Algae blooms, seen in this July 2005 satellite image, have created the world's largest dead zone in the Baltic Sea. Image courtesy Jeff Schmaltz, NASA

By James Owen in Stockholm, for National Geographic News Published March 5, 2010 This story is part of a special series that explores the global water crisis. For more clean water news, photos, and information, visit National Geographic’s Freshwater Web site.
 … An explosion of microscopic algae called phytoplankton has inundated the Baltic’s sensitive waters, sucking up oxygen and choking aquatic life. Though a natural phenomenon at a smaller scale, these blooms have recently mushroomed at an alarming rate, fed by nutrients such as phosphorous and nitrogen from agricultural fertilizers and sewage. When it rains, farm fertilizers are washed into the sea. Sewage-treatment facilities also discharge waste into the Baltic ecosystem. As a result, the Baltic is now home to seven of the of the world’s ten largest marine “dead zones”—areas where the sea’s oxygen has been used up by seabed bacteria that decompose the raining mass of dead algae. “We’ve had enormous algal blooms here the last few years which have affected the whole ecosystem,” Westman said. Overfishing of Baltic cod has greatly intensified the problem, Westman said. Cod eat sprats, a small, herring-like species that eat microscopic marine creatures called zooplankton that in turn eat the algae. (Related: “Overfishing is Emptying World’s Rivers, Lakes, Experts Warn.”) So, fewer cod and an explosion of zooplankton-eating sprats means more algae and less oxygen. This vicious cycle gets worse as the spreading dead zones engulf the cod’s deep-water breeding grounds, he added. …

World’s Largest Dead Zone Suffocating Sea