A team of researchers have reported the discovery of moving ‘dead zones’ in the tropical North Atlantic, several hundred kilometres off the coast of West Africa. The areas have so little oxygen that most marine life cannot survive in them and, if they approach land, they could cause mass fish die-offs.
The large, swirling eddies of water contain the lowest levels of oxygen ever recorded in open water in the Atlantic and are moving slowly westward, according to research published in the open access journal Biogeosciences.
In addition to the obvious environmental implications, there are serious potential economic impacts. Low oxygen content in ocean waters has been linked to declining commercial fish stocks in the Baltic Sea and elsewhere.
“Before our study, it was thought that the open waters of the North Atlantic had minimum oxygen concentrations of about 40 micromol per litre of seawater, or about one millilitre of dissolved oxygen per litre of seawater,” says lead-author Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Centre for Ocean Research Kiel, in Kiel, Germany in a statement.
Had those estimates proven to be correct, it would have been enough for most fish species to survive. However, the levels proved to be up to 20 times lower than the researchers suspected, with some areas containing almost no oxygen at all.
Dead zones sometimes occur near populated coastlines due to water pollution. Fertilizers and other nutrients, trigger algae blooms and when the algae die they sink to the seafloor. The algae then decompose and the bacteria that feed on the algae use oxygen in the process of consuming them, depleting the water and endangering marine life.
However, this is the first time dead zones have been discovered in the open ocean. The zones have formed within masses of swirling water called eddies.
“The few eddies we observed in greater detail may be thought of as rotating cylinders of 100 to 150 km in diameter and a height of several hundred metres, with the dead zone taking up the upper 100 metres or so,” explains Karstensen.
The researchers also noted that the areas immediately surrounding the dead zones remained rich in oxygen.
“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,” said Karstensen.
The team found that plant growth within the areas is similar to the coastal dead zones that result from fertilizer runoff and algae blooms.
“From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions,” explained Karstensen.
The researchers report that oxygen levels in the dead zones range from almost no oxygen at all to 0.3 millilitres of oxygen per gallon. Meanwhile, just outside the eddies, the levels reach up to 100 times those levels.
The researchers have been studying the waters off West Africa and the Cape Verde Islands for seven years, measuring oxygen levels, water movements, temperature and salinity using drifting floats and satellite data.
