It is important to only let dogs swim in water that appears fresh, as algae can cause diseases in pets, a vet has said.
Pete Wedderburn, writing for the Metro, explained that algae can produce poisonous chemicals, which can cause liver failure in pets.
He had been contacted by a reader who was concerned about her Springer Spaniel swimming in lakes that contain blue-green algae.
The vet wrote: "Pets are at risk when they drink or swim in water where algal blooms have occurred: these are more likely to be concentrated on the windward side of ponds, lakes or reservoirs.
"Ensure that dogs only swim in water that appears fresh, and remember that running water is safer than stagnant pools."
He added that while the risk is very low and only a few dogs die from this every year, death can take place very quickly and treatment often does not work.
Blue-green algae is also known as Cyanobacteria. As well as pets, it can be harmful to humans and marine life.