Algae is a particular problem in Bassenthwaite
Efforts are being made to reduce the amount of chemicals from household detergents and animal waste filtering into lakes in Cumbria.
Phosphates encourage the growth of algae which cut off the oxygen supply to plant and animal life, a particular problem in Bassenthwaite lake.
A campaign has been encouraging owners of bed and breakfast accommodation to use phosphate-free detergents.
Farms have also been filtering out animal effluent from water courses.
John Pindar, from the Environment Agency, said work was well under way to improve the situation.
He said: "We are already halfway between the position that we were in and the position that we need to be for an entirely healthy lake.
"For the other lakes that haven't been impacted quite as bad then we are right ahead of the game, and the issue there is about prevention rather than cure."