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Yu's efforts have paid off. According to local environmental-protection organizations that have monitored the lake, the water quality has been raised to Levels I and II, which both meet the national standards for drinking water. In 2011, Liangzi Lake was regarded as the cleanest freshwater body in China and ranked top out of nine "significant lakes" by the Ministry of Environmental Protection.

In 2002, Yu also brought macrophyte seeds and propagules - vegetable structures, such as buds, that are capable of forming new plants - to Changtan Reservoir, a lake in Zhejiang province that had been afflicted by blue-green algae for three years. The team grew 666 hectares of aquatic plants, and a year later, the quality of water in the lake had risen to Level II.

Although the method has been successful in Liangzi Lake and other pilot regions, it has not been widely used in other polluted lakes.

That's because of a shortage of labor and the fragile nature of ecological systems, according to Yu.

"Machine operations are unable to replace human labor," he said. "We hope to promote the method in other areas, but not everyone is willing to devote so much effort. If a natural disaster hit the region, the only thing that we could do would be to start all over again."

Yu's years of effort were almost destroyed by floods in 1998, when the plants he had introduced were killed off. In 2010, disaster loomed again when the lake broke its banks and flooded the surrounding area for 47 days, leaving a trail of dead snails, shrimps and rotting vegetation, which threatened the water quality, when the waters receded.

In response, Yu and his team planted more than 5,000 kg of seeds, covering more than 80 percent of the lakebed.

Although the elements and natural disasters can make the work seem an unending and thankless task, Yu is determined to maintain his pace.

"The eutrophication of the water is likely to continue, and we can never be too careful about controlling water pollution in the lake. We must make great efforts to make it work," he said.