Continuous rainfall and relatively high temperatures are the main reason behind recent reports of blue-green algae in some major lakes in eastern China, according to experts.
Blue-green algae recorded on July 1 near Bakou and Chuanchang, both on the eastern shore of Chaohu Lake in Anhui province, had increased by 852 per cent and 479 per cent respectively from levels reported last year, according to the provincial Environmental Protection Department.
Chaohu Lake is the country's fifth-largest freshwater lake.
Environmental authorities have issued two red alarms - the highest alert - for blue-green algae outbreaks in the lake since April, with the latest one announced late last month still in force. This level of alarm was rarely issued previously.
Xinhua News Agency said the algae has been hard to contain and the authorities are attempting to ensure clean water supplies for local residents, as the two areas are sources of drinking water.
"We are watching the water quality closely. Microcystin, a toxin that threatens drinking water, has not been detected. We have also stepped up purifying procedures to ensure water supply," said Zhu Yu, deputy director of the Environment Monitoring Center in Anhui.
Zhu said about 50,000 tons of emergency water supplies can be obtained from other water sources in 10 days' time.
Taihu Lake, the country's second-largest freshwater lake in neighbouring Jiangsu province, has also reported its first major blue-green algae outbreak for seven years.
Zhu said, "Heavy rainfall in June and July produced a lot of nitrogen, phosphorus and potassium in Chaohu Lake, while the high temperatures in recent days have provided fine conditions for the algae's growth."
Earlier reports said that Dianchi Lake in Yunnan province had seen nine days of moderate algal blooms and another four days of moderate and severe blooms as of July 8.
Fang Shengzhong, deputy director of the environmental monitoring centre in Kunming, the capital of Yunnan, said, "The average temperature in Dianchi Lake has risen by one degree centigrade and accumulated sunlight in the first half of the year increased by 170.6 hours from previous years."
To ease the situation, authorities for the three lakes have assigned additional workers to collect the algae. About 120,000 tons of it have been collected by dozens of environmental workers at Chaohu Lake.
More than 220 million yuan (S$49 million) has been spent to contain blue-green algae in Chaohu Lake this year, according to authorities.
Xinhua contributed to this story.