Environmental authorities in east China's Anhui Province Friday warned of the danger of a blue algae outbreak in Chaohu Lake, the country's fifth largest fresh water lake.

"The hot weather since late July, the sun light, wind and nitrogen and phosphorus contents in the water, are very favorable for the algae outbreak," said Lu Xiaoping, an official with the provincial environmental protection bureau.

"Floodwater in the past month has washed more pollutants of nitrogen and phosphorus from nearby farmlands into the lake," he said.

Thursday's satellite pictures showed that about 20 square kilometers of blue algae has formed on the northwestern and southeastern parts of the lake.

The bureau also said the algae boom has so far not affected drinking water supplies, as water intake areas remain clear of algae.

The bureau has sent five ships to fish out the algae, according to officials.

Chaohu Lake is frequently hit by blue algae outbreaks between June and September due to its high contents of nitrogen and phosphorus following years of fish farming in the water and waste discharged from nearby agriculture and industries.

Environmental officials said that the algae is not poisonous and can soon dissipate due to wind, rainfall, or heat if temperatures rise above 37 degrees Celsius.

The major harm caused is in cutting off oxygen to the lake's surface,killing aquatic animals and causing pollution.

Hefei City, located along the lake, has invested heavily in sewage treatment facilities after a serious algae outbreak affected drinking water in 2004.

It has also restricted fish farming in some lake areas, according to the bureau.