Toxic blue green algae, or Cyanobacteria, can bloom on ponds when the weather has been hot and humid and the water is stagnant. Blue green algae can grow on both fresh and brackish water. This toxic algae is difficult to distinguish from common, non-toxic algae without a microscope and an expert. The vast majority of algae is pretty harmless but since it is not possible to be 100% sure by sight alone that the algae is safe it is best to just avoid ponds with algae blooms. This algae can be a problem anywhere that there are hot temperatures and stagnant water but has especially been a problem on Cape Cod in recent years.
The toxic blue green algae can produce two different toxins, both of which can be fatal in small quantities. One toxin leads to rapid liver cell death and the other is a neurotoxin. Once a dog has ingested the algae signs can start very rapidly and can include vomiting, diarrhea, lethargy, tremors, drooling, paralysis, seizures, coma and death. Dogs that manage to get to the hospital very quickly while still alive will need 24 hour intensive care and still may not survive. Those that do survive may have permanent liver impairment.
If you come across a pond with algae blooms on the surface it is best to just keep your dog out of the water. People should also avoid swimming in the water. The algae is not only toxic to dogs but also humans if ingested. If you think your dog has ingested blue green algae then seek medical attention immediately.