Pollution levels in many Chinese lakes have declined somewhat from high levels in the past decade, helped by billion-dollar investments in urban sewers and waste water treatment.

Concentrations of phosphorous fell by a third from 2006 to 2014 in 862 freshwater lakes in China, although they remain above clean water levels, according to an article published this week in the journal Nature Geoscience.

Phosphorous is vital to life, but high concentrations can trigger blooms of toxic algae that choke fish and other life. Man-made sources of phosphorous include waste water, livestock farming, aquaculture and chemicals.

Improved sanitation facilities are key

"The current decline in the most populated areas is due to improved sanitation facilities such as pipelines, waste water treatment plants and improved rural toilets," says Yan Lin, one of the authors of the report according to Reuters. Lin is a researcher at the Section for Catchment Processes at the Norwegian Institute for Water Research (NIVA).

Building good sanitation and sewage infrastructure is, according to Lin, key to stop phosphorous pollution, and these findings could guide other developing nations seeking ways to clean up vital freshwater resources.

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Still high levels

The study, the first to have common measurements of phosphorous across China's lakes, showed the median level fell to 51 micrograms per 1 liter in 2014 from 80 in 2006.

That is still high, however: A level of 25 is considered good quality water in European legislation.

Phosphorous builds up in lake sediments and lingers long after sources of pollution have dried up.

A long time is to be expected before Chinese lakes could reach good ecological status, according to the report, which is written by NIVA scientists together with Chinese colleagues.

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Regional differences

Despite overall declines in most parts of China, the researchers discovered that phosphorous levels had risen from low levels in the sparsely populated northeast.

According to the researchers, some of the reasons for this is diffusion from aquaculture and livestock farming, in addition to deforestation caused by logging.

"The rise could also be linked to climate change, which is causing more downpours and erosion of soils," Lin said.

In light of these regional differences, the researchers suggest that a set of spatially flexible policies for water quality control would be beneficial for the future health of Chinese lakes.