In what’s been a summertime rite of passage since 2007, the Chinese coastal city in the Shandong province reluctantly welcomes an ocean of sea lettuce to its beaches whenever the water temperature climbs into the 70s.
Biologists say the green tide is connected to a combination of seaweed farming and water pollution to the south of Qingdao. Seaweed farmers grow porphyra on large rafts along the coast in the Jiangsu Province. According to aNew York Times writeup on the phenomenon in 2013, “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.” Although seaweed farming is nothing new, The Times adds that the seaweed rafts “expanded much farther offshore starting in 2006.”
In the U.S., algae blooms in the Great Lakes have grabbed headlines in recent years, but the Yellow Sea’s are often considered the largest (annual estimates suggest that the blooms total one million tons of biomass). Qingdao’s blooms received international attention in 2008 when Chinese soldiers and volunteers were sent to gather up all the algae they could before an Olympic sailing event.
Such large quantities of algae choke marine life and cost millions in damages and cleanup, but algae is also fun to play with. Every summer, Qingdao beachgoers are spotted by photographers—sitting on the algae, swimming in it, tasting it, and even turning themselves into sea monsters.
The fun stops once the blooms start decomposing; that’s when they produce smelly, toxic hydrogen sulfide gas.