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Biogeosciences An interactive open-access journal of the European Geosciences Union

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Biogeosciences, 12, 2597-2605, 2015
http://www.biogeosciences.net/12/2597/2015/
doi:10.5194/bg-12-2597-2015
© Author(s) 2015. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
30 Apr 2015
Open ocean dead zones in the tropical North Atlantic Ocean
J. Karstensen1, B. Fiedler1, F. Schütte1, P. Brandt1, A. Körtzinger1, G. Fischer2, R. Zantopp1, J. Hahn1, M. Visbeck1, and D. Wallace3 1GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
2Faculty of Geosciences and MARUM, University of Bremen, Bremen, Germany
3Halifax Marine Research Institute (HMRI), Halifax, Canada
Abstract. Here we present first observations, from instrumentation installed on moorings and a float, of unexpectedly low (<2 μmol kg−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.

Citation: Karstensen, J., Fiedler, B., Schütte, F., Brandt, P., Körtzinger, A., Fischer, G., Zantopp, R., Hahn, J., Visbeck, M., and Wallace, D.: Open ocean dead zones in the tropical North Atlantic Ocean, Biogeosciences, 12, 2597-2605, doi:10.5194/bg-12-2597-2015, 2015.
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This study is the first report of the formation of dead zones in the open ocean. A combination of multiple ocean observing system elements (mooring, floats, satellites, ships) allowed us to reconstruct the generation of the dead zones and to connect the formation to enhanced respiration within mesoscale ocean eddies. The dead zones present specific threats to the ecosystem, such as the interruption of the diurnal migration of zooplankters.
This study is the first report of the formation of dead zones in the open ocean. A combination of multiple ocean...
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