Beijing (AsiaNews/SCMP) –  Two red tides of algae have engulfed more than 5,000 sq km of the northeast sea, forming the largest toxic slick in history of Bohai sea, the area closest to Pechino, and threatening to contaminate important fishing waters.

One tide started on Friday near the mouth of the Yellow River, the mainland's second longest, affecting an area of 1,850 sq km. The other tide started on Saturday in the middle, east and north of the Bohai Sea, affecting 3,200 sq km, the State Oceanic Administration said on Tuesday.

" These red tides are a sort of  biological cancer because they threaten environmental quality, aquatic life forms and even human life," said Zhao Zhangyuan, of the Chinese Research Academy of Environmental Sciences.

The toxic slicks consist of a densely populated algae bloom that breeds in abundance, and  consumes urban pollution, industrial discharge, farm waste and fertiliser run-off that flows into coastal waters from rivers and streams.

Their large presence in the sea saps the water of oxygen while producing toxins that can paralyse fish and contaminate seafood. The administration urged authorities in Liaoning , Hebei , Tianjin and Shandong , which all lie along the Bohai coast, to monitor the algae and ensure all seafood was free of toxins before being sent to market.

 "Red tides are just one of the environmental problems the mainland is facing. It will lead to an unimaginable disaster if no clever action is taken quickly ... and the government has been slow to act," said Mr Zhao.

Another urgent problem to solve is desertification. According to Xinhua, more than 1.7 million sq km - 18.2 per cent of the mainland's land area - is now classified as desert and the area is expanding at an alarming pace - 3,464 sq km a year, compared with 2,460 sq km in 1994.

More than 730 hectares of farmland has been converted back into forest in four years as part of a project to bring the dust and sand that often whips through the city under control, Beijing forestry officials said. In order to protect population of the worst-affected areas, the government has relocated families in safer areas, as it has done for people in six districts and counties on the outskirts of Beijing, who would be relocated by 2010. Compared with last year, the number of relocated people is increased, from 480 to 2,000 of this year.

Forestry officials added 56 billion yuan had been spent since 2000 on sand-control projects in more than 70 counties and districts, like Beijing, Tianjin, Hebei and Shanxi