Breathlessness under the keel: Why the Baltic Sea is running out of air at depth

The probability of a massive influx of oxygen into the Baltic Sea was 80 per cent at the beginning of 2026. Dr Michael Naumann from the Leibniz Institute for Baltic Sea Research Warnemünde (IOW) explains why this did not happen. An insight into the invisible processes from the depths of the Baltic Sea.

At the beginning of the year, the media reported that a major oxygen exchange was imminent in the Baltic Sea. What is it all about?

A lot of water has flowed out of the Baltic Sea this winter. This means that easterly winds have pushed low-oxygen Baltic Sea water into the Atlantic over a long period of time. That's why on 5 February we had a historic low water. Minus 67 centimetres compared to the average water level is a historic low. This is actually the perfect starting position for oxygen-rich and saltier water to subsequently flow in from the North Atlantic.

In view of the data on similar inflow scenarios in the past, the probability of a major saltwater intrusion was assumed to be 80 per cent. Saltwater inflows from the North Sea in turn supply the oxygen, which then reaches the deeper regions of the Baltic Sea in particular. Saline water is basically the transport company for the oxygen from the North Atlantic to the deep waters of the Baltic Sea.

What can you tell us now, has there been a large influx of oxygen?

No! We can clearly see that from our data. We actually had a small inflow. Small amounts of oxygen-rich water with a high salt content entered the Arkona Sea, north-east of Rügen, via the Öresund. Unfortunately, this was nowhere near enough water volume for a major event, but rather a saltwater inflow of low to medium intensity. Over the next few weeks, it will be interesting to see whether the inflow water can reach the more easterly regions of the Baltic Sea.

Why did that fail?

There are several factors at play here. Basically, you can say that it failed because there was no westerly wind. After we had this strong easterly wind in January and February, the weather calmed down. It would have been normal for us to have our typical north German 3 to 5 degrees, cold and wet with wind. This weather often occurs when low-pressure systems from the North Atlantic move across northern Europe and create westerly winds. Ultimately, that would have been the weather we needed, and for several weeks.

What other factors are there?

In order to understand why long-lasting westerly winds are needed to push salty and oxygenated water from the Atlantic into the Baltic Sea, it is important to know what the seabed of the Baltic Sea looks like. It is basically divided into a chain of successive basins, such as the Arkona, Bornholm and Gotland basins. The elevations between them, also known as sills, prevent heavy salt water from flowing unhindered along the bottom into the deeper regions of the Baltic Sea. Examples include the Darss Sill and the Stolperrinne.

Accordingly, a large volume of water is needed to overcome these thresholds. If you look at the depth profile of the Baltic Sea from the side, you will see these sills again and again. You can visualise it like the edges of a bathtub: we take bathtubs, put them all next to each other and the water always has to flow from one bathtub into the other. To do this, I need enough volume so that the water can flow over the edges of the bathtub.

Where are the biggest hurdles to overcome?

There are several. One large bath rim in particular is the threshold between the Bornholm pool and the Stolper Rinne. This is the so-called Stolper Schwelle. It is 58 metres deep. However, the Bornholm Basin further west has a water depth of almost 95 metres. It takes around 170 cubic kilometres of inflow water to overcome this 37 metre difference. Such quantities of water are not pushed over the sills of the western Baltic Sea by rising winds within a day and then sink into the deep water of the Arkona Basin. In total, we need about six days of inflow and water with a salt content of at least 17 grams per kilogramme of salinity at the Darss Sill. Unfortunately, this has not been possible in the weather conditions in recent months.

Could there be more frequent natural events such as fish kills in the future if the oxygen exchange continues to fail?

You have to be careful, these are correlations that do not fit together directly. In general, the water in the Baltic Sea is divided into two layers. One is the surface water, which tends to have a low salt content, and the deep water, which is richer in salt and sinks due to its higher density. Oxygen is constantly introduced into the surface water via the atmosphere, for example through wind and temperature mixing.

Water is transported from the top to the bottom within the surface water at the water surface, which heats up and cools down over the course of the year. This works like a heating circuit in a house or in a freshwater lake, in which water is also shifted between the surface and the bottom via temperature changes. In the Baltic Sea, however, this process stops at the saline layer; the deep water cannot be aerated beyond this limit and is dependent on sporadic inflows of salt water from the North Sea. And that is the important distinction.

A fish kill off the coast is linked to a different process. This occurs in late summer when the oxygen at the bottom in the shallower sea regions is depleted and the surface water is pushed away from the coast by rare wind events. In this case, the deep water, which is low in oxygen at this time of year, can rise to the surface for a short time, thus restricting the habitat for breathing organisms.

So the low-oxygen deep water of the central Baltic Sea basins does not reach the surface at all?

Correct. The saline layer is a strong boundary between surface and deep water, we also call it a halocline. It means that there is no gas exchange between surface water and deep water. For example, the halocline in the Bornholm Basin is around 50 metres deep and in the Gotland Basin around 70-80 metres. As long as we continue to have sufficient rainfall in our climate zone and fresh water is carried from the rivers and lakes into the Baltic Sea, the thermocline remains in the same place and the oxygen-poor water cannot rise any further or reach the coasts.

So the phenomena that we perceive as sailors are mainly related to the nature of the surface water?

Yes, unfortunately the oxygen deficiency in the deep Baltic Sea is often lumped together with the shallow-water oxygen deficiency near the coast, although they don't have much to do with each other per se. In fact, the oxygen content stagnates below a water depth of 70 metres. However, the oxygen deficiency in the deep basins has nothing at all to do with the coastal and shorter-term processes, but is naturally determined by the characteristics of the Baltic Sea.

When people talk about a major fish kill in the context of an oxygen deficiency in the Baltic Sea, they are referring to the deep waters close to the coast. If there is a lack of oxygen in shallow waters in the Baltic Sea, this is a local phenomenon and has nothing to do with the larger areas in the central Baltic Sea basins. This should not be confused.

How has the trend in Baltic deep water developed in recent years?

The oxygen deficit is becoming increasingly apparent in the deep Baltic Sea in particular. However, the area affected by this is not changing. It varies to a certain extent, but the maximum area of 70,000 - 80,000 square kilometres was already reached in the 1970s. This is due to the cover of the salt layer. As long as it rains enough and the surface water layer does not become thinner, nothing will change.

What we have been able to observe, however, is the approximate doubling of oxygen depletion rates over the last 30 years. This means that the same amount of oxygen that reaches the deep Baltic Sea today is being used up twice as quickly as before. One of the reasons for this is the large amount of undecomposed organic matter deposited on the seabed, which leads to high oxygen consumption as a result of decomposition processes. These are the consequences of the high nutrient inputs from us humans into the sea via the river systems.

Since the 1940s, more and more fertilisers have been used in agriculture that are not completely absorbed by plant growth. The surplus fertilisers find their way into river waters via the groundwater and then into the sea. There, the increased supply of nutrients stimulates plankton growth in the surface water. This occurs cyclically from spring and dies off again in autumn. Dead plankton then trickles to the bottom and has to be decomposed. This process consumes a lot of oxygen.

What is being done to combat the influence of humans?

Since the mid-1970s, the Helsinki Commission has been in existence as an association of all countries bordering the Baltic Sea, which endeavours to improve the environmental status of the Baltic Sea. It provides analyses for the political level on which environmental measures are based. Indicators are developed together with us in the scientific community and each neighbouring state has its own voice there.

What has been achieved since then?

As part of the Helsinki Commission's guidelines, nutrients have been massively reduced in agriculture since the early 1990s. This is where agricultural science meets marine research. If we manage to ensure that only as much fertiliser is applied as the plants can fully absorb, nothing would end up in the groundwater via the soil. If we look at ecosystem models driven by the measurement data and assume the implementation of environmental measures, then we will see the first effects from around 2050. We must therefore continue to stick to the reduction of nutrients.