The inflow of saline water from the North Sea to the Baltic has begun

The negative trend of declining water levels in the Baltic Sea is reversing. According to scientists from the Institute of Oceanology of the Polish Academy of Sciences, water is starting to flow into our sea from the North Sea – cold, saline and highly oxygenated. What does this mean for the natural environment?

A marine system of connected vessels

In February, we reported in Wodne Sprawy on a record-low level of the Baltic Sea – a reading taken at the Swedish gauge in Landsort-Norra showed a level 67 cm below the long-term average, the lowest in the 140-year history of measurements. According to scientists from IO PAN, the cause of this situation was the prevailing easterly winds since the beginning of the year, which pushed more than 270 km3 of water through the Danish Straits into the North Sea.

Dr Tomasz Kijewski from IO believes that the drop in the Baltic Sea level was linked to ongoing climate change, more specifically to the breakdown of the polar vortex caused by Arctic warming. A stable high-pressure system formed over the North Sea, and the lack of atmospheric fronts facilitated the outflow of water.

The situation is beginning to reverse

According to scientists from the Leibniz Institute for Baltic Sea Research Warnemünde (IOW), drops in Baltic Sea levels of more than 20 cm below the mean sea level create favorable conditions for major inflows of saline water from the North Sea. As predicted, a change in wind direction has reversed the vector of water movement – to the benefit of our sea.

Data released by IO PAN indicate that the first signs of the inflow were observed as early as 17 February. In the Arkona Basin, bounded by the Danish islands and the southern coast of Sweden, advection was recorded, that is the horizontal transport of saline water from the west. It was triggered by a large difference in water levels between the Baltic and the North Sea, known as the barotropic gradient.

The inflow intensified in the second half of March. The latest measurements taken in the Arkona Basin indicate a strong inflow of highly saline water, which is a natural result of the equalization of levels between the two seas. Cold, saline and well-oxygenated water has already reached the Bornholm Basin and is now moving towards the deeper parts of the Baltic.

Why are inflows from the North Sea so important for the Baltic?

Data collected by IOW show that major inflows to the Baltic (MBI, Major Baltic Inflows) exhibit significant multi-decadal variability – in some periods they are very rare, in others they intensify. In 2014, an exceptionally strong inflow occurred, when 320 km3 of water from the North Sea passed through the Belt and Sound straits. Only in 1922 and 1951 was transport on a larger scale observed.

Oceanographers from the Institute of Oceanology of the Polish Academy of Sciences, who conducted research cruises in the Baltic in 2014, found a significant increase in salinity in the Gdańsk Deep. In the Bornholm Basin, record salinity levels of around 20 PSU units and high oxygen concentrations (10 mg/l) were confirmed. Why is this important? Oxygen is necessary for the regeneration of the Baltic depths, where the decomposition of organic matter leads to the formation of anoxic zones and the release of hydrogen sulfide, while the salt contained in ocean water supports Baltic ecosystems. Without inflows from the North Sea, the deep waters of the Baltic would quickly become a biological desert, according to IO PAN scientists.

Unfortunately, the inflow from the North Sea will not solve all the problems of our semi-enclosed basin. The key issue remains eutrophication, which leads to the formation of dead zones and the destruction of underwater life. According to IOW experts, the 2014 inflow removed only about 30 percent of phosphates from the Baltic, while another 5 percent was permanently buried in sediments.