The Baltic Sea is running out of air

Oxygen deficiency in the Baltic Sea is ten times worse than 100 years ago

If the oxygen content on the seabed falls too low, only bacteria survive there. Here white sulfur bacteria form a veritable shroud. © Peter Bondo Christensen
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Death zones at the bottom of the sea: in the deep regions of the Baltic Sea, areas are spreading where water hardly contains any more oxygen and higher life is no longer possible. According to Scandinavian researchers, this ecological catastrophe has reached alarming proportions in the last 100 years. In their study in the journal "Proceedings of the National Academy of Sciences" they also mention the causes: over-fertilization and global warming.

The deep water layers of the Baltic Sea are a sensitive habitat: oxygen has always been in short supply here. Because fresh deep water only flows through a few accesses to this ocean and towering ribs on the seabed block this way in many places. In addition: Above the sinks with the low-oxygen deep water, a less salty layer of water blocks the exchange with the surface. If these zones remain small, living beings can avoid them. For in most cases only bacteria can survive in these "death zones".

Scientists from Denmark and Sweden have now examined these hostile zones on the Baltic Sea soil in more detail and followed their development. They come to an alarming conclusion: In just over a hundred years, the size of areas of extreme oxygen deficiency has increased more than tenfold. If around 5, 000 square kilometers were still around 1900, this is now a dead area of ​​60, 000 square kilometers. This is one and a half times the size of the land area of ​​Denmark.

Nutrients cause oxygen deficiency

In their research, the researchers also found the reason for the dramatic development: "We have analyzed data from the last 115 years on the content of oxygen in the water, temperature and salinity, " explains first author Jacob Carstensen from the University of Aarhus. He and his colleagues compared this data with data on nitrogen and phosphorus fluxes into the Baltic Sea. "Based on this analysis, we can see that the high nutrient inputs from land to sea are the main cause of widespread oxygen deficiency."

The main source of these nutrients is sewage, which is discharged into water and the sea, but also fertilizers from agriculture, which are then washed into the rivers with rain. This rich supply of food leads to over-fertilization of the seawater. Algae multiply uncontrollably. If they die off, they sink into the depths and are decomposed by microorganisms. The oxygen demand in this decomposition exceeds the replenishment, until the last remainder is used up. Because only a little supply of oxygen-rich water reaches the Baltic Sea and these sinks, they become an ever larger death zone. display

The figure shows areas of low oxygen (red) or zero oxygen (black) in the Baltic over the period 1906 to 2012. Aarhus University

Rising temperatures make death zones grow

The climate warming of the last 20 years also contributes further to the oxygen shortage, as the researchers report. At warmer temperatures, less oxygen is dissolved in the water. However, microorganisms grow better consumption continues to rise. And without human action, no improvement is in sight: "The water temperature is increased and will continue to rise in the coming years, " Carstensen emphasizes. "Therefore, it is all the more important that all countries commit themselves to the action plan for the Baltic Sea, to which they have joined and purposefully meet the necessary action to reduce nutrient input into the Baltic Sea."

In the suffocating regions, bacteria spread, which can survive without oxygen. Many of these microbes produce methane and toxic hydrogen sulfide. They transform the seabed into a desert. The resulting gases can also rise into higher water layers, thereby releasing toxic sediment from the seabed, as the researchers explain. So they also endanger the fish life in higher water layers the entire ecosystem is affected. Studies show that it can take decades for such a death zone to recover.

(Proceedings of the National Academy of Sciences (PNAS), 2014; doi: 10.1073 / pnas.1323156111)

(University Aarhus, 01.04.2014 - AKR)