Tong Yindong and his colleagues at Namtso Lake in the summer of 2015. [Photo provided to chinadaily.com.cn]

The risk level of Chinese lake eutrophication has been significantly decreased. Total phosphorus concentration levels have declined by a third, with the number of lakes with heavy phosphorus pollutants greatly reduced. The results are recorded in a recent study, The effective control of phosphorus discharge in China brings down the concentration of total phosphorus in lakes, published in Nature Geoscience.

This is the first study of phosphorus pollution in China's lakes for nearly a decade. It was completed by the team of Tong Yindong of the College of Environment at Tianjin University and his international partners, who spent more than two years tracking and analyzing 862 lakes across the country.

Since they are highly sensitive to human activities, lakes are often referred to as the "sentinels" of the terrestrial ecosystem, reflecting the risk levels of their local ecologies.

"Nothing else is more appropriate than lakes to reflect the impacts of human activities," Tong explained. Unlike flowing rivers, lakes are relatively static.

Although phosphorus is essential for the existence of life, it is also a common pollutant in industrial and agricultural production areas. Livestock waste water, aquaculture, and chemical industry emissions are all important phosphorus sources.

In the nearly ten years between 2006 and 2014, the concentration of phosphorus in about 60% of lakes studied decreased from 80 μg l?1 to 51 μg l?1. In 2006, about 22% of the sampling sites had a TP concentration of more than 200 μg l?1 (water quality standard V grade in China). By 2014, only 7% of the lakes had such high concentrations of phosphorus.

The effects of declination of phosphorus in the lakes on daily life are readily apparent. "For example," said Tong, "In summer, the number and frequency of phytoplankton blooms in water bodies have dropped significantly."

Study of regional distribution shows that the total phosphorus concentration of lakes in eastern, central and western China, and in China's five largest freshwater lakes (Chaohu, Dongtinghu, Hongzehu, Poyanghu, and Taihu), has decreased significantly.

"The decrease in phosphorus concentrations in water bodies indicates that the risk of eutrophication is reducing, which is one of the important indexes for the improvement of lake water quality," said Tong.

Although the specific reasons for water quality improvement in different parts of China are not exactly the same, there is no doubt that urban development and sanitation facilities have made significant contributions.

Since the regulation Assessment of Reduction of the total Emission of Major Pollutants came into force in 2006 pollution control and emission reduction have been incorporated into environmental performance appraisals in China. According to the data, from 2006 to 2014, China invested about 792.3 billion yuan to control urban sewage and industrial wastewater. By the third quarter of 2016, nearly 5,000 sewage treatment plants were in use in China.

However, there are still issues causing concern. While the phosphorus concentration of lakes in China has decreased by almost one third, the median level of 51 μg l?1 still remains high. According to European water quality standards, the phosphorus content should be lower than 25 μg l?1 for water to be considered high quality.

There are still some sources of pollution in the east and middle parts of China, such as domestic wastewater and cage aquaculture in rural areas. As the use rate of baits exceeds 30%, superfluous baits and fish faeces increase phosphorus concentrations in water bodies.

In the western part of China the phosphorus elements are mainly from planting and phosphating industrial emissions. Dianchi lake in Yunnan province has an annual intake of 30 tons from phosphate and related industries. In Northeast China, other potentially important causes of increased phosphorus load should be considered, such as surface erosion caused by heavy rainfall.

"At present, the sewage treatment plants in our country have created a decrease of nutrient elements. With a higher sewage treatment rate, the plants can try to adopt flexible effluent targets as the next step and turn the plants from 'reducers' of nutrients into 'regulators' of nitrogen and phosphorus concentrations in the lakes," Tong said.

Tong is very concerned about human environmental impact. "For developing countries, improving the quality of the water environment through urban sewage treatment is the first priority. It can bring about a change of water environment in a short time. Water pollution policy needs to reflect a country’s economic development level, natural conditions and other factors."

Environmental protection requires action by all of us, Tong suggested. "It is best not to develop remote clean lakes for the tourism industry. Once the environment of these clean lakes is damaged, it will be very difficult to restore it. In addition, tourism and catering businesses need better control and planning so that the pursuit of economic benefit does not come at the cost of the environment."

"It is expected to be a long time before Chinese lakes reach an acceptable ecological status. At least, we have made a good start."Tong added.

For future environmental protection, Tong said, "It is not easy to restore or protect the environment on a large population basis. The decrease of total phosphorus concentration in the water body and the recovery of the aquatic ecosystem can lag behind, and we need a little patience to realize the overall improvement of the water function in our country. But from our results, our water pollution policy is effective. We should have a positive attitude towards improvement of the overall water environment."