Toxic algae found in waterways throughout California has reached record levels this year and was detected in San Luis Obispo's Laguna Lake in late September.
The Central Coast had been relatively free of this problem until now.
Samples collected from the lake, located just off Madonna Road, show levels high enough to post warning signs, based on state recommendations.
Blue green algae can make humans sick and be fatal for pets and wildlife. Laguna Lake is adjacent to a dog park.
The City of San Luis Obispo says the issue is limited to water contact at Laguna Lake and does not have an impact on drinking supplies. All three of the City's reservoirs, including Nacimiento, Salinas, and Whale Rock are routinely monitored for the problem, according to the City.