But their owners have been warned that the otherwise idyllic open water could prove perilous to pets. Dogs Trust notes that many cases were related to dogs – or people attempting to rescue them – needing help after entering the water. View image in fullscreen Sign alerting walkers at Earlswood to dangers of blue-green algae. Even on a picturesque summer’s day Earlswood is far from hazard-free, with a small yellow sign alerting walkers to dangers of blue-green algae. Emily Mills, veterinary projects manager at Dogs Trust, said: “Dogs can become unwell from drinking contaminated water or simply cleaning it from their fur after swimming.