Spread of the death zones in the Baltic Sea

Climate change promotes the spread of Baltic Sea death zones

07/02/2012

The low-oxygen zones of death in the Baltic Sea will presumably continue to expand significantly as a result of climate change, according to a recent study by the Leibniz Institute for Baltic Sea Research Warnemünde (IOW). Because the expected increase in temperature has a significantly greater impact on the formation of the livelier underwater areas, than previously thought.

Above all, the excessive intake of nutrients was previously suspected as the cause of the formation of so-called dead zones in the Baltic Sea. Overfertilization of the fields leads to too many nutrients in the rivers, the latter transport the residues of the fertilizer towards the Baltic Sea, which here massive algae growth is favored. As a result, the oxygen content of the sea sometimes drops so drastically that no life is possible in the affected areas, according to the previous theory. But temperature, according to IOW, plays a far greater role than previously thought. According to the results of the current study, the intake of nutrients would not be a comparable problem if the temperatures remained constant or fell. Rising temperatures, however, promote the rapid spread of the death zones.

Influence of climatic fluctuations on the dead zones of the Baltic Sea
According to IOW is the „Baltic Sea a stable stratified inland sea“ with light fresh water from the rivers at the surface and heavy salt water at greater depths. The exchange between the two areas is severely limited. „Only when oxygen-rich North Sea water passes through the Danish straits into the Baltic Sea, the deep water ventilated and refreshed with new O2“, explain the researchers of the IOW. However, these saltwater infiltrations from the North Sea are rather rare, which is why „Oxygen-deficient or even oxygen-free conditions are regularly found in wide areas of the deep Baltic Sea“ prevail, according to the IOW. In these „dead zones“ according to the researchers no higher life is possible anymore. In their interdisciplinary study, the scientists of the IOW have now investigated, „which influence climatic fluctuations in the past on the dissemination of dead zones in the Baltic Sea.“ For this purpose, they analyzed several sediment cores that were recovered, inter alia, from the particularly deep area of ​​the Gotland Basin. „Because sediments at the bottom of the ocean store the conditions prevailing in the water at the time of their deposition as an archive, the scientists were able to reconstruct the last 1,000 years in the history of the Baltic Sea“, reports the IOW in its latest release.

Clearly recognizable sediment layers Indication of missing life in the warm phases
As the first author of the study, Karoline Kabel, explained, are reflected „In the reconstructed temperature curve for the last 1,000 years, the Medieval Warm Period, the Little Ice Age and the recent warming since 1900 are clearly reflected.“ During the medieval warm period in the years 950 to 1250 AD, the average summer temperature of the Baltic Sea water, according to the researchers with more than 16 degrees Celsius on a similar level as today (about 17 degrees Celsius). „Exactly in the warm periods we find in our cores a clearly recognizable stratification“, stressed Karoline Kabel. According to the expert „This undisturbed deposit is a sure sign of low-oxygen ocean floor conditions“, otherwise the stratification would have been disturbed by multicellular organisms ransacking the soil. There are no such creatures in the death zones, so that the individual sediment layers remain clearly recognizable. Conversely, it can be assumed that there were already larger death zones in the Baltic Sea during the medieval warm period.

In the cold period, the spread of the death zones decreased
During the „Little ice age“ Between the 15th and 19th centuries, the reconstructed temperature curves of the IOW study show summer values ​​for Baltic Sea waters of around 15 degrees Celsius. The sediment cores from this period, according to the researchers, a homogeneous structure, which speaks for the presence of multicellular organisms. „There was obviously enough O2 in the deep water and higher life possible“, according to the researchers. The results of her study have Karoline Kabel and colleagues in the journal „ Nature Climate Change“ under the title „Effects of climate change on the ecosystem of the Baltic Sea over the past 1,000 years“ released. The study also uses different computer models to show the redrawing of death zones from the 19th century to their present state. In addition, the researchers calculated possible effects of increased nutrient input during the cold period to check whether it had comparable effects, as in the today's warm season. Kabel and colleagues found that the massive nutrient input was indeed in the „Little ice age“ would have led to the formation of death zones, but these would have been much smaller than in the warm periods.

Cyanobacteria give rise to the origin of the death zones?
The „connecting mechanism of action between surface water temperature and oxygen depletion in depth“ According to the researchers, they are probably producing so-called cyanobacteria. This mistakenly often called „blue-green algae“ called bacteria, can become „in the Baltic Sea only multiply at a temperature higher than 16 degrees Celsius and calm wind conditions and form the characteristic flowers that we can see in warm summers“, explained Karoline Kabel. In the sediment cores of „In warm periods, we find a lot of organic carbon, indicating, among other things, increased biomass production“, so cable on. The organic material formed by the flowering of cyanobacteria sinks to the bottom and is degraded at the expense of oxygen at depth, especially by other bacteria, the expert explained. Thus, the cyanobacteria give rise to the direct connection between long-term temperature fluctuations and the spread of so-called „dead zones“ in the deep Baltic Sea. In the opinion of the IOW, this suggests that the death zones will significantly increase in the future, „because all major global climate models expect rising temperatures over the coming decades.“ (Fp)

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Picture: Andreas Hermsdorf