The number of “dead zones” in coastal regions around the world continues to rapidly increase, according to a study conducted by researchers from the Virginia Institute of Marine Science and the University of Gothenberg, and published in the journal Science.
“It’s not sort of a local or regional problem, which is how it was thought of in the past,” researcher Robert Diaz said. “It is actually a global problem.”
Dead zones are areas where oxygen has become so depleted that little or no marine life is able to survive. They form when excessive plant nutrients, particularly nitrogen and phosphorus, run off from the coast and lead to an explosion of algae blooms. When this vastly increased biomass dies and sinks to the bottom, its decomposition leads to the proliferation of oxygen-consuming bacteria.
In some cases, this may lead to increased crowding pressure in other parts of the ocean.
“Fish are the best at avoiding dead zones,” Diaz says. “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.”
Many slower moving animals such as clams, worms and small crustaceans, however, simply die.
In the current study, researchers found that the number of dead zones has steadily increased from 39 at the end of the 1960s through 63 at the end of the 1970s, 132 at the end of the 1980s and 301 at the end of the 1990s to the current number of 405. The total area consumed by dead zones now measures no less than 95,000 square miles.
The major sources of the pollutants that produce dead zones are fertilizer runoff from industrial agriculture and nitrogen-based byproducts of fossil fuel use.
“Most of it is agricultural-based, but there is a lot of industrial nitrogen in there, too, if you consider electric generation,” Diaz said.
Dead zones now function as one of the primary stresses on marine biodiversity, along with overfishing and habitat loss.