People are being urged not to let their pets swim in lakes on Southampton Common.
The boating lake has been fenced off after concerns about health risks from an algae bloom.
Residents are also being asked not to feed the ducks, as crumbs can encourage algae growth.
The council says the fences will remain in place until heavy rain can flush away any toxins.
A sign in place reads: 'Following a large algal bloom, this temporary fence has been put in place whilst the algal bloom disperses.'
According to the Blue Cross, algae can produce harmful toxins which stop a dog’s liver from functioning properly.
They add that exposure to toxic blue-green algae is often fatal, and 'can also cause long term health problems in dogs that survive after drinking or swimming in algae-contaminated water'.
A spokesperson for Southampton City Council said: “We are continuing to monitor the boating lake on Southampton Common. Temporary fencing has been installed around the area to prevent people and animals from coming into contact with the water.
“This will remain until the algae has broken down and there has been some heavy rain to flush away any toxins. To reduce the likelihood of algal blooms in the future, we are asking people to restrict duck feeding to the Cemetery Lake only and to not allow dogs to swim in any of the lakes on the Common.
"Cemetery Lake has a specific duck feeding area. We also ask for you to please not throw bread for the birds as uneaten bread and crumbs can encourage algae to grow.”