Officials have been investigating reports of dead fish in the lagoon at Gedling Country Park.
Officers from the agency visited the site on Sunday after concerned visitors contacted them via their emergency hotline after hundreds of dead fish were spotted floating in the waters.
Residents have also contacted Gedling Eye to report that a large number of birds, like ducks, who use the waters regularly have also not been seen for days.
A spokesman for Gedling Borough Council said: “Looking at the evidence presented, the issue was likely to do with an unusual spike, then drop in oxygen levels, brought about by the recent mixed weather along with pond weed and algae.
“The Environment Agency visited site at the weekend and as a result have appointed a member of their staff to come up with mitigation measures that should prevent the issue happening again in the future.
“Gedling Borough Council Rangers have removed any remaining fish from the area today and will be monitoring the situation closely.”
The Environment Agency said the cause of the deaths was ‘unknown’.
Matt Buck, fisheries technical specialist for the Environment Agency, said: “Thanks to a call from a member of the public, we worked with Gedling Borough Council to respond to a report of dead fish in the lake at Gedling Country Park at the weekend.
“The cause of the fish deaths is unknown at present and we’re continuing to monitor.
“Gedling Borough Council, as park landowners will shortly be removing the dead fish.
“We rely on members of the public to report possible pollutions to us, so if anyone sees an environmental incident, such as fish in distress, an oil slick on the water or any river with a strong odour, please report it to us 24 hours a day on 0800 80 70 60.”