A massive bloom of algae has spread to a size of almost 20,000 square kilometers in the waters off of China’s eastern coast, according to a report by the country’s state-controlled Xinhua news agency.

China’s principal maritime authority, The North China Sea Marine Forecasting Center of State Oceanic Administration, said that its surveys indicated that about 19,050 square km of seawater in the Yellow Sea were covered with algae.

The Center believes the algae will continue to spread in a northward direction, while some will likely wash ashore.

Officials caution that while the algae are not toxic, because of their ability to consume significant amount of oxygen, they could pose a threat to marine life as well as local tourism.

The New York Times reported that the algae may be flourishing due to the presence of high level of nitrates in China’s coastal waters (which are the result of agricultural and industrial run-off).

The green algae were initially reported in the Yellow Sea four years ago. Soldiers and local people had removed thousands of tons of the foul-smelling material in 2008 prior to Olympic sailing events held in the city of Qingdao.

The algae returned to the Qingdao area in 2009 and 2010.