Large-scale algae outbreaks are once again posing a threat to the drinking water supplies from Taihu and Chaohu lakes.

As the temperature has continued to rise over the past three days, the blue-green algae has gathered at the western end of Taihui Lake, which sits on the border of East China's Jiangsu and Zhejiang provinces, just 11 km from Gonghu water plant, the main water source for Wuxi, Jiangsu.

And as the mercury climbs above 30 C, the thick blanket of foul-smelling algae in the 2,400-sq-km freshwater lake - the country's third largest - will expand rapidly, experts from the Wuxi agriculture and forestry bureau said.

Winds from the east have been pushing the algae toward the western part of the lake since Monday.

Zhang Xianzhong, an official with the bureau, said between 1,000 and 2,000 people have been involved each day, trying to clear the algae from the water.

"We have been pulling out more than 1,000 tons of algae from the lake every day, first using baskets and then pumps," Zhang said.

Now, special boats fitted with advanced filtration technology are being used to clear away the unwanted plant.

More than 100 such boats will be mobilized over the coming months, Zhang said.

About 300 km to the west of Taihu, Chaohu Lake in nearby Anhui Province is also under threat of an algae outbreak.

The local environmental protection bureau said dozens of 4- and 5-sq-km "belts" of algae had been seen floating on the 780-sq-km freshwater lake, the country's fifth largest.

The western part of Chaohu is said to be most at risk, and a tap water factory 3 km from its western bank stopped using it as a water source earlier this week.

"Once the conditions for algae growth are ripe, it is very difficult to effectively control it in the short term," Zhang Zhiyuan, an engineer with the Anhui environment protection bureau, said.

"If the situation gets worse, we will divert water from the Yangtze River to dilute the concentration of algae to prevent a large-scale outbreak," Zhang said.

As the quality of its water has been steadily degraded, Chaohu Lake has experienced small- and medium-size algae outbreaks each June and July over recent years, an expert has said.

Professor Lu Jianjian, a marine expert from East China Normal University in Shanghai, said: "Excessive nutrients, including nitrogen and phosphate from fertilizers, industrial runoff and untreated sewage, combined with the hot weather, provide good growing conditions for the algae.

"More effort should be made to stop factories discharging wastewater directly into the lakes," Lu was quoted as saying by People's Daily.

In May, algae contaminated Taihu Lake leaving some 2 million Wuxi residents without tap water for more than a week.

(China Daily 06/22/2007 page5)