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Biogeosciences An interactive open-access journal of the European Geosciences Union
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Volume 12, issue 24 | Copyright

Special issue: Hydrography, biogeochemistry, and biology of "dead-zone eddies"...

Biogeosciences, 12, 7467-7482, 2015
© Author(s) 2015. This work is distributed under
the Creative Commons Attribution 3.0 License.

Research article 21 Dec 2015

Research article | 21 Dec 2015

Hidden biosphere in an oxygen-deficient Atlantic open-ocean eddy: future implications of ocean deoxygenation on primary production in the eastern tropical North Atlantic

C. R. Löscher1,2, M. A. Fischer1,*, S. C. Neulinger1,*, B. Fiedler2, M. Philippi1, F. Schütte2, A. Singh2,a, H. Hauss2, J. Karstensen2, A. Körtzinger2,3, S. Künzel4, and R. A. Schmitz1 C. R. Löscher et al.
  • 1Department of biology, Institute for General Microbiology, Kiel, Germany
  • 2GEOMAR, Helmholtz Centre for Ocean Research Kiel, Kiel University, Kiel, Germany
  • 3Christian-Albrechts-Universität zu Kiel, Kiel, Germany
  • 4Max Planck Institute for Evolutionary Biology, Plön, Germany
  • anow at: Physical Research Laboratory, Geosciences Division, Ahmedabad, 380 009, India
  • *These authors contributed equally to this work.

Abstract. The eastern tropical North Atlantic (ETNA) is characterized by a highly productive coastal upwelling system and a moderate oxygen minimum zone with lowest open-ocean oxygen (O2) concentrations of approximately 40 μmol kg−1. The recent discovery of re-occurring mesoscale eddies with close to anoxic O2 concentrations (< 1 μmol kg−1) located just below the mixed layer has challenged our understanding of O2 distribution and biogeochemical processes in this area.

Here, we present the first microbial community study from a deoxygenated anticyclonic modewater eddy in the open waters of the ETNA. In the eddy, we observed significantly lower bacterial diversity compared to surrounding waters, along with a significant community shift. We detected enhanced primary productivity in the surface layer of the eddy indicated by elevated chlorophyll concentrations and carbon uptake rates of up to three times as high as in surrounding waters. Carbon uptake rates below the euphotic zone correlated to the presence of a specific high-light ecotype of Prochlorococcus, which is usually underrepresented in the ETNA. Our data indicate that high primary production in the eddy fuels export production and supports enhanced respiration in a specific microbial community at shallow depths, below the mixed-layer base. The transcription of the key functional marker gene for dentrification, nirS, further indicated a potential for nitrogen loss processes in O2-depleted core waters of the eddy. Dentrification is usually absent from the open ETNA waters.

In light of future projected ocean deoxygenation, our results show that even distinct events of anoxia have the potential to alter microbial community structure with critical impacts on primary productivity and biogeochemical processes of oceanic water bodies.

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The waters of the tropical Atlantic Open Ocean usually contain comparably high concentrations of oxygen. Now, it became clear that there are watermasses related to eddies that are nearly anoxic. We surveyed one of those eddies and found a biosphere that largely differed from the usual biosphere present in this area with a specific community responsible for primary production and for degradation processes. Further, we found the very first indication for active nitrogen loss in the open Atlantic.
The waters of the tropical Atlantic Open Ocean usually contain comparably high concentrations of...