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Green TideA child plays on an algae-matted beach in the coastal city of Qingdao (map) in east China's Shandong Province earlier this month.

Local authorities and residents in the popular tourist destination have been struggling over the summer to remove a large mass of green algae that has washed ashore. As of late June, the algae bloom—or "green tide"—covered more than 170 square miles (440 square kilometers) of coasts south of Qingdao.

The algae blanketing the city's beaches belongs to a species of marine plankton known as Enteromorpha prolifera. The algae can be found in waters all around the world, and can explode in so-called macro-algal blooms if conditions are right, said Steve Morton, a marine biologist at the National Oceanic and Atmospheric Administration (NOAA).

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 isn't toxic, big blooms can create oxygen-poor "dead zones" in the water and leave an unpleasant odor on beaches.

(See "World's Largest Dead Zone Suffocating Sea.")—Ker Than

Photograph from Imaginechina/AP