Open ocean dead zones in the tropical North Atlantic Ocean.

Karstensen, Johannes , Fiedler, Björn, Schütte, Florian, Brandt, Peter , Körtzinger, Arne , Fischer, G., Zantopp, Rainer J. , Hahn, Johannes , Visbeck, Martin and Wallace, Douglas W.R. (2015) Open ocean dead zones in the tropical North Atlantic Ocean. Open Access Biogeosciences (BG), 12 . pp. 2597-2605. DOI 10.5194/bg-12-2597-2015.

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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.

Document Type: Article
Additional Information: WOS:000353840500021
Keywords: OXYGEN MINIMUM ZONES; MESOSCALE EDDIES; SYSTEMS; EDDY; PERU
Research affiliation: OceanRep > SFB 754 > A3
OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > SFB 754
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-CH Chemical Oceanography
OceanRep > SFB 754 > A4
OceanRep > SFB 754 > B9
OceanRep > The Future Ocean - Cluster of Excellence > FO-R10
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
OceanRep > SFB 754 > B8
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-12-2597-2015
ISSN: 1726-4170
Projects: CVOO, CARBOOCEAN, CARBOCHANGE, FixO3, SOPRAN, AWA, SFB754, GROOM, Future Ocean
Date Deposited: 15 Dec 2014 14:42
Last Modified: 23 May 2019 08:46
URI: http://oceanrep.geomar.de/id/eprint/26507

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