© NASA Earth ObservatoryThe dead-zone eddies found in the Biogeosciences study are somewhat similar to the one seen in this picture, which was captured by the Moderate Resolution Imaging Spectroradiometer on NASA's Terra satellite in late 2011.
A team of German and Canadian researchers have discovered areas with extremely low levels of oxygen in the tropical North Atlantic, several hundred kilometres off the coast of West Africa.
The levels measured in these 'dead zones', inhabitable for most marine animals, are
the lowest ever recorded in Atlantic open waters. The dead zones are created in eddies, large swirling masses of water that slowly move westward. Encountering an island, they could potentially lead to mass fish kills. The research is published today in
Biogeosciences, an open access journal of the European Geosciences Union (EGU).
Dead zones are areas of the ocean depleted of oxygen. Most marine animals, like fish and crabs, cannot live within these regions, where only certain microorganisms can survive. In addition to the environmental impact, dead zones are an economic concern for commercial fishing, with very low oxygen concentrations having been linked to reduced fish yields in the Baltic Sea and other parts of the world.
"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. 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.
Dead zones are most common near inhabited coastlines where rivers often carry fertilisers and other chemical nutrients into the ocean, triggering algae blooms. As the algae die, they sink to the seafloor and are decomposed by bacteria, which use up oxygen in this process. Currents in the ocean can carry these low-oxygen waters away from the coast,
but a dead zone forming in the open ocean had not yet been discovered.
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