Large ocean dead zones are on the move and could someday affect sea life around islands in the Atlantic.
Scientists suprised by low oxygen level in massive swirls of water
FRISCO — Scientists probing the mysteries of the sea probably weren’t expecting to find giant oxygen-sucking eddies, but now that they have, the research could help understand, and even predict, fish kills, which sometimes seem to happen almost randomly.
The team of German and Canadian researchers discovered the areas with extremely low levels of oxygen in the tropical North Atlantic, several hundred kilometers 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.
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.
The oxygen levels in the newly discovered areas are 20 times lower than the previously estimated minimum, not even close to enough oxygen to sustain life.
Normally, dead zones are found 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.
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 metres, with the dead zone taking up the upper 100 metres or so,” said Karstensen. The area around the dead-zone eddies remains rich in oxygen.
“The fast rotation of the eddies makes it very difficult to exchange oxygen across the boundary between the rotating current and the surrounding ocean, Karstensen explained.
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,” he said.
The researchers have been conducting observations in the region off the West African coast and around the Cape Verde Islands for the past seven years, measuring not only oxygen concentrations in the ocean but also water movements, temperature and salinity. To study the dead zones, they 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, the researchers noticed that 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,” said Karstensen. “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. 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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