Children swim through algae-choked waters off the coast of Qingdao, in eastern China's Shandong Province, on July 17.
An algae bloom, or "green tide," has clogged nearly 7,700 square miles (20,000 square kilometers) of the Yellow Sea (see map), Chinese authorities said Sunday, according to the state-run media outlet Xinhua.
The algae blanketing the beaches belongs to a species of marine plankton known as Enteromorpha prolifera, found in waters all around the world. In the right conditions, the algae can explode into so-called macro-algal blooms, Steve Morton, a marine biologist at the U.S. National Oceanic and Atmospheric Administration (NOAA), told National Geographic News in 2010.
Such massive blooms require warm ocean temperatures and waters rich in the elements phosphorus and nitrogen, which are found in fertilizers and can be carried to the coasts by water runoff. While the algae aren't toxic, big blooms can create oxygen-poor "dead zones" in the water and leave an unpleasant odor on beaches.
(See pictures of algae coating China's beaches in 2010.)