An open ocean eddy shows what the dead zones look like in action. Photo by NASA/Earth Observatory
KIEL, Germany, May 4 (UPI) -- Dead zones are large expanses of oxygen deprived water where little to no life can survive. In a newly published study, researchers from Germany and Canada describe a number of dead zones recently identified in the tropical North Atlantic.
As detailed in the new paper -- published this week in Biogeosciences, the open access journal of the European Geosciences Union -- the concentrations of oxygen-deficient water develop in eddies. The swirling dead zones accumulate more lifeless water as they churn slowly westward across the open ocean. Their structure is similar to that of a hurricane.
"The fast rotation of the eddies makes it very difficult to exchange oxygen across the boundary between the rotating current and the surrounding ocean," Johannes Karstensen, a researcher at the Helmholtz Centre for Ocean Research Kiel, in Germany, explained in a press release. "Moreover, the circulation creates a very shallow layer -- of a few tens of meters -- on top of the swirling water that supports intense plant growth."
The researchers also discovered that zooplankton, the tiny marine organisms that anchor large ocean-based food chains, congregate on the surface of these drifting dead zones.
Most dead zones are found near the coast, where rivers deltas deliver large concentrations of fertilizers used on inland farms upstream. Fed by the excess phosphorous, the growth of summertime algae blooms accelerate -- sucking the oxygen from large swaths of ocean water. This lifeless water can be swept out to sea, but until now, dead zones have never found in the open ocean.
Researchers worry the whirlpool-like masses of lifeless water could result in large fish kills if they encounter islands and reefs where marine life flourishes. In previous studies, low oxygen levels have been linked with low yields for commercial fishermen in places like the Baltic Sea.
Researchers used special buoys to track the formation and westward movement from their genesis off the coast of West Africa out into the open ocean.
"Given that the few dead zones we observed propagated less than 100 kilometers north of the Cape Verde archipelago, it is not unlikely that an open-ocean dead zone will hit the islands at some point," Karstensen said. "This could cause the coast to be flooded with low-oxygen water, which may put severe stress on the coastal ecosystems and may even provoke fish kills and the die-off of other marine life."