When Jiangsu seaweed farmers won the right to sell their produce to Japan in 2005, few expected their victory to bring such big changes to one of the mainland's most popular seaside resorts. But as the tourist city of Qingdao in neighbouring Shandong copes with its sixth massive summer algal bloom in as many years, scientists are pointing to the expansion of seaweed farming along the coast as a possible culprit. Qingdao's beaches began to look more like grasslands as they were swamped by the annual "green tide" last month, with this year's outbreak covering an unprecedented 28,600 square kilometres. While the algae appears harmless to humans, it can choke off oxygen supplies for marine life, and the smell as it rots does little to attract tourists to a city long famed for its local Tsingtao beer and beaches. The algae first hit in earnest in 2008, weeks before Qingdao was in the international spotlight as host of the sailing events for the Beijing Olympics, yet no convincing explanation for the algal bloom has emerged. Many cite coastal pollution as a likely factor. However, a study published in May in the journal Estuarine, Coastal and Shelf Science concluded that the "green tide" originated from the Jiangsu coast and proliferated in the Yellow Sea before being swept north by the tide towards Qingdao. The study by scientists from the Yantai Institute of Coastal Zone Research under the Chinese Academy of Sciences cited satellite images and field observations to support its claims. But the findings are already proving controversial, with scientists involved in the research reluctant to speak publicly due to its "sensitivity". Not only does the study cover relations between two provinces, but it also has implications for a fast-growing and lucrative industry. The massive algal bloom, consisting of the seaweed species Ulva prolifera , usually forms in the Yellow Sea about 300 kilometres south of Qingdao in mid-April, and arrives on the city's coast between mid-June and late July, according to Professor Zhu Mingyuan , of the Qingdao-based First Institute of Oceanography under the State Oceanic Administration. "The small-scale algal bloom can expand really fast as the natural conditions - seawater temperature and sunlight - are right. Then the dominating southern winds, starting in April, bring them to the Shandong Peninsula, with Qingdao as a major destination," Zhu said. The nearby cities of Weihai and Rizhao have also been plagued by the green tide in past years thanks to a slight change in wind direction. "Where the initial biomass of Ulva prolifera comes from remains a complicated issue and there are still debates among researchers from different backgrounds," he said. The recent published study sheds some light as it tracks the origin of Ulva prolifera to the tens of thousands of hectares of tidal flats devoted to growing seaweed, which it says offer a "hypothetical nursery bed" for the outbreak. Scientists believe the algae grows on rafts used by Jiangsu seaweed farmers to grow porphyra, an edible species popular in Japan. When the seaweed is harvested in April, the Ulva prolifera is removed from the rafts, discarded and swept into the sea by waves, researchers believe. "Certain biological traits of Ulva prolifera - efficient photosynthesis, rapid growth rates, high capacity for nutrient uptake and diverse reproductive systems - allow the growth of the original 5,000 tonnes of Ulva prolifera biomass into more than one million tonnes in just two months," the study says. A study published in 2009 by the same group of researchers ruled out the possibility that the 2008 algae outbreak was caused by a sudden surge in the levels of nutrients such as phosphate and nitrogen in the seawater. But if seaweed farming is the problem, why was there no algal bloom before 2008? The study's lead author, Liu Dongyan , declined an interview, describing the finding as too sensitive as it involved "different departments". A researcher with knowledge of the studies in Qingdao, who asked not to be named, said Jiangsu's seaweed farming industry boomed after 2005, when Japan, the world's biggest consumer of dried seaweed, was forced to open its markets to Chinese producers under World Trade Organisation rules. Previously, Japan had only allowed imports of seaweed from South Korea. But a February 2004 application by the provincial seaweed producers' association in Jiangsu led to an investigation by Beijing's Ministry of Commerce that forced Tokyo to change its stance. The case was hailed as Beijing's first big victory in removing foreign trade barriers under WTO rules after China joined the trade body in 2001. Official statistics from the China Fishery Yearbook show that seaweed farming in Jiangsu expanded from 12,040 hectares in 2004 to 38,260 hectares in 2010. The provincial seaweed association, which now represents more than 350 enterprises, says annual sales of seaweed products are now worth more than 3 billion yuan (HK$3.76 billion). "There was a small-scale algal bloom reported in 2007, but not as significant as in the years that followed," the researcher said. "This coincidence in time leads us to believe the expansion of seaweed farming [in Jiangsu] and the algae bloom are related." But the hypothesis was, perhaps not surprisingly, flatly rejected by researchers in Jiangsu, who said the findings lacked "direct evidence", the source said. There are also alternative hypotheses in Shandong. Pang Shaojun , a researcher from the Institute of Oceanology under the Chinese Academy of Sciences, also based in Qingdao, told the Jinan -based Dazhong Daily that the algae could be caused by an increase in the use of fermented chicken manure as fertiliser in the area. Wang Yamin , an associate professor of marine studies at Shandong University in Weihai , said the excessive amount of nutrients in seawaters, a result of worsening coastal pollution, was to blame for the wild algal bloom. "I think the studies are showing the traces of local protectionism ... By blaming Jiangsu's seaweed industry, Qingdao is trying to escape from its own responsibility," said Wang. "Don't forget it is the high level of nutrients in the waters off Qingdao that offers favourable conditions for fast growth of the algal bloom, the same reason for rampant outbreaks of blue algae in inland lakes," he said. Even without the algal bloom, Wang said, the state of the Qingdao coastline offers alarming evidence that unchecked coastal pollution caused by human activities will, in turn, lead to disasters beyond human control. Research by the Ministry of Environmental Protection reveals a grim picture for the nation's coastline. The ministry grades seawater on a scale of one to four, with a fifth category considered worse than grade IV, meaning that the marine ecosystem has been destroyed and will not be able to recover in the short term. Some 18.9 per cent of waters are rated worse than grade IV, with a further 5.3 per cent rated grade IV. In Qingdao, as the clean-up operation continues, the bloom barely raises curiosity any more. "The city was caught completely off guard in 2008 right ahead of the summer Olympics, when thick seaweed covered the waters used as sailing venues," said Wang Yongqing , 61, a frequent beach-goer. "It was the first massive outbreak. "But it has become routine since then, no longer posing too much of a disturbance for us, as the algae seems not to be toxic and quickly gets removed," he added, as he rubbed traces of the algae in the sand with one foot on the already cleared No 3 bathing beach. Yet there has been a heavy price for the city's government, which spent an estimated US$30 million on cleaning up more than one million tonnes of algae before the Olympics. Six years on, the city government is more experienced at dealing with the problem and deployed 2,000 people in clean-up operations earlier this month, as officials rushed to keep up with the strands swept in from the sea. But with the root cause of the problem still unclear and vested interests involved, the chances of preventative measures being put in place remain slim, the researcher said. "It is impossible for Shandong to pay Jiangsu compensation and ask it to stop seaweed farming as the industry involves billions of yuan," the source said. "But if the findings are true, why can't we try fishing out the algae biomass before it expands to an uncontrollable scale? It's like leaving a tumour unheeded until it grows into the late stage of cancer." But until the source of the algae becomes clear, the city of Qingdao can expect to receive, if not exactly welcome, its green visitor for many years to come.