Residents who live on or near Baptiste Lake, or visitors to the lake, are warned to not drink the water, nor let pets drink the water.
Health officials also say people and pets should also not swim or wade in the lake, which is 167 kilometres north of Edmonton.
Any fish caught in the lake should not be eaten and people should also avoid contact with blue-green algae along the shoreline.
The colourful algae produce a poison that can cause serious illness to animals or humans who drink or have skin contact with water containing this toxin.
People who come in contact with or ingest water containing the blue-green algae may experience skin irritation, rash, sore throat, sore red eyes, swollen lips, fever, nausea and vomiting and/or diarrhea.
Symptoms usually appear within one to three hours and resolve in one to two days. Symptoms in children are often more pronounced because they spend more time in the water and are more likely to accidentally ingest contaminated water.
People who consume contaminated water as a primary source for drinking water can develop more serious illnesses, such as liver damage, over time.
Although weather and wind conditions can cause algae blooms to move from one location in the lake to another, the toxin can stay in the water even after algae have moved or disappeared.