The dead zones are spreading. No, it’s not a sign of the zombie apocalypse, though the consequences could be just as dire. A recent study by the European Geosciences Union’s journal Biogeosciences describes a dead zone off the coast of Africa which contains the lowest levels of oxygen ever observed in the Atlantic. To complicate matters, the Atlantic-African dead zone is on the move, threatening all life in its path.
Often located in coastal waters, dead zones are caused by the runoff of chemicals and fertilizers used in agriculture, though poorly designed disposal of liquid human waste also contributes to the problem. A particularly infamous zone lurks in the Gulf of Mexico, a deadly consequence to farming runoff down the Mississippi River. Nitrogen and phosphorus rich runoff settles in river deltas or in the ocean, where it feeds the growth of algal blooms. As the algae dies, it becomes food for bacteria, which in turn consumes the oxygen present in the water. This environment is extremely inhospitable to marine life. Oxygenless water is usually diluted by current and tidal movement, yet the presence of enormous eddies in the Atlantic-African dead zone prevents this dispersal.
“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,” says Johannes Karstensen, lead author of the report. “From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions.”
Eddies slowly move westward and if these particular eddies make landfall, it would spell disaster for the local marine ecosystem. “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,” says Karstensen.
The outlook is not all doom and gloom. Dead zones can be rehabilitated, as demonstrated by the transformation of the Black Sea dead zone. Previously the largest in the world, the Black Sea dead zone disappeared in the decade following the collapse of the Soviet Union, which made fertilizers cost prohibitive for farmers. Although this revitalization was coincidental, deliberate efforts have been made to eliminate dead zones along the Hudson River and in San Francisco Bay. However, no clean-up will be able to revive now-extinct organisms that have perished in the dead zone.