The State Oceanic Administration said an area larger than Connecticut had been affected by the mat of “sea lettuce,” as it is known in Chinese, which is generally harmless to humans but chokes off marine life and invariably chases away tourists as it begins to rot.

Some beachgoers appeared to be amused by the outbreak, at least according to the Chinese news media, which in recent days have featured images of swimmers lounging on bright green beds of algae, tossing it around with glee or piling it atop of one another as if it were sand.

Local officials, however, are less enthusiastic. Last month, they declared a “large-scale algae disaster,” sending hundreds of boats and bulldozers to clean up the waters off Qingdao, a former German concession in Shandong Province that is famous for its beer and beaches. As of Monday, about 19,800 tons of the algae had been cleared, the Qingdao government said. While valued for its nutrition — or as an ingredient in fertilizers and biomass energy production — algae in large quantities can prove dangerous as it decomposes, producing toxic hydrogen sulfide gas. It also smells like rotten eggs.

The green tide, spread over 7,500 square miles, is thought to be twice the size of an outbreak in 2008 that threatened sailing events during the Beijing Olympics, which took place near Qingdao. Officials deployed boats, helicopters and 10,000 workers to keep the waters clear for the competition.

The cleanup costs were later estimated at more than $30 million. Abalone, clam and sea cucumber farms suffered more than $100 million in damage, according to a 2011 study by researchers from the Chinese Academy of Fishery Sciences. A 2009 outbreak was bigger.

Although biologists are at a loss to explain the most recent algae bloom, scientists suspect it is connected to pollution and increased seaweed farming in the province just south of Shandong. While similar green tides have been reported around the world, the annual bloom in the Yellow Sea is considered the largest, growing to an estimated million tons of biomass each year.

The green tides were first reported in Qingdao in 2007. A central factor is the high supply of nutrients from agricultural runoff and wastewater. But those pollutants have been in the Yellow Sea for decades, leading scientists to look for new triggers.

A group of researchers believe that the algae that washes up around Qingdao originates farther south in seaweed farms along the coast of Jiangsu Province. The farms grow porphyra, known as nori in Japanese cuisine, on large rafts in coastal waters. The rafts attract a kind of algae called Ulva prolifera, and when the farmers clean them off each spring they spread the algae out into the Yellow Sea, where it finds nutrients and warm conditions ideal for blooming.

“It feeds off those nutrients and grows bigger and bigger, and eventually you can see it from satellites,” said John Keesing, a scientist at the Csiro Division of Marine and Atmospheric Research in Australia who is studying the green tide with Chinese researchers. “The currents gently move the algae in a northeastern direction out into the center of the Yellow Sea. You get a huge amount, and eventually it starts to wash on shore.”

While farmers have long grown seaweed along the Jiangsu coast, the rafts expanded much farther offshore starting in 2006, which may have contributed to the recent blooms, according to an article published by Dr. Keesing and his colleagues. The answer to curtailing the blooms may lie in disposing of the algae that clogs the nori rafts on land.

“We haven’t suggested people stop growing porphyra, but proper husbandry methods to prevent much of the waste algae from going into sea, that’s probably the only preventive measure that could be deployed,” he said. 

 © 2013 The New York Times Company

...and I am Sid Harth