Blue-green algae has been confirmed to be present in the Loch of Cliff in Unst.
The council were alerted to the problem by the Scottish Environment Protection Agency and warning signs are set to be erected beside the loch.
People are being advised that that contact with algal scum should be avoided and it isn’t safe for animals to drink or enter the water.
Parents with young children and dog owners are advised to be cautious around any bodies of fresh water which may present a toxic risk if blue-green algae is present.
Blue-green algae exist in fresh waters and can collect to form ‘blooms’ looking like blue-green paint, in the water or on the shore line.
This form of algae can be toxic and may give rise to adverse medical effects, including skin rashes, eye irritations, vomiting and diarrhoea, fever and pains in muscles and joints.
Toxic algae has proven fatal in the past with the deaths of livestock and dogs.