Shortness of breath in the open Atlantic

Marine researchers discover "dead zones" without oxygen in the open ocean

The death zones in the tropical North Atlantic are similar to this vortex photographed by the Terra satellite in 2011 (left). © The death zones in the tropical North Atlantic are similar to this vortex photographed by the Terra satellite in 2011 (left).
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No oxygen to breathe: Even in the open Atlantic there are almost oxygen-free zones. They are located in the center of huge eddy currents, as an international research team has now discovered. So far, such "death zones" have only been known from the bottom of inland seas like the Baltic Sea - they are a surprise near the surface of the tropical Atlantic. How the regions are formed without oxygen, the researchers describe in the journal "Biogeosciences".

Oxygen is one of the basic conditions for survival for most creatures on our planet. This also applies to fish that absorb the dissolved oxygen in the water. However, water behaves differently than the air that surrounds us: Different layers of water with different temperatures and different oxygen contents often do not mix, but remain stable stratified. In addition, the activity of microorganisms contributes to oxygen depletion. In the deeper regions of the Baltic Sea, but also in the Gulf of Mexico, this leads to so-called death zones. The water at the bottom is so low in oxygen that apart from some microorganisms, nothing lives there anymore.

Surprise in the tropical Atlantic

Also on the eastern edges of the tropical oceans, in a few hundred meters of water, there are vast areas where significantly lower oxygen concentrations can be found. In these so-called oxygen minimum zones (SMZ), the concentration was low, but is sufficient for most marine life completely.

For Johannes Karstensen of the GEOMAR Helmholtz Center for Ocean Research in Kiel and his colleagues, the discovery of an extremely low-oxygen zone in the open tropical North Atlantic, hundreds of kilometers away from the African coast, was unexpected. The area with about half the area of ​​Schleswig-Holstein is still close to the water surface, not in depth.

Lack of oxygen in the giant vortex

Such oxygen deficient regions evidently occur in gigantic vertebrae of 100 to 150 kilometers in diameter, which occur in almost all areas of the world ocean. "We knew that large swirls were forming in front of the African coast, moving west across the Atlantic, " explains Karstensen. "But we were very surprised by the prevailing levels of oxygen depletion." In the center of these vortices, the amount of oxygen is up to 20 times lower than the previously assumed minimum. display

Low-oxygen eddies migrate from the African coast westward across the tropical North Atlantic. European Geosciences Union

Scientists discovered one of these low-oxygen vortices at a long-term monitoring station, the Cape Verde Ocean Observatory, north of the Cape Verde Islands. Afterwards, they were able to observe several vortices from their formation off the coast of Africa to the west of the Cape Verde Islands using various modern measuring systems including satellites and freely floating oceans .

Bacteria consume existing oxygen

As it turned out, the vortices harboring the oxygen depleted zones form due to changing flow conditions off the West African coast. At first, they do not show particularly low oxygen concentrations, but are rich in nutrients. Sunlight promotes strong plankton growth under these conditions. The plankton forms a dense carpet near the surface, which then hardly allows light to penetrate into deeper layers.

Cross-section through a vortex: The death zone is formed in the center, approximately 20 to 100 meters deep. White lines show water layers of the same density. Measured by the Cape Verde Ocean Observatory, this vortex has a diameter of 120 to 140 kilometers. Kar J. Karstensen / GEOMAR / Biogeosciences

Underneath this plankton rug, dead organic particles sink, which are decomposed by bacteria. They consume the available oxygen. Since there is hardly any exchange between the vertebrae and the surrounding ocean water, there is no oxygen supply from outside. "This explains the extremely low oxygen concentrations near the surface, " says Karstensen.

Self-produced oxygen depletion

The process is similar to the emergence of death zones in the Baltic and other inland waters. In the open ocean, however, the phenomenon was previously unknown. "We now have the first evidence that these vortices produce an oxygen-depleted to oxygen-free state and do not simply take it from the coastal region into the open ocean, " explains the oceanographer.

Eventually, the vortices break up on the way across the Atlantic and mix with the surrounding water. The role of the vortices in the overall oxygen balance of the North Atlantic, how organisms react to the oxygen-depleted zones, and what happens if these vortices collide with an island chain, such as the Cape Verde Islands, will be investigated further. (doi: 10.5194 / bg-12-1-2015)

(GEOMAR Helmholtz Center for Ocean Research Kiel / European Geosciences Union, 30.04.2015 - AKR)