Blue-green algae has been spotted in Pullar’s Loch in Lerwick.
Sepa has notified the environmental health team at Shetland Island Council that the algae is present in Ness of Sound.
At this stage, there is no adverse effect on water supplies.
The blue-green algae can show up in lochs and bodies of water in Shetland, especially in warmer weather.
It has varying levels of toxicity and resembles blue-green paint.
Notices by the loch warn that all contact with the algal scum should be avoided and people are urged to exercise caution if they see these blooms.
Dogs and livestock should be kept away from the lochs to prevent contact with the algae and any potential side effects.
The public are advised not to bathe in the affected waters and fisherman should exercise caution.
Both adjoining landowners and NHS Shetland have been informed.
Anyone that comes across what they believe is blue-green algae should report sightings through the Bloomin Algae app.
The public can also report sightings directly to Sepa via their online form or by calling their Pollution Hotline on 0800 80 70 60.
Reports can also be made to the SIC environmental health team on 01595 745250 or by emailing ehadmin@shetland.gov.uk.