Investigations are underway after a suspected blue green algae was discovered at the Spiggie Loch.
Samples have been taken for testing, however the council’s environmental health department have advised people to avoid the algal scum evident at the water’s edge.
Notices are being posted in shops in the area and at various points close to the loch. Adjoining landowners and those with fishing interests are also being told of the situation.
The Scottish Environment Protection Agency (Sepa) and Shetland NHS board have also been notified, although there has so far been no adverse effect on water supplies.
Blue green algae exist in fresh waters and are noticed when their concentrations increase to form blooms.
Possible effects of people coming into contact with toxic scums include skin rashes, eye irritations, vomiting and diarrhoea, as well as fever and pains in muscles and joints.
In the past toxic algae have also caused deaths among livestock and dogs.
The treatment of water supplies removes blue green algae and additional treatment may be applied to destroy or remove toxins should they arise. The behaviour of algae is erratic, and the level of its toxicity can fluctuate.