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
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.
A “green tide” swamped the shores of Qingdao in June 2008
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.