"Many people and water agencies have been caught off guard by the growing extent of the algae, which can cause gastrointestinal upsets, skin rashes and allergic reactions in people, and sickness or death among dogs, livestock, sea otters and other wildlife."

http://www.eastbaytimes.com/2016/09/17/toxic-algae-troubles-many-california-lakes-and-waterways/

We have had a bout of it this last week. Smells bad.

There are even suspicions that it can cause brain plaques similar to Alzheimers.