People should err on the side of caution when going near the water at Thrapston Lake following the discovery of blue-green algae.
The warning comes from the Environment Agency, which has been monitoring the situation at the lake.
It says that, although the conditions have stabilised, people should still avoid the water as a precaution.
Environment Agency area duty manager Mike Dugher said: “After reports of dead fish in Thrapston Lake, our teams tested the water and confirmed the fish died as a result of low oxygen levels in the water, which occurs naturally in hot summer weather, and they also confirmed blue-green algae in the lake.
“We aerated the water, removed the dead fish and monitored the situation for more than a week, and now that conditions have stabilised, we have removed our equipment so we’re ready to respond to other incidents that may arise.
“As the hot weather continues, we may see more incidents like this so we’d encourage people to report fish in distress or suspected blue green algae to our incident hotline on 0800 80 70 60.”
An agency spokesman added that they have worked with the owners of the lake to make sure appropriate warnings are in place to urge people and their pets to stay out of the water, as blue-green algae can be dangerous to pets and make people ill.
They said it’s difficult to tell blue-green algae from other forms of algal growth by sight, so people should still avoid the water as a precaution, and if anyone suspects blue green algae in another location, please report it to their incident hotline – 0800 80 70 60.