The blue green algae blooms are extremely toxic and dangerous to dogs. A dog only needs to ingest a small amount to kill him. There isn’t really much a vet can do for the dog to save a dog who has ingested the poisonous algae. Many symptoms including seizures, weakness, collapse, confusion, drooling etc. It will destroy their liver and they will die. Not all algae blooms are toxic but you can’t tell which are toxic just by sight. Testing is necessary. I don’t mess around with his stuff. If there is algae floating on a lake, river or pond my dog doesn’t get near the water Since you can’t tell just by observation if it is toxic or not.
I had a friend years ago who was throwing a ball into a lake for his dog to fetch. There was algae there but he wasn‘t aware that algae could be toxic. His dog was dead the next day, it died on the table at the ER vet. Just be super careful . Check each body for algae before you let your dog in it.