this is an increasing problem, its blue (or is it green?) algae, soon as the temps raise, it appears, and stays until weather goes cold. Its very stringy, and is real bad for model boats as it chokes the props that cannot cut through it. St Helens have had this problem with their large lake, over past 5 years its got worse and worse. Last year, the council spent thousands on contractors that used special cutting equipment in the water, it lasted a few months, then the algae came back. The bird population started to die in the water, but it was never proved if it was due to something such as the algae, or someone had put something in the water. Apparently the lake had green flag status, and was investigated as the lake became stagnant and an eyesore, locals complained and this drove the council into action, until then they had ducked and dived away from the problem.
In Crosby, near Liverpool, there is a salt water boating lake, this now has a sign forbidding its use due to algae