When summer arrives, local people around Dianchi Lake know it's time to get ready for something unpleasant.  

Often, they wake in April or May, to find sections of the lake green in color and releasing an awful smell.

Almost overnight, the surface of the water is covered by a thick layer of tiny, green algae.

Sometimes, the "green oil paint," as local residents have taken to calling the algae, covers so large an area and grows so intensively that fish are killed due to the lack of oxygen.

People in the city of Kunming, in southwest China's Yunnan Province, say the blue and green algae have bloomed almost every year over the past decade on Dianchi, the largest lake in the province.

Sometimes, the algae become so serious that the whole city is mobilized to get rid of the problem.

In 1999, Dianchi suffered the worst outbreak in history. The algae went wild and spread over 20 square kilometers, or 7 percent of the lake's surface.

For months, the city government tried to get rid of it but the situation eased only slightly before coming back the next year.

Widespread threat

Dianchi is not the only lake in China that suffers from the wildly growing algae.

Major lakes and rivers in China, such as Taihu Lake in eastern China, have almost all experienced regular outbreaks of the algal bloom over the past few years. In the worst cases, even drinking water supplies suffered because of the water pollution caused by the bloom.

In the coastal area, explosive blooms of algae have also occurred from time to time, forming the so-called red tide that severely threaten the fishing industry.

In the past two weeks, the waters around the Zhoushan Islands and in Bohai Bay have suffered from red tide.

Public calls for treatment of the biological disaster have been running high and scientists have been asked to work out effective solutions, the quicker the better.

So far, a couple of solutions have been developed and applied with some success. Still, many puzzles remain and may frustrate efforts to look for a definitive cure, said Xie Ping, a researcher with the Institute of Hydrobiology in Wuhan, capital of central China's Hubei Province.

According to Xie, Chinese scientists have conducted on-site investigations and research in almost every major lake in China that has suffered algal blooms.

Dianchi and Taihu, both close to large cities and industrial areas, have become the focus of the nationwide scientific effort.

Xie and his colleagues are among China's pioneer researchers in the field.

Their labs are located by East Lake, the largest in Wuhan where the algal bloom used to be a constant threat to the lake fish and a smelly nuisance to local residents.

Scientific effort

"The algal bloom used to be so common that local people had got used to its smell," Xie said.

Yet the algae threat to human health is not limited to its odour, said Zhao Zhangyuan, a researcher at the Chinese Academy of Environment Science.

He said that algal blooms are also the source of another far more serious threat to human health: algal toxins. The blue-green algae produces toxins that have been associated with liver and other cancers.

Microcystins and nordularin, which belong to a set of toxins known as phycotoxins, are the two main toxins present in the algae-polluted water that have been linked to cancer, said Zhao.

These two toxins, however, represent only a few of the phycotoxins present in such polluted waters.

In fact, more than 80 percent of 480 algae samples he and his colleagues took from surface waters of lakes and rivers throughout China were found to contain a variety of toxins.

A number of studies, including epidemiologic surveys, have demonstrated the algal toxins have played a role in the pathogenesis of liver cancer, he said.

Even when the presence of hepatitis B viral infections, ethnic or genetic susceptibilities, and alcohol consumption were factored in, the risk of liver cancer associated with the drinking water taken from water sources polluted by blue and green algae was still two times greater than that of unpolluted water.

"That's why the algal bloom is called 'biological cancer,'" Zhao said.

Years of research have identified the excess nutrients in the water as the major cause of the water bloom, said Xie, who uses a term, ultrophication, to describe the situation.

Cause and cure

Investigations have found that closed or half-closed lakes that received water with excess nitrogen and phosphorus are most susceptible to the outbreak of water blooms.

Take Taihu for example. The water ultrophication there was caused by the very high volume of nitrogen, phosphorus and carbon in the waste water released from chemical factories and households nearby, resulting in rapid growth of blue and green algae.

Consequently, the algal blooms occurred while the water turned murky and the levels of soluble oxygen in the water declined, leading to the death of a great deal of aquatic life.

Yet scientists are still wondering about the exact scenario that leads to algal blooms, said Kong Fanxiang, a researcher at the Nanjing Institute of Geography and Lake Research in east China's Jiangsu Province.

Kong and his team have been doing research on the water blooms in the mid- and lower reaches of the Yangtze River, including the Taihu Lake area.

He said that there has been controversy among researchers over how the algal blooms emerge and spread.

Traditional thinking suggests algal blooms are hard to predict because they break out in very short periods of time. Kong, however, believes that may not be the case.

Rather, the emergence of the algal bloom may go through what he described as four phases. And the algae may not have grown during the process.

He said large amounts of the algae already exist in the water before it condenses and clumps on the water surface under proper conditions.

The algae goes down to the lake bed and enters a dormant period between October and February. Then the algae resurrects, proliferates and clumps on the water surface, causing the bloom.

This finding implies the dormant period - the least active of the algae's life cycle - may be the best time for people to curb or remove it, he said.

Xie, from the Wuhan Institute, has found a way to remove the algae blooms effectively.

His research found the red and bighead carps eat blue and green algae and effectively prevent it from blooming.

Xie and his team were researching the mysterious disappearance of algal blooms in the East Lake a couple of years ago when they found the unique trait among this species of fish.

Unique trait

The East Lake had suffered severe water blooms for decades, but none since the early 1990s, rousing the interest of many researchers.

After finding red and bighead carps abound in those once algae-intensive water areas, Xie's team carried out experiments with the fish in lab conditions and confirmed their ability to eat up the algae.

The method was later applied in Dianchi and Taihu and, so far has proven to be the most effective.

Compared with the chemical or physical removal of the algae, which has been widely used but proven all but futile, this method is cheap and easy to conduct, Xie said.

Yet it still is not a final solution, he admitted. The carps can only spawn and proliferate in rivers with fast-flowing water, rather than in the slow-moving lakes.

"So we have to bring large amount of fry of the red and bighead carps to the lake each year and that may not be enough," Xie said.

And this method does not deal with the excess nutrient in the water, either, which is the direct cause of the water bloom, he added.

A comprehensive scheme, combined with control of waste water input, may turn out to be the practical cure in the long run, he said.

(China Daily May 25, 2004)