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Other research product . 2018

Open ocean dead zones in the tropical North Atlantic Ocean

Karstensen, J.; Fiedler, B.; Schütte, F.; Brandt, P.; Körtzinger, A.; Fischer, G.; Zantopp, R.; +3 Authors
Open Access
Published: 27 Sep 2018

Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low (−1) oxygen environments in the open waters of the tropical North Atlantic, a region where oxygen concentration does normally not fall much below 40 μmol kg−1. 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, at about 4 to 5 km day−1, from their generation region off 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.

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Funded by
Fixed Point Open Ocean Observatories Network
  • Funder: European Commission (EC)
  • Project Code: 312463
  • Funding stream: FP7 | SP4 | INFRA
Gliders for Research, Ocean Observation and Management
  • Funder: European Commission (EC)
  • Project Code: 284321
  • Funding stream: FP7 | SP4 | INFRA
Changes in carbon uptake and emissions by oceans in a changing climate
  • Funder: European Commission (EC)
  • Project Code: 264879
  • Funding stream: FP7 | SP1 | ENV
Related to Research communities
European Marine Science Marine Environmental Science : Fixed Point Open Ocean Observatories Network
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