It was as far back as the year 2015 that a research work published in Biogeosciences warned of ocean dead zones swirling off Africa, with the threat to kill mass fishes along the coastlines.

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’ were discovered to be swirling off the coast of Africa in satellite photos.

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.

Six years after the publication of this report, some coastal communities including Axim in the Western Region and Osu in the Greater Accra region of Ghana woke up to the bizarre incident of fish mortality last weekend. At Axim alone, more than 60 dolphins were washed ashore. Also, large species of fish spotted in Osu were washed ashore, while, in Keta in the Volta Region, hundreds of dead tuna were washed ashore.

This has prompted government agencies and institutions to commence investigations into the immediate causes. One of the institutions that have so far conducted some research about the incident is the Marine and Fisheries Sciences Department of the University of Ghana.

The researchers have concluded that some fish that washed ashore last week was as a result of low oxygen concentration in the waters.

“Preliminary investigations conducted by the Marine and Fisheries Sciences Department revealed that most parameters required for life in the ocean were within acceptable limits, with the exception of Chemical Oxygen Demand (COD), which was significantly higher than expected. This oxygen demand would most likely create a condition of stress on living organisms that depend on dissolved in the water body”

The department’s press release says, it observed a sudden drop in sea surface temperature in the coastal waters from Cote d’Ivoire to Togo a few days to the washing ashore of the fish.

“This is most likely an indication of upwelled water from the bottom of the ocean, probably carrying low oxygen concentration,” per the researchers.

The statement signed by the head of department used the opportunity to appeal to government to procure a ferries geared towards monitoring marine for research purposes as global warming presents new challenges within the field.

Read below the article published in Biogeosciences in 2015

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.

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.

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.

(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.)
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.

Karstensen notes: “…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.