Ocean dead zones roughly double since 1960's

The number of "dead zones" in Earth's oceans have roughly doubled since the 1960's, and a recent research places the blame on fertilizers and other chemical pollutants in river runoff that generate blooms of algae that eat up oxygen when they die.
This was revealed by Robert Diaz, Virginia Institute of Marine Science and Rutger Rosenberg, University of Gothenburg, Sweden, in their findings in the Friday's issue of the journal Science.
There are 405 dead zones in coastal waters worldwide, affecting about 95,000 square miles (245,000 square kilometers) of ocean, an area about the size of New Zealand.
When fertilizer runoff dumps excess nutrients, such as nitrogen and phosphorous, into coastal waters, providing food for algae, these dead zones developed. When these microscopic plants die and sink to the ocean bottom, bacteria feed on them and subsequently consume all the oxygen dissolved in the water, the researchers said.
Fish and other bottom-dwelling sea creatures are left without enough oxygen to survive, resulting in displacements and mass die-offs. Usually, the researchers noted, these events aren't noticed until they threaten valuable fish stocks.
Diaz and Rosenburg said dead zones now rank as one of "the key stressor[s] on marine ecosystems," along with over-fishing and habitat loss.
"There is no other variable of such ecological importance to coastal marine systems that has changed so drastically over such a short time as dissolved oxygen," they wrote.
Baltic Sea has the world's largest dead zone and the mouth of the Mississippi River at the Gulf of Mexico is in the United States.
Source:Xinhua/Agencies