Local authorities have confirmed that Dianchi Lake, the largest freshwater lake in Southwest China's Yunnan Province, is suffering from a blue-green algae boom.
However, the outbreak is not as serious as the one that hit Taihu Lake last month, polluting the water supply for millions of residents in Wuxi.
Large areas of the northern part of Dianchi Lake were covered in the foul-smelling pond life yesterday, despite heavy rains on Monday, which it was hoped would dilute the algae.
But officials were quick to point out the situation was not serious.
"It happens every year. In fact it is much less serious than last year," Ma Wensen, director of the Dianchi Lake Administration, said.
Ma said the concentration of chlorophyll was about 703 micrograms per liter in June 2006, while this month it is 620 micrograms per liter.
He attributed the lower levels to the early arrival of the rainy season.
The administration has mobilized about 100 workers to help clean up the affected areas, Ma said.
The algae boom does not pose a threat to the drinking water supply for Kunming, the capital of Yunnan, which has a population of about 6 million.
A water supply facility designed to divert water from the Zhangjiu River, was completed and put into service in March.
With a total area of nearly 300 sq km, Dianchi has been suffering from severe pollution since the 1980s, and there have been several short-term threats to the water supply. The lake absorbs most of Kunming's industrial and domestic wastewater.
Ma said there were two severe algae outbreaks in 1999 and 2001.
"The algae was so thick you couldn't even push a stick through it," he said.
Zhang Yuxiang, a farmer living near the lake, said the pungent smell was affecting people.
"Many people and even some poultry have fallen sick due to the poor environment," he said.