Scientists claim to have discovered the first deep ocean ‘dead zones’, which are areas of water with virtually no oxygen-in immense whirlpools in the tropical North Atlantic.
Canadian and German researchers claim that the slowly moving eddies can be the cause of deaths of a large population of fishes and economic ruin if they reach Western African Communities.
Lead author Johannes Karstensen, of the Helmholtz Centre for Ocean Research in Kiel, said the team had found dead zones less than 100 kilometres from the Cape Verde archipelago. “It is not unlikely that an open-ocean dead zone will hit the islands at some point,” he said.
In the shallow waters where fertilizers and other various chemicals trigger algal blooms, dead zones are a general phenomenon. They line the coasts of the US, Western Europe and Japan, and there are around 10 known dead zones around the southern states of Australia.
But the new ones, reported in the journal Biogeosciences, are the first found hundreds of kilometres out to sea.
Dr Karstensen siad that the dead zones lie inside the whirling cylinders of the ocean, up to 150 kilometers across and many hundred meters in depth, which had formed when a current along the West African coast became unstable. They had been pushed slowly westward by rotation of the earth.
He said the water, initially fairly oxygenated, had become more and more depleted until dead zones evolved inside the swirling water. Oxygen concentrations were up to 20 times lower than the previously recorded minimums in the North Atlantic, making the areas uninhabitable to all marine life other than certain microorganisms.
Coastal dead zones occur when the bacteria consume all the oxygen while having dead algae sinking towards the sea floor. Dr Karstensen said the deep ocean zones appeared to be operating in a similar manner.
He said a shallow layer on top of the swirling water supported “intense plant growth” similar to the coastal algal blooms. Bacteria in the deeper waters consumed the available oxygen as they decomposed the sinking plant matter. “The oxygen consumption within the eddies is some five times larger than in normal ocean conditions,” he said.
The findings have emerged amid growing concern that increasing ocean temperatures could lead to the expansion of some 400 known dead zones.