Massive 'Dead Zones' Discovered Moving West in North Atlantic Waters

Dead zones may be coming to the coast. A team of researchers have discovered areas with extremely low levels of oxygen in the tropical North Atlantic, only a few hundred kilometers off the coast of West Africa. These "dead zones" have levels of oxygen that are the lowest ever recorded in Atlantic open waters.

Dead zones are areas of the ocean that are depleted of oxygen. Most marine animals, like fish and crabs, cannot live within these regions. Dead zones are also an economic concern for commercial fishing, since very low oxygen concentrations have 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 liter of seawater, or about one milliliter of dissolved oxygen per liter of seawater," said Johannes Karstensen, the lead author of the new study, in a news release.

Dead zones are most common near inhabited coastlines where rivers often carry fertilizers and other chemical nutrients into the ocean. This nutrient load triggers algae blooms. As these algae die, though, they sink to the sea floor and are decomposed by bacteria, which use up oxygen in the process. Dead zones in the open ocean, though, have not been seen-until now.

The newly discovered dead zones form within eddies, which are large masses of water that spin in a whirlpool pattern.

"The fast rotation of the eddies makes it very difficult to exchange oxygen across the boundary between the rotating current and the surrounding ocean," said Karstensen. "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. From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions."

The findings are important for understanding the environment and even predicting fish yields. The fact that these swirling masses of water are moving westward means that, if they encounter an island, they could cause mass fish kills.

The findings are published in the journal Biogeosciences.

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