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The world ocean is now a region of expanding oxygen-deprived dead zones.
It’s an upshot of a human-warmed ocean system filled with high nutrient run-off from mass, industrialized farming, rising atmospheric nitrogen levels, and increasing dust from wildfires, dust storms, and industrial aerosol emissions. Warming seas hold less oxygen in solution. And the nutrient seeding feeds giant algae blooms that, when they die and decompose, further rob ocean waters of oxygen. Combined, the two are an extreme hazard to ocean health — symptoms of a dangerous transition to stratified, or worse, Canfield Ocean states.
(Geographical extent of more than 405 coastal dead zones worldwide. New dead zones discovered by scientists are now traversing mid-ocean regions. Image source: No Fish Left.)
In total, more than 405 dead zones now occupy mostly coastal waters worldwide. Covering an area of 95,000 square miles and expanding, these anoxic regions threaten marine species directly through suffocation or indirectly through the growth of toxin-producing bacteria which thrive in low-oxygen environments.
Mobile, Anoxic Underwater Cyclones
Now, according to new research published in Biogeosciences, it appears that some of these dead zones have gone mobile.
The report finds zones of very low oxygen covering swirls of surface water 100-150 kilometers in diameter and stretching to about 100 meters in depth. The zones churn like whirlpools or eddies. Encapsulated in their own current of water with oxygen levels low enough to induce fish kills, these ‘dead pools’ have been discovered swirling off the coast of Africa in recent satellite photos.
The ‘dead pools’ form as strong ocean eddies break off from West African ocean currents. The eddies create mixing environments near the surface which fuels algae blooms (seen as the light blue coloration in the image below). Large algae blooms are trapped in the eddy and as the algae die, they rob the water column of oxygen. The flows of the eddy form as a kind of wall to mixing with higher oxygen surrounding waters. As a result, the oxygen readings within the dead pool plummets.
(Newly discovered ocean dead pools like the one shown above are propagating off the coast of West Africa. These eddies are mobile dead zones of low oxygen water. A new phenomena, they represent a unique threat to ocean health in addition to the 405 other, mostly stationary, dead zones in the world’s coastal waters. Image source: Biogeosciences.)
According to lead-author Johannes Karstensen, a researcher at GEOMAR, the Helmholtz Centre for Ocean Research Kiel, in Kiel, Germany:
“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. From our measurements, we estimated that the oxygen consumption within the eddies is some five times larger than in normal ocean conditions.”
Researchers found levels in these swirls to be less than 0.3 millilitres of oxygen per litre of seawater or about 1/100th the oxygen content of surrounding ocean. These are readings low enough to produce mass fish kills and to support production of toxin-producing bacteria harmful to oxygen dependent life.
Azores Downrange of Dead Pools
The zones were observed moving through the Tropical North Atlantic west of Africa. They propagated toward the north and west, finally petering out about 100 kilometers north of the Azores. This puts that East Atlantic archipelago directly in the line of fire of these new, low-oxygen eddies. A cause for concern. If one of these eddies were to enter the Azores the result could be a massive fish die off around the island chain.
“…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.”
Observations of these dead pools seems to indicate they are a new event. A possible result of nutrient enrichment of the surface waters in West African currents due to increased run-off or surface water nitrogen and dust seeding. As extreme rainfall events related to climate change wash more sediment down rivers and into the oceans, as more nitrogen compounds and particulate matter hit the atmosphere due to fossil fuel emissions, wildfire burning, and dust storms, and as sea level rise starts to flood nutrient-rich low lying areas, it is possible that the Tropical Atlantic dead pools represent an emerging ocean state that will grow more prevalent as time moves forward.