Samples of water taken at Chorlton Water Park by the Environment Agency have indicated the presence of blue-green algae.
It is thought that the naturally forming blue-green algae blooms usually occur after a warm, wet period of weather in late summer or autumn. They usually disappear in cold weather.
Notices have been posted next to the lake warning that contact with the algae should be avoided, the lake is closed to all water sports and angling.
Weekly samples will continue to be taken by the Environment Agency until the algae levels are considered safe. Blue-green algae exist in fresh waters in Great Britain and throughout the world; they are noticed when their concentrations increase to form 'blooms' and when they form scums-looking like blue-green paint - or when they collect on the shore line.
The behaviour of algae is erratic and the level of toxicity can fluctuate; it can appear one day, be dispersed by the wind and mixing and re-accumulate at any time. Manchester City Council is working with the Environment Agency to minimise the impact of this naturally occurring algae.
The samples are at a level of concentration, which is potentially harmful to livestock, pets and humans if ingested, according to the threshold set.
Swallowing the water or algae scum can cause stomach upsets or more serious health effects. Contact with the water or with algae scum can cause skin problems. It is a sensible precaution for all visitors to the centre and their animals to avoid contact with the scum and the water close to it.
Councillor Mike Amesbury, assistant executive member for arts and leisure, said: "Manchester City Council has a duty of care to members of the public and employees; to prohibit all contact with the water until the algae is at a safe level. We hope to resume a normal service as soon as possible. We are urging people to cooperate and stay away from the water."
Margaret Blackburn, 0161 234 4014