Water sports are off limits at Gunwade Lake in Ferry Meadows after a positive test for blue-green algae.
The algae was detected last Friday with the Environment Agency returning since to see if it has disappeared.
The results of the re-test are expected later this week.
Blue-green algae can be toxic for animals and cause severe illnesses in humans.
Gunwade Lake plays host to water sports and activities at Nene Outdoors.
James Sainsbury, centre manager at Nene Outdoors, said: “We’ve asked the Environment Agency to come back and do a re-test. We’re waiting for the results.
“Ninety-nine per cent of the water sports and activities around the water are still restricted because the highest risk is to people accessing the edges of the lake.”
A statement on the website of Nene Park Trust, which manages Ferry Meadows, added: “Blue-green algae can be dangerous to dogs who may swim and drink affected water.
“It is very important to keep dogs out of the algae and ensure they drink only clean water. There is also a risk to humans from rashes and potential illnesses if swallowed.”
The growth of blue-green algae has been fuelled by the recent hot weather, with other lakes around the country also having the same problem.
It had been suspected earlier in the summer that there was blue-green algae in Lynch Lake at Ferry Meadows, but tests came back negative.