Oxygen – it’s pretty important, even for fish who live in the ocean. That’s why scientists from Germany and Canada publishing a study in the European Geosciences Union’s journal Biogeosciences were shocked to find large eddies of water with the lowest levels of oxygen ever recorded in the Atlantic off the coast of Africa. What’s worse, these “dead zones” – uninhabitable to virtually all marine life – are moving west, where they have the potential to ravage sea life populations.
“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,” says lead-author Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Centre for Ocean Research Kiel, in Kiel, Germany.
Earlier research had begged the minimum oxygen level at about one millilitre of dissolved oxygen per litre of seawater. That’s still awfully low, but survivable by most fish. The new study, however, found oxygen levels in the eddies to be 20 times lower, an environment in which practically nothing can survive.
Most dead zones are found in coastal areas. Fertilizers and other chemicals are carried out into river deltas and oceans, which allow algae to bloom. When these algae die they’re consumed by bacteria, and that metabolic process causes the bacteria to consume the water’s oxygen stores. Tides and currents disperse the de-oxygenated water out into the ocean, but until now, a concentrated eddy of low-oxygen water had not been observed in the open ocean.
An eddy is better known as a whirlpool, and these occur on a massive scale. Those observed in the study were 100 to 150km wide, and ran several hundred meters deep. The dead zones themselves were found in the upper-most 100 meters, Karstensen said. That whirlpool action is what allows these eddies to become – and remain – so low 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.”
Due to Earth’s rotation, the eddies make their way westward, slowly, over the course of several months. If they do manage to encounter a body of land, the results could be disastrous. Shallow waters tend to be even more oxygenated than deeper ocean water, making it an ideal environment for the kind of fish that island economies depend on – the shallow waters around and just below the dead zones, for instance, is up to 100 times more oxygenated. Already, the researchers have observed zooplankton changing their behavior: Where they normally only come to the surface at night to feed, they choose to remain at the surface of the eddies rather than descend into the lower-oxygen water below. Zooplankton are a big part of the marine food web, and staying at the surface in daylight makes them easy pickings for larger marine life.
