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.

Image Qingdao’s algae is generally harmless to humans, but not marine life. Credit... The New York Times

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.