Example of an ocean eddy (not from the study) as seen from space (Credit: NASA Earth Observatory)
We are aware of the dead zones along inhabited areas of coast line, particularly in the US East Coast Gulf of Mexico and the Baltic Sea. These dead zones are a result of fertilizer, oil (think BP's Gulf of Mexico oil spill) and other chemicals. No sea life can survive in them as the pollutants trigger massive algae blooms. Once the bloom dies it sinks to the sea bed and are consumed by bacteria that depletes the oxygen in the water. For the first time dead zones have been found in deep open water in the tropical Atlantic ocean.
The European Geosciences Union reports that a team of German and Canadian researchers have discovered dead zones hundreds of miles off the coast of Africa. These dead zones are created by large eddies and have a probability of encountering islands killing all the ocean life that the residents need to survive.
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,” explains 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. 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,” explains Karstensen. 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.”
The eddies studied in the Biogeosciences article form where a current that flows along the West African coast becomes unstable. They then move slowly to the west, for many months, due to the Earth’s rotation. “Depending on factors such as the [eddies’] speed of rotation and the plant growth, the initially fairly oxygenated waters get more and more depleted and the dead zones evolve within the eddies,” explains Karstensen. The team reports concentrations ranging from close to no oxygen to no more than 0.3 millilitres of oxygen per litre 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.