Greater restrictions have been placed on a nearby sewage treatment works following last summer's algae outbreak
A "perfect storm" of nutrients from sewage effluent and unusual weather conditions has been blamed for a 2009 outbreak of toxic algae on a lake.
Llyn Padarn at Llanberis was closed to leisure users for much of last summer.
A report commissioned by Environment Agency Wales calls for stricter limits on effluent discharged from the local sewage treatment works.
A water sports businesses said the closure had been "pretty devastating" but the lake was now fully open.
The report by Professor Glen George described the warm spring, followed by a still June and a wet July and August as "the perfect storm".
He said the weather conditions, combined with nutrients in the lake, not only created the right conditions for the algal bloom to form but also meant it continued through the summer.
His report makes a number of recommendations for further scientific studies of the lake and more work to find out how the management of the sewage treatment works in Llanberis is affected by heavy rain.
A recommendation to tighten the consent limits on the sewage effluent has been pre-empted by a modification to the sewage plant's consent while more analysis of the lake takes place.
Prof George said: "From an ecological point of view we could almost describe the weather conditions experienced in 2009 as the perfect storm.
"We now need to establish the likelihood of these conditions being repeated in another year.
"Since nothing can be done to control the weather, the only practical solution is to improve the quality of the effluent discharged from the sewage treatment works."
Environment Agency Wales area manager for north Wales, David Edwell, said the consent limits at the sewage plant were already stricter.
But he warned another algal bloom could not be ruled out.
"Nobody wants to see a repeat situation," he said.
"But the science behind these blooms is extremely complex and if we have similar weather this year we could well see the algal blooms reappear."
Llanberis based canoe and kayaking company Surf-Lines is holding an open day on the lake later this month aimed at encouraging people back into the water.
Managing director Phil Nelson said: "Last year the lake was off limits to kayakers and canoeists for much of the summer because of a problem with blue green algae, which was pretty devastating for local businesses situated on and around Llyn Padarn.
"This year things are looking better and the lake is currently fully open to recreational users."