WASHINGTON (Reuters) - "Dead zones" in coastal waters -- regions of ocean floor so deprived of oxygen that most marine life cannot survive -- are spreading worldwide at an alarming pace, scientists said on Thursday.
Driving the trend are nitrogen and phosphorous from chemical agricultural fertilizers that reach coastal waters after flowing off farm fields and into streams and rivers, according to the study published in the journal Science.
Nitrogen compounds from burning fossils fuels, particularly from power plants and cars, also are settling back to the ground and eventually wash into coastal waters, they said.
This decade alone, the number of coastal dead zones has risen by about a third to 405 worldwide, with clusters on the coasts of the United States and Europe. Combined, they take up an area of at least 95,000 square miles.
The biggest one measures about 30,000 square miles in the Baltic Sea, the researchers said. This is followed in size by one in the Gulf of Mexico starting at the mouth of the Mississippi River in the United States and one at the mouth of China's Yangtze River in the East China Sea.
"It's not sort of a local or regional problem, which is how it was thought of in the past," Robert Diaz of the College of William and Mary's Virginia Institute of Marine Science said in a telephone interview. "It is actually a global problem."
"Most of it is agricultural-based, but there is a lot of industrial nitrogen in there, too, if you consider electric generation industrial," added Diaz, who tracked the proliferation of the dead zones along with Rutger Rosenberg of the University of Gothenburg in Sweden.
The number of dead zones started to approximately double every 10 years starting in the 1960s, the researchers said.
There were 301 such dead zones at the end of the 1990s, 132 at the end of the 1980s, 63 at the end of the 1970s and 39 at the end of the 1960s, Diaz said.
The researchers said dead zones must be considered an important source of stress on marine ecosystems, ranking alongside over-fishing, habitat loss from human development and harmful algal blooms as global environmental problems.
Dead zones are formed when excess nutrients, mostly nitrogen and phosphorus, enter coastal waters and help fertilize blooms of algae. When these tiny plants die and sink to the sea bottom, they provide a food source for bacteria, which consume dissolved oxygen from surrounding waters.
As a result, there are large areas of sea floor with insufficient oxygen to support most marine life.
"Fish are the best at avoiding dead zones. When the oxygen starts to decline, they're smart -- they leave, they don't hang around. Crabs and shrimp are pretty good at getting away, too, as are lobsters," Diaz said.
But slower-moving creatures on the sea floor often die, including worms, clams and small crustaceans. "These are the animals that are the fundamental food base for the commercial crabs, shrimp and fish that feed on the bottom," Diaz said.