Forfar Loch and Backwater Reservoir are among the local water courses being monitored for blue-green algae.
NHS Tayside has issued a reminder for people in the area to be on the lookout for the potentially hazardous algal blooms which form naturally in lochs, ponds, reservoirs, rivers and in the sea.
They are a common seasonal occurrence and waters affected by agricultural, domestic or industrial discharges are more at risk.
In still waters, the algae can multiply during the warmer summer months and discolour the water which then appears green, blue-green or greenish brown. At the shoreline, algal crusts may appear brown to almost black in colour.
Humans and animals can suffer as a result of direct contact with affected water and the health authority is advising water sport enthusiasts, anglers and dog owners to be alert to algae as temperatures rise.
Dr Jackie Hyland, public health medicine consultant, said: “Canoeists, wind surfers and swimmers who come into contact with the algal scum or who accidentally swallow affected water can suffer from complaints such as skin rashes, eye irritation, vomiting, diarrhoea, or pains in muscles and joints, which are usually mild but in some cases can be severe.
“The risk to animals is significant over the summer months as they tend to drink more water in the heat and may eat shoreline algal crusts, so dog owners should keep an eye on their pets especially if they live, or are walking near water which could be affected. Public drinking water supplies are treated to prevent any harmful effects to health from blue-green algae.”
Backwater, Den of Ogil, Glenogil and Monikie reservoirs will all be monitored along with and Forfar Loch, Loch Lee, Rescobie Loch and Lintrathen Loch
Where monitoring reveals higher than acceptable levels of algal bloom, warning notices will be posted. Anyone who finds a water course which they suspect is affected and which is not displaying a warning sign, should contact the local environmental health service.