Potentially dangerous blue green algae has now developed in River Witham and Brayford Pool in Lincoln too.
It has now been spotted across Brayford Pool from Tuesday morning.
North Kesteven District Council said that the algae spread in bloom proportions due to the continued warm weather and could be dangerous to people and pets if they were to go in the water.
There could be concern for the swans in the Brayford, as blue green algae can be toxic to animals.
It spreads from Brayford Pool itself, all the way down stream through the underpass at High Bridge.
The Lincolnite has approached City of Lincoln Council for a statement.