Scientists discovered ominous "dead zones" in the tropical North Atlantic off the coast of West Africa.
The oxygen levels measured in these areas are the lowest ever recorded in the open Atlantic, and they make the regions uninhabitable to most creatures, the European Geosciences Union reported. They are created by eddies, which are swirling masses of water that move towards the west.
"Before our study, it was thought that the open waters of the North Atlantic had minimum oxygen concentrations of about 40 [micromole] per [liter] of seawater, or about one [milliliter] of dissolved oxygen per [liter of seawater]," said lead-author Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Centre for Ocean Research Kiel. This concentration of oxygen is low, but still allows most fish to survive. In contrast, the minimum levels of oxygen now measured are some 20 times lower than the previous minimum, making the dead zones nearly void of all oxygen and unsuitable for most marine animals.
The newly-measured oxygen levels are 20 times lower than the previous minimum. The massive eddies that cause these dangerous dead zones are as large as 90 miles in diameter.
"The fast rotation of the eddies makes it very difficult to exchange oxygen across the boundary between the rotating current and the surrounding ocean. 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," Karstensen said.
The recent observations allowed the researchers to measure the properties and extent of these dead zones, as well as how they influence local ecosystems. They found zooplankton within eddies remain at the surface, instead of hiding in deeper waters during the day to avoid predators.
"Another aspect related to the ecosystem impact has a socioeconomic dimension," Karstensen said. "Given that the few dead zones we observed propagated less than 100 km north of the Cape Verde archipelago, it is not unlikely that an open-ocean dead zone will hit the islands at some point. 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."
The findings were published in a recent edition of the journal Biogeosciences.
