Scientists have discovered what are called "dead zones" in the tropical North Atlantic, and they could potentially lead to mass fish kills, according to new research.
Dead zones are areas of the ocean with extremely low levels of oxygen. Most marine animals, like fish and crabs, cannot live within these regions, where only certain microorganisms can survive. These zones not impact the environment, but also are an economic concern for commercial fishing. For example, very low oxygen concentrations having been linked to reduced fish yields in the Baltic Sea and other parts of the world.
And now, several hundred kilometers off the coast of West Africa, a team of German and Canadian researchers have discovered the lowest oxygen levels ever recorded in Atlantic open waters.
"Before our study, it was thought that the open waters of the North Atlantic had minimum oxygen concentrations of about 40 micromole per liter of seawater, or about one milliliter of dissolved oxygen per liter of seawater," lead author Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Centre for Ocean Research Kiel, in Kiel, Germany, said in a statement.
Although this concentration of oxygen is low, most fish can still survive in such waters. However, this latest study shows that the minimum levels of oxygen are actually about 20 times lower than the previous estimate, making the dead zones nearly void of all oxygen and unsuitable for most marine animals.
So how exactly are dead zones created? They are most common near inhabited coastlines where rivers often carry fertilizers 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.
Lake Erie, for instance, has been suffering from its largest dead zone in decades thanks to harmful algae blooms. (Scroll to read on...)
It is known that ocean currents can carry these low-oxygen waters away from the coast, but this study is the first to find a dead zone forming in the open ocean. The newly discovered dead zones are unique in that they form within eddies, large masses of water spinning in a whirlpool pattern.
"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," explained Karstensen.
"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," he added. "From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions."
To study the dead zone, which is located off the West African coast, the research team used drifting floats and used satellite observations to measure oxygen levels and plant growth.
They discovered that the dead zone contains concentrations of almost no oxygen, up to only 0.3 milliliters of oxygen per liter of seawater. For comparison, oxygen levels outside the dead zone can be up to 100 times higher than those within it.
Already the researchers noticed that zooplankton are affected by these oxygen-depleted zones, which is worrisome considering that these small animals play an important role in marine food webs.
But what's more concerning is that this open-ocean dead zone could move westward and hit the islands at some point, which "could cause the coast to be flooded with low-oxygen water," Karstensen warned.
This could put severe stress on the coastal ecosystems and lead to mass fish kills as well as mass die-offs of other marine life.
The findings are described in more detail in the journal Biogeosciences, an open access journal of the European Geosciences Union (EGU).
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