Cardiff's Roath Park Lake has been closed due to a potential outbreak of a toxic algae in the lake that can kill dogs and harm humans.
Parkwood Outdoors, which manages Roath Park Lake Boathouse, said there is a possibility that there is blue-green algae present in the lake.
According to Public Health Wales, blue-green algae is a type of bacteria that is found in lakes, ponds and inland water sources.
The bacteria forms in blooms, and can persist for several weeks, sometimes months, depending on the weather and flow conditions.
The park was temporarily closed last year when Natural Resources Wales confirmed the presence of the potentially toxic algae.
There are no quick remedies for the control of blue-green algae once blooms appear.
Blue-green algae is capable of producing several different toxins that can cause skin rashes, nausea, vomiting, stomach pains, fever and headaches.
In more serious cases, it may result in illnesses such as liver and brain damage.
Cardiff council said: "Please look out for warning signs around the lake and stay away from the water's edge to avoid contact with the algae."
Anyone who has come into contact with water containing the algae should shower immediately and seek medical attention if they become ill.
Parkwood Outdoors said it will announce further details on Monday.
The park will remain closed until the water levels return to normal.
