A group of researchers led by Dr Johannes Karstensen of the GEOMAR Helmholtz Centre for Ocean Research Kiel in Germany has discovered unexpectedly low (< 2 micromol per liter of seawater) oxygen environments in the open waters of the tropical North Atlantic, several hundred km off the coast of West Africa, a region where oxygen concentration does normally not fall much below 40 micromol per liter.
An ocean eddy (bottom of the image) as seen from space. Image credit: NASA Earth Observatory.
‘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, these 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.
The newly-discovered dead zones in the tropical North Atlantic are generated in either cyclonic or anticyclonic-modewater eddies – large masses of water spinning in a whirlpool pattern. Tracking of eddies reveals them to be generated off the northwestern African coast. They propagate westward, with a speed of about 4.5 km day.
“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 meters, with the dead zone taking up the upper 100 meters or so. The area around the dead-zone eddies remains rich in oxygen,” said Dr Karstensen, the lead author on the study published in the journal Biogeosciences.
“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.”
This plant growth is similar to the algae blooms occurring in coastal areas, with bacteria in the deeper waters consuming the available oxygen as they decompose the sinking plant matter.
“From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions.”
“Before our study, it was thought that the open waters of the North Atlantic had minimum oxygen concentrations of about 40 micromol per liter of seawater, or about 1 ml of dissolved oxygen per liter of seawater,” Dr Karstensen said.
“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.”
Dr Karstensen and co-authors report concentrations ranging from close to no oxygen to no more than 0.3 ml of oxygen per liter of seawater. These values are all the more dramatic when compared to the levels of oxygen at shallow depths just outside the eddies, which can be up to 100 times higher than those within.
To study the dead zones, the scientists used several tools, including drifting floats that often got trapped within the eddies. To measure plant growth, they used satellite observations of ocean surface color.
Their observations allowed them to measure the properties of the dead zones, as well as study their impact in the ecosystem.
Zooplankton – small animals that play an important role in marine food webs – usually come up to the surface at night to feed on plants and hide in the deeper, dark waters during the day to escape predators. However, within the eddies zooplankton remained at the surface, even during the day, not entering the low-oxygen environment underneath.
“Another aspect related to the ecosystem impact has a socioeconomic dimension. 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,” Dr Karstensen said.
“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.”
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Karstensen, J. et al. 2015. Open ocean dead zones in the tropical North Atlantic Ocean. Biogeosciences, 12, 2597-2605; doi: 10.5194/bg-12-2597-2015
