The swan – who was known by locals as Missy – was put to sleep by the RSPCA after being found unwell on Monday in a boating lake in Southampton Common.
An RSPCA spokesperson told the Daily Echo:
Sadly, an inspector collected an adult bird, which he said he was told was mum to cygnets. She appeared extremely unwell and had leeches feeding off both her eyes and an independent vet made an instant decision to put her to sleep.
The cause of contamination in the lakes has been confirmed as blue-green algae, which is extremely toxic and harmful to animals if ingested.
The British Veterinary Association have reported that there has been more reports of the toxic blue-green algae found in lakes than in previous years.
They have advised dog walkers to stay extra vigilant if they are walking dogs in the vicinity of a contaminated lake.
To prevent animals or people from getting near the contaminated water, Southampton City Council have installed temporary fencing around the lake.
A Southampton City Council spokesperson told the Daily Echo:
We are continuing to monitor the boating lake on Southampton Common. [The fence] will remain in place until we are confident the issue has been fully resolved.