Fresh oxygen for the Baltic Sea

Measuring devices report the largest drop in oxygen-containing water in 60 years

Algal bloom in the Baltic Sea (July 2001). Overfertilization creates "death zones" with extreme oxygen deficiency. © SeaWiFS Project, NASA / Goddard Space Flight Center, ORBIMAGE
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Hope for the "death zones"? In December, the Baltic Sea received the largest influx of saltwater from the North Sea for 60 years - and with it, fresh oxygen. Such an influx is enormously important for the ecosystem of the Baltic Sea, but rarely occurs. Researchers hope that the large oxygen deficiency areas at the bottom of the Baltic Sea can now recover somewhat.

Oxygen is scarce at the bottom of the Baltic Sea in large areas: Due to the comparatively low salinity of the inland sea, there are very stable layers of water. As a result, the oxygen-rich surface water hardly mixes with the saltier water at depth. If oxygen consumption increases in the low-lying basins, for example due to over-fertilization, real dead zones can develop there: The resulting lack of oxygen hardly allows a higher life there.

Almost 200 trillion liters of salt water

These zones only receive replenishment when oxygen-containing salt water flows in through the narrow and shallow connection to the North Sea. However, these burglaries, which are important for the ecosystem, are rather rare. As a result of the over-fertilization of recent decades, the oxygen-depleted zones have grown considerably. From 13 to 26 December 2014, however, scientists from the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) measured the largest saltwater inflow of the last 60 years.

This event was only possible after long-lasting east winds, which initially drove a large amount of water out of the Baltic Sea. On 5 December 2014, the wind then turned in the opposite direction and continued unchanged for three weeks. The previously emanated water was replaced by a volume of almost 200 trillion liters of North Sea water, which flowed first through the Öresund, a little later by the Great and the Little Belt into the Baltic Sea. Since the beginning of oceanographic observations in 1880, this is the third largest measured saltwater intrusion in the Baltic Sea.

Oxygen for the deficient areas

In the coming months, the researchers want to investigate regularly how these water masses spread in the Baltic Sea and what effect they will have. Already now they know that the incoming water is very high saturated with oxygen. They therefore expect it to have a positive effect on the deficient areas in the Bornholm and Gotland Basins. On the 12th of January an expedition with the research vessel "Elisabeth Mann Borgese" will start in order to investigate this effect on site. The oceanographers assume that until then first parts of the injected salt water will have reached the western part of the Bornholm Basin. display

The fact that the scientists were able to measure the extent and course of the saltwater inflow in good time is due to a "early warning system": the Federal Maritime and Hydrographic Agency (BSH) operates the environmental monitoring network MARNET with several autonomous measuring stations. Three of the five stations stationed in the Baltic Sea are looked after by the IOW. This includes a station on the "Darsser Threshold" and another in the Arkona Basin, which delivered the first signals of the inflow on 12 December. Both stations measure around the clock temperature and salinity in different water depths and send their data by satellite to the IOW and the BSH.

(Leibniz Institute for Baltic Sea Research Warnemünde, 08.01.2015 - AKR)