It may sound strange to many, but if the Baltic Sea is to do well, it needs the water from the North Sea. More precisely, the very oxygen-rich, more saline deep water. As an inland sea, which is also often shallow, the Baltic Sea has been suffering from increasing oxygen depletion for centuries. This occurs because the excessively nutrient-rich water forms a lot of algae in summer, which then die off later in the year, sink to the bottom and begin to rot there. This results in a lack of oxygen. This has been a huge problem in the Baltic Sea for years and has led to square kilometre-sized "death zones" at the bottom where life is no longer possible. This is also dramatically decimating fish stocks, such as cod.
This is why the Baltic Sea is biologically dependent on a regular, strong influx of cold, much saltier North Sea water. Especially at depth. This mixes and displaces the oxygen-poor water at the bottom.
Largest saltwater intrusion since 2014
It is precisely this process that the Leibniz Institute before Christmas, as reported by the BSH: " The autonomous measuring station Darßer Schwelle operated by the BSH has been measuring a strong inflow of salty water in the entire water column since 20 December 2023. This is one of the strongest saltwater intrusions since 2014. "
On top of this, long south-easterly winds prevailed before "Zoltan", which had previously pushed the water out of the Baltic Sea and significantly lowered the water level. The return current, fuelled by the hurricane, was all the stronger. This allowed oxygen-poor deeper basin areas such as the Gotland Basin to be aerated with oxygen. This at least temporarily prevented the formation of toxic hydrogen sulphide at the bottom.
For decades, marine biologists had observed that these inflow events had become fewer and weaker and that the formation of algae in the Baltic Sea was becoming increasingly dramatic at the same time because too many nutrients were being introduced from agriculture and wastewater. We can only hope that the current strong influx will perhaps improve the situation somewhat.