The month-long algae outbreak on a tributary of the Yangtze River, blamed on large numbers of phosphor mines and processing factories, has sent an alert to environmental authorities to raise water treatment standards in the Three Gorges Dam area.

A resident scoops up a bowl of polluted water in Zigui, Hubei province. [China Daily]

Large areas of algae bloomed on June 16 on a 25 km section of the Xiangxi in Xingshan county, Hubei province, forcing thousands of residents to stop drawing water from it, the Xinhua News Agency reported Monday.

Cai Qinghua from the Institute of Hydrobiology under the Chinese Academy of Science, said: "This is the first time the Three Gorges Dam area has seen such a blue-green algae outbreak.

"The pollutants from many phosphor mines and chemical plants in the upper reaches of the Xiangxi River resulted in water deterioration."

An excess of blue-green algae removes oxygen from the water, killing fish and other aquatic life, which then decay and release toxins. Smelling foul, the water cannot be consumed by human and animals.

After the algae was found in the Xiangxi River, the local government took measures to improve water quality monitoring, including banning residents from taking water from the river, preventing fishing and organizing people to collect algae.

The algae expansion has slowed, Xinhua said.

Ma Jun, director of the Beijing-based Institute of Public and Environmental Affairs, told China Daily that besides checking on industrial polluters along the river, the environmental watchdog should reset wastewater treatment standards in the dam area.

"Construction of the dam has slowed the velocity of many rivers in the area, which weakens the self-purification capability of water body," Ma said.

"It is another reason why the Xiangxi River has rampant algae.

"The amount of pollution emissions the dam area is able to purify naturally should be recalculated. With a big reduction of pollutant emission budget, the pollution credits need to be allocated again," he said.

Ma also said wastewater treatment plants should strengthen treatment capability of phosphor and nitrogen.

The Xiangxi and six tributaries of the Yangtze saw algae blooms last year because of excessive amounts of pollutants pumped into them, a report by the Ministry of Environmental Protection said.