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Baltic Sea ‘dead zone’ development since 1969: IOW proviedes detailed map material based on long-term data


Oxygen minimum zones – deep water layers with so little dissolved oxygen that no higher life form can survive in these areas – are caused by intense microbial degradation processes of organic matter. These zones are a characteristic feature of the Baltic Sea. The hostile living conditions of the so-called ‘dead zones’ become even more pronounced, when toxic hydrogen sulphide is produced by bacterial activity. To visualise the spatial and temporal distribution and dynamics of these areas, the IOW regularly publishes maps in a unique collection of several hundered, thereby covering data of more than five decades.


Collection of maps of the dead zones, 1969 – 2021, information on the map material, and the PDF download of the 100th issue of the “Marine Science Reports”:

Dead zones and salt water inflows in the Baltic Sea – a natural interplay
Oxygen minimum zones are a natural feature of the Baltic Sea caused by its situation as an almost completely enclosed inland sea and its particular topography: Narrow straits as well as a highly structured sea floor with submarine sills and deep basins only permit very little water exchange with the much saltier North Sea. Furthermore, due to high freshwater runoff from the large catchment area with its many tributaries, outflow conditions into the North Sea are generally dominating. Only under a very specific wind regime the direction of the main current through the Danish Belt Sea gets reversed for a short period by so-called salt water inflows, which transport large amounts of high salinity water from the North Sea into the Baltic Sea. Here, a characteristic and very stable stratification develops due to the different densities of salt and fresh water: The low salinity surface water is well oxygenated because it is permanently and well mixed by wind. The much heavier high salinity water accumulates at the bottom of the deep basins, where it is not affected by surface mixing processes. Bacteria and other microorganisms use up the available oxygen in those deep water layers by degrading organic material sinking down from the surface, thus creating oxygen poor or even completely oxygen free conditions. If toxic hydrogen sulphide develops through further microbial processes, only highly specialized microorganisms can survive in these areas, which are therefore also called “dead zones”.

Map of the distribution of oxygen minimum and hydrogen sulphide polluted zones in the Baltic Sea, March 2016
Ideal basis for various research issues: the IOW maps showing the distribution of oxygen minimum and hydrogen sulphide zones in the Baltic Sea, here - as an example - for March 2016

Only salt water inflow events are capable to supply the deep areas of the Baltic Sea with dissolved oxygen as the inflowing water from the North Sea usually is oxygen saturated. Because of its high salinity it is heavy enough to form a layer directly at the sea floor. However, salt water inflows only reach the deep basins of the central Baltic Sea, if their volume is large enough to overflow the submarine sills. The essentially natural phenomenon of the oxygen minimum zones increasingly becomes problematic as sufficiently large inflow events have become rarer: While six to seven of such events could be registered per decade from the beginning of the records in 1880 to the middle of the 1980s, only three major Baltic inflows have reached the deep central basins in the last 30 years. Because of the longer pauses in between, the conditions of oxygen deficiency last longer and spread over a larger area. The problem is further aggravated by on-going eutrophication and climate change, as high nutrient levels and increasing water temperatures lead to an increase of algal growth and therefore to more biomass flux into the deep waters, where it increasingly stimulates oxygen depleting processes.

Visualisation of long-term data:

IOW publishes a comprehensive set of maps and will supply regular online updates
For many decades, IOW and its predecessor institute have been collecting hydrographical and physico-chemical data in the Baltic Sea several times per year at up to 60 stations. The cartographic processing of data on oxygen minimum and hydrogen sulphide zones for scientific purposes, too, has a long standing tradition: Since 1969 respective maps are being published yearly in the institute’s monograph series “Marine Science Reports”. To provide an overview of long-term changes, all available data sets from 1969 to 2015 have now been used to produce maps and have been published collectively in the 100th issue of the series. This allows a visual assessment of inflow events, of the extent of oxygen depleting processes and of the development of anoxic areas – with regards to seasonal dynamics as well as longer periods. To improve the rendering of the maps, a software-based method was developed to directly link the sources for high-resolution data of the Baltic Sea’s shoreline and its underwater topography with IOW’s monitoring databank. The new method is a transparent and reproducible way that provides a graphical representation of hypoxic and anoxic areas at the sea floor in a very good temporal and spatial approximation. The maps therefore are an ideal basis for various research issues, which depend on this kind of information.

For the timespan 1969 – 2015, the 100th issue of the “Marine Science Reports” provides three to five maps per year on the distribution of oxygen deficit and hydrogen sulphide polluted zones plus one additional map that sums up the changes within one year. Furthermore, the entire map collection, which is already available online, will be updated on a yearly basis with scientifically quotable new map material as a service for scientists and other experts.

For questions on the method for generating the maps, please contact:
Dr. Susanne Feistel | | Phone: +49(0)381 – 5197 456

For questions regarding the Baltic Sea hydrography, please contact:
Dr. Günther Nausch | | Phone: +49(0)381 – 5197 332
Dr. Michael Naumann | | Phone: +49(0)381 – 5197 267