the ingesting the algae can be deadly. Rebecca Fajola was walking her dog on Tuesday and making sure her pet didn't wander close to the water. "I have to yank him back because he wants to drink the water," she said. This is the second summer officials have blocked access to the water because of the algae bloom. Last year, the lake which attracts families and pet owners was closed for much of the swimming season. It is the second regional park swim lake to be closed this year due to blue-green algae. The swim area at Quarry Lakes Regional Recreation Area in Fremont was closed Feb. 8 because of the algae.