Deoxygenation of the Baltic Sea during the last century

Edited by David M. Karl, University of Hawaii, Honolulu, HI, and approved March 4, 2014 (received for review December 12, 2013)
March 31, 2014
111 (15) 5628-5633

Significance

Oxygen-deficient waters are expanding globally in response to warming and coastal eutrophication. Coastal ecosystems provide valuable services to humans, but these services are severely reduced with decreasing oxygen conditions. In the Baltic Sea, oxygen-deficient waters have expanded from 5,000 to over 60,000 km2 with large decadal fluctuations over the last century, reducing the potential fish yield and favoring noxious algal blooms. This increase is due to the imbalance between oxygen supply from physical processes and oxygen demand from consumption of organic material, enhanced by nutrient inputs and temperature increases. Further nutrient reductions will be necessary to restore a healthier Baltic Sea and counteract effects from warming.

Abstract

Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. We report a 10-fold increase of hypoxia in the Baltic Sea and show that this is primarily linked to increased inputs of nutrients from land, although increased respiration from higher temperatures during the last two decades has contributed to worsening oxygen conditions. Although shifts in climate and physical circulation are important factors modulating the extent of hypoxia, further nutrient reductions in the Baltic Sea will be necessary to reduce the ecosystems impacts of deoxygenation.

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Data Availability

Data deposition: Data have been deposited with the Baltic Nest Institute’s Baltic Environmental Database (BED), http://nest.su.se.

Acknowledgments

We thank the many people from research institutions and monitoring agencies who over the years have sampled and provided water quality data to the Baltic Environmental Database. Karsten Dromph and Cordula Göke helped with database and geographic information system programming. Bo Riemann is acknowledged for commenting on the paper. This study is a contribution from the HYPER project under the BONUS+ program and the TARGREV project under the Helsinki Commission. Funding for these projects was obtained from the European Community’s Seventh Framework Programme research program, the Danish and Swedish research councils, the Nordic Council of Ministers, and Aarhus University Research Foundation.

Supporting Information

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Supporting Information

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Information & Authors

Information

Published in

Go to Proceedings of the National Academy of Sciences
Go to Proceedings of the National Academy of Sciences
Proceedings of the National Academy of Sciences
Vol. 111 | No. 15
April 15, 2014
PubMed: 24706804

Classifications

Data Availability

Data deposition: Data have been deposited with the Baltic Nest Institute’s Baltic Environmental Database (BED), http://nest.su.se.

Submission history

Published online: March 31, 2014
Published in issue: April 15, 2014

Keywords

  1. biogeochemistry
  2. climate change

Acknowledgments

We thank the many people from research institutions and monitoring agencies who over the years have sampled and provided water quality data to the Baltic Environmental Database. Karsten Dromph and Cordula Göke helped with database and geographic information system programming. Bo Riemann is acknowledged for commenting on the paper. This study is a contribution from the HYPER project under the BONUS+ program and the TARGREV project under the Helsinki Commission. Funding for these projects was obtained from the European Community’s Seventh Framework Programme research program, the Danish and Swedish research councils, the Nordic Council of Ministers, and Aarhus University Research Foundation.

Notes

This article is a PNAS Direct Submission.

Authors

Affiliations

Jacob Carstensen1 [email protected]
Department of Bioscience, Aarhus University, DK-4000 Roskilde, Denmark;
Jesper H. Andersen
Department of Bioscience, Aarhus University, DK-4000 Roskilde, Denmark;
Bo G. Gustafsson
Baltic Nest Institute, Stockholm University, SE-106 91 Stockholm, Sweden; and
Daniel J. Conley
Department of Geology, Lund University, SE-223 62 Lund, Sweden

Notes

1
To whom correspondence should be addressed. E-mail: [email protected].
Author contributions: J.C. designed research; J.C., J.H.A., B.G.G., and D.J.C. performed research; J.C. analyzed data; and J.C., J.H.A., B.G.G., and D.J.C. wrote the paper.

Competing Interests

The authors declare no conflict of interest.

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    Deoxygenation of the Baltic Sea during the last century
    Proceedings of the National Academy of Sciences
    • Vol. 111
    • No. 15
    • pp. 5445-5754

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