Blue-green algae are not algae at all, but types of bacteria called cyanobacteria. They often begin to affect a dog almost immediately. Symptoms usually develop within 30 to 60 minutes. Many dogs get sick on the way home usually with vomiting and diarrhea.
Note: The National Centers for Environmental Information provides an online Harmful Algae BoomS Observing System that maps harmful algae species including cyanobacteria <Link>.
Source: Whole Dog Journal, April 19, 2023. Link.