Algae blooms are places where algae has greatly overgrown in a short period of time. They’re dangerous because they create “dead zones” in the water by blocking out sunlight and consuming too much oxygen for any other underwater organisms to survive. These are harmful to drinking water and to dogs, because dogs who ingest it can be fatally poisoned by certain types of algae (i.e. blue-green algae). So, if your dog likes to play in the water, be careful and make sure to keep water sources clean!