Dundee Council bosses have issued an urgent warning for visitors to Clatto Reservoir.
It comes after a potentially deadly blue-green algae bloom was spotted in the water.
The plant is made up of tiny organisms which develop naturally in lochs, ponds, reservoirs, rivers and in the sea.
They are a common seasonal occurrence and waters which have been affected by agricultural, domestic or industrial discharges are most at risk of developing the algae.
The algae can multiply during the summer months and discolour the water which then appears green, blue-green or greenish brown and, occasionally they clump together to form a scum on the surface of the water.
At the shoreline, algal crusts may appear brown to almost black in colour.
Contact with algal scum or water close to it can cause skin rashes.
More serious health effects can include stomach upsets, eye irritations and pain in muscles and joints.
These symptoms are usually mild, but in some cases can be severe.
The risk to small animals like dogs is significant over the summer months as they tend to drink more water in the heat and may eat shoreline algal crusts.
Dog owners should keep an eye on their pets, especially if they come in to contact with water which could be affected.
If anyone has been in contact with the water and experiences health problems they should contact their GP or NHS 24 on 111.
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