Sometimes science sees something change – there is more of something, or less, or more importantly, there is a change in the rate of some phenomenon or in its pattern of variability. But sometimes science looks out there in the world and observes something that was probably there all along (though there may be changes in the past or future) but it just wasn’t noticed before.

There is a new study that describes and documents such a phenomenon. The thing we are talking about is over 100 kilometers across, several meters thick, moves at about 4.5 km/h, exists just below the surface of the ocean, can potentially be detected in satellite data as a very small change in sea level, there are probably a lot of them, and they are continuously forming, moving across certain parts of the sea, and disappearing.

Oh, and the are Neptunic Grim Reapers. They are potentially fatal to any animal that ends up inside them, and they may explain previously observed mass die-offs around oceanic islands.

Is this some kind of fish? A red tide gone amok? Glowing blobs of nuclear waste spreading from Fukushima? No, but they do remind me of a 1960s era but still extant musical group that frequently toured with Frank Zappa.

The phenomena is a dead zone that forms inside an eddy, as the eddy flows across the ocean’s surface. An eddy is a vortex with a non linear pattern of kinetic energy, which causes properties that might normally transcend the water in and near the eddy to be broken in to discrete areas. The eddy can have a very rich biota near its center, and that includes animals that respirate much of the oxygen into CO2, but fresh dissolved Oxygen does not transfer across the outer boundary of the eddy. When this happens, the eddy loses most of its dissolved Oxygen and you get a big flat round traveling dead zone. The dead zone isn’t very thick, and the ocean layer above it has O2 because it is directly interacting with the atmosphere. But since the eddy is caused by living breathing organisms, it obviously exists in a 3 dimensional space where living breathing organisms (mostly plankton) would normally be. From the abstract of Open ocean dead zones in the tropical North Atlantic Ocean by Karstensen, Fiedler, Schütte, Brandt, Körtzinger, Fischer, Zantopp, Hahn, Visbeck, and Wallace:

Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low … oxygen environments in the open waters of the tropical North Atlantic … The low-oxygen zones are created at shallow depth, just below the mixed layer, in the euphotic zone of cyclonic eddies and anticyclonic-modewater eddies. Both types of eddies are prone to high surface productivity. Net respiration rates for the eddies are found to be 3 to 5 times higher when compared with surrounding waters. Oxygen is lowest in the centre of the eddies, in a depth range where the swirl velocity, defining the transition between eddy and surroundings, has its maximum. It is assumed that the strong velocity at the outer rim of the eddies hampers the transport of properties across the eddies boundary and as such isolates their cores. This is supported by a remarkably stable hydrographic structure of the eddies core over periods of several months. The eddies propagate westward … from … the West African coast into the open ocean. High productivity and accompanying respiration, paired with sluggish exchange across the eddy boundary, create the “dead zone” inside the eddies, so far only reported for coastal areas or lakes. We observe a direct impact of the open ocean dead zones on the marine ecosystem as such that the diurnal vertical migration of zooplankton is suppressed inside the eddies.

“The simple appearance of dead-zones in an ocean that typically has relative high oxygen concentrations is a local “extreme environment” – and extreme environments can always help us to better understand how for example ecosystems have reacted in the past, or how quickly they can develop strategies to adapt,” lead author Johannes Kartensen told me. “This is to us an exciting question for future research.”

I asked Kartensen why these eddies had not been discovered before. He said, “I think that open ocean dead-zones have been “overlooked” in the past.” He showed me a map of “all historical oxygen observation at 50 m depth (core of the dead-zone we observe) available in the year 2005 (but dating back to the early 1900). Large areas can be seen that virtually have no (or maybe 1) observation – easy to miss a dead-zone eddy with 100km diameter.” Here’s the map:


There are an awful lot of dots on this map, but the spaces between them are huge. He added, “in the past oxygen was measured only a discrete depth and maybe at 24 points over a water depth of 4000m – say one data point every 200m – such a vertical resolution makes interpretation of the data challenging. Extreme anomalies resolved with one data point only have often be disregarded, as an ‘outlier.’ In fact, we also disregarded the data from the first observation in 2007 as an outlier and only after repeating observations considered the data to be real.” He also noted that this also happened with the eddies themselves. “before the satellite era it was not expected that the ocean is populated by eddies and that the ocean is such a “turbulent” place.”

Now, what about the death thing? I would like to know, ultimately, how much organic material is contributed to deeper water components of the carbon cycle, or deposited in the deep ocean, by these eddies ocean wide. Clearly more research has to be done to establish the distribution of the dead zone eddies, as well as variation in their occurrence across space and time. Meanwhile, there is the current but seemingly low-probability thread of a deadly eddy flowing into the near shore environments of oceanic islands. Since they are over 100 km long and move at a rate of a few km a day, such a dead zone could have a huge impact on the local marine ecology. From the paper:

Eddies were observed less than 100 km north of the Cabo Verde islands; thus a possible interaction of a dead zone eddy with an island must be considered. Given the shallow depth of a few tens of metres where lowest [dissolved Oxygen] concentrations are found, a sudden flooding of a coastal areas with [low oxygen] waters may occur. A dramatic impact on the local ecosystems and sudden fish or crustacean death may be the consequence. In retrospect, such eddy–island interactions may explain events that have been reported in the past.

I look forward to more research into these eddies. Meanwhile, this should keep us entertained:


  1. […] data indicates the oceans are full of eddies that travel with the major currents, creating areas of oxygen depleted water, possibly hazardous to […]