A "green tide" swamped the shores of Qingdao in June 2008
Scientists have discovered the source of the gigantic green tide of algae that almost derailed the Beijing Olympics sailing regatta.
The huge algal bloom was triggered by a rapid expansion of farmed seaweed almost 200km down the coast, satellite images reveal.
The green tide then grew and grew as it moved closer to the regatta city of Qingdao.
At one point, it became the largest ever recorded anywhere in the world.
Initially, the international media and many scientists suggested that excess nutrients (eutrophication) in coastal waters caused the algal bloom.
We suspect that the reason the bloom had not occurred previously was that the growth of aquaculture in this region has been so rapid
Dr John Keesing, CSIRO
The new finding, published in Marine Pollution Bulletin, disproves that idea and suggests that similar tides could happen again.
In late June 2008, the waters and shores at the Qingdao venue hosting the Olympic sailing regatta experienced a massive green tide covering about 600 sq km. Lasting over two weeks, it took more than 10,000 people to clean up, removing over one million tonnes of algae from the beach and coast.
The algae responsible is called Enteromorpha prolifera.
Satellites saw the algae appearing off Yancheng and moving north
"It's not a dominant or common species in the local area," says Dongyan Liu, a marine biologist at the Chinese Academy of Sciences in Yantai, Shandong.
"The rapid appearance and sheer scale made us suspect it had accumulated offshore and been transported in."
E. prolifera pollutes aquaculture farms that grow Porphyra seaweed on semi-floating rafts made of bamboo and net curtain.
The seaweed is mainly sold as food and also grown to prevent eutrophication, as the seaweed takes up nutrients that might otherwise pollute the water, allowing weeds to grow, starving other plants of oxygen.
Photos passed to Liu and John Keesing of the Australian research organisation CSIRO showed the Qingdao green tide contained bamboo poles used in Porphyra aquaculture.
The researchers then spoke to farmers growing the seaweed across the Yellow Sea on the coast of Jianngsu province. The timing of their harvest suggested the bloom could have originated there.
To investigate further, Liu, Keesing and colleagues examined photos of China's north-eastern coast taken by instruments on board Nasa's Terra and Aqua satellites, which view the entire Earth's surface every one to two days.
On the 15 May 2008, small green patches of algae, covering around 80 sq km, appeared off the coasts of Yancheng and Lianyungang in Jianngsu province, the images reveal.
The bloom (in red circles) grows as it heads towards Qingdao
Within 10 days, these patches had moved away from the coast and into the Yellow Sea, covering 1,200 sq km, and impacting about 40,000 sq km of ocean, making it the largest algal boom, or green tide, ever recorded.
On the 18 June, algae patches began to move towards the coast at Qingdao, landing on the shore on 28 June.
Further satellite data confirmed that the right temperatures, wind speeds and oceanographic conditions existed at the time to favour the rapid growth of the algae, and transport it across the Yellow Sea.
"We suspect that the reason the bloom had not occurred previously was that the growth of aquaculture in this region has been so rapid," said Keesing.
From 2003 to 2008, the coastal area used for seaweed aquaculture off the coast of Jianngsu has more than doubled to 23,000 hectares.
"It has now reached a critical size where it has the capacity to produce enough Enteromorpha to cause the problems we saw in 2008," the CSIRO researcher said.
To prevent a reoccurrence "we should carefully manage the distribution and production of coastal aquaculture and educate farmers not to discard unwanted Enteromorpha into the water."
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