The seas off China have been hit by the largest ever growth of algae. The vast swath of green growth is washing onto the shores of the Yellow Sea, near the eastern city of Qingdao, where bulldozers shoveled up tons of algae from the sand.

The State Oceanic Administration said on its website that the algae, enteromorpha prolifera, started to appear a week ago and had spread across an area of 28,900 square kilometers. Two years ago, a 410-square-km patch of algae floated into the city’s beaches. Shandong suffered severe green algae invasions in 2007, 2008 and 2010 that decimated its aquaculture and cost millions of yuan in damage. The previous largest bloom was in 2008 when it affected around 13,000 square kilometers, and threatened the Olympic sailing events that were held in Qingdao’s waters.

The algal phenomenon is usually caused by an abundance of nutrients in the water, especially phosphorus, although the triggers for the enormous blooms that began to appear in the Yellow Sea in 2007 remain uncertain. “It must have something to do with the change in the environment, but we are not scientifically sure about the reasons,” said professor Bao Xianwen, of the Qingdao-based Ocean University of China.