An outbreak of potentially toxic blue-green algae has led to the closure of a boating lake in Southampton.
Visitors to Southampton Common are being advised not to go in the water and pet owners have been urged to keep animals away from the lake.
Consumption can cause vomiting, diarrhoea and breathing problems.
The RSPCA said one of a pair of cygnets found unwell at the lake had died and the second one was under specialist care.
A spokeswoman for the charity said: "We cannot be sure what caused the death of this cygnet as the parent birds seem in good health at the moment.
"The second cygnet also appears to be in good health but is now being cared for by a specialist."
Warning signs have been put up at the lake and specialist equipment is being used to get oxygen into the water in a bid to improve the lake's quality.
A Southampton City Council spokeswoman said: "We are in talks with the Environment Agency and our ecology team to look at a safe way of dealing with the bloom.
"We will continue to monitor all birds and fish on the site while the issue is being resolved."
The Environment Agency said water samples had been sent to a laboratory for analysis to confirm the type of algae.