Large areas of algae bloom in an upper Yangtze River tributary have forced thousands of residents in the central province of Hubei to stop drawing drinking water from the river.

The bloom first appeared on Monday in the Xiangxi River. By Friday, it had expanded from 5 km to about 23 km, stretching from Gaoyang Township of Xingshan County to Zigui County, where it converged with the Yangtze River.

The river water has turned emerald green and opaque. Experts from the Yangtze River pollution regulatory agency had determined the water bloom to be blue algae.

Experts found excessive phosphate and a slow current in the river, which they said accounted for the nutrition enrichment phenomenon.

Local governments are closely monitoring the water quality and the spread of the algae and have advised residents against eating fish or drinking water from the river.

Blue algae choked east China's Taihu Lake last summer, triggering a drinking water crisis for residents in Wuxi City in Jiangsu Province.

Blue algae, which exists widely in water bodies and is not harmful itself, grows easily in polluted water with high concentrations of nitrogen and phosphorous and a temperature of about 18 degrees Celsius.

An excess of blue-green algae removes oxygen from the water, killing fish and other aquatic life, which then decay and release toxins.

(Xinhua News Agency June 21, 2008)