Deoxygenation of the Baltic Sea during the last century

Die toten Zonen in der Ostsee haben sich von 5000 Quadratkilometern vor 110 Jahren auf heute 60.000 Quadratkilometer durch Düngermitteleintrag und Klimawandel verzehnfacht.

Oxygen-deficient waters are expanding globally in response to warming and coastal eutrophication. Coastal ecosystems pro- vide valuable services to humans, but these services are se- verely 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 coun- teract effects from warming.


PNAS Proceedings of the National Academy of Sciences of the United States of America


Jacob Carstensena,1, Jesper H. Andersena, Bo G. Gustafssonb, and Daniel J. Conleyc
aDepartment of Bioscience, Aarhus University, DK-4000 Roskilde, Denmark; bBaltic Nest Institute, Stockholm University, SE-106 91 Stockholm, Sweden; and cDepartment of Geology, Lund University, SE-223 62 Lund, Sweden
Edited by David M. Karl, University of Hawaii, Honolulu, HI

Freely available online through the PNAS open access option.


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 warm- ing 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 stratifica- tion 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.