World’s Largest Dead Zone Suffocating SeaBy James Owen in Stockholm
March 5, 2010, National Geographic News
Algae blooms (seen in a July 2005 satellite image) have created the world’s largest dead zone in the Baltic Sea. Image courtesy Jeff Schmaltz, NASA
Algae blooms (seen in a July 2005 satellite image) have created the world’s largest dead zone in the Baltic Sea.
Image courtesy Jeff Schmaltz, NASA
"Eagle!" The shout goes up as a great shadow sweeps over our boat. The white-tailed eagle makes its descent to one of the 24,000 islands that make up Sweden‘s pine-covered, rocky Stockholm Archipelago.
The tourists on board for this nature tour in August 2009 mostly miss the photo opp. But local wildlife expert Peter Westman, of the conservation group WWF Sweden, assures the group that there will be others.
Numbers of this once-threatened predator have soared from 1,000 to more than 23,000 in the Baltic Sea (map) since pollutants including DDT, an eggshell-thinning pesticide, and PCBs, chemical compounds used in electrical equipment, were banned in the 1970s, Westman said.
But there is a new danger to the eagle and many other marine species: 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 Adding to Algal Blooms
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.
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.
The algal blooms, which can be toxic to animals and human swimmers, leave behind an ugly layer of green scum that fouls tourist beaches and starves seaweeds of light.
"Other species have taken the place