The water quality at Taihu Lake, China's third largest body of freshwater, was basically stable, amid renewed concerns on an algae bloom, a Ministry of Water Resources official said on Friday.
No large areas of algae had been spotted in the lake and water supply to neighboring cities was normal, said the ministry's Taihu Basin Authority chief Ye Jianchun.
Blue-green algae was only detected in the lake's southern parts, with sporadic swathes along the shore and isolated spots in small rivers linked with the lake, he said.
No algae was found at water intake sites and other areas.
Officials said on Wednesday a re-emergence of algae growth could affect the lake that supplies water to about 30 million people, renewing concerns on water pollution.
An algae outbreak in May rendered tap water undrinkable for about 10 days, affecting more than 1 million residents in Wuxi City, Jiangsu Province. Jiangsu, Zhejiang and Shanghai get much of their water from Taihu.
The lake's water quality from January to March was basically at the same level as that in the same period last year, Ye said.
In the first three months, the indices of algae-related chemicals such as ammonia and nitrogen were as low as qualifying the lake as a drinking water source. Chlorophyll concentration varied between 11.6 to 17.6 milligrams per cubic meters, below the critical level of 40 that leads to an algae outbreak, according to Ye.
A new project was launched in January to facilitate water flow in the lake and improve water quality, he added. From then through Thursday, 460 million cubic meters of water had been injected into the lake from the Yangtze River.
Satellite monitoring showed blue algae first began to appear in the west and south of the lake on April 3, showing it had entered a sensitive period of annual algae outbreaks, according to Qin Boqiang, a Nanjing Institute of Geography and Limnology researcher.
Jiangsu stepped up emergency monitoring by provincial, city and county agencies and demanded better pollution control efforts.
High levels of nitrogen and phosphorus are believed to be major causes of algae outbreaks, which develop in water that is rich in organic matter.