The Baltic Sea is brackish, a combination of fresh and salt water, and its balance is ensured as little water is exchanged. However, this can also create problems.

The last time a significant amount of saltwater came through the Danish Strait from the North Sea was in 2014, almost a decade ago. The new inflow was detected on December 20.

As the phenomenon is still gaining momentum, scientists do not know how big the impact will be. In 2014, the impact did not extend beyond the Swedish island of Gotland.

TalTech marine scientist Urmas Lips said several specific events must take place for a saline pulse to occur.

"First of all, easterly winds must last for a while, as a result of which the water level of the Baltic Sea becomes a little lower than the North Sea. If after that another good westerly storm arrives, which raises the water level in the Kattegat Strait, then a lot of oxygen-rich salt water will enter the Baltic Sea at the same time," Lips told ERR.

"The Baltic Sea is a very enclosed sea, and salt water only enters through the narrow Danish straits. The water changes in the upper layers and interlayer, but not near the bottom. The Baltic Sea is such that there is no oxygen near the bottom in deep areas. The only way to access oxygen is to bring in large amounts of salt water, which is dense enough to sink into the deeper regions and squeeze out or mix with the oxygen-free water," he explained.

The inflow is considered a necessary phenomenon as it allows oxygen to be replenished in the deepest parts of the Baltic.

"It has been 10 years since the last major inflow, and if one comes again, it will certainly temporarily improve the situation in the central Baltic. At the moment, it is only recorded in the southern part of the Baltic Sea and we do not yet know exactly what volume it will come in and whether it will reach the Gotland Basin and other Baltic basins. But it looks like it will," the scientist said.

A problem for the Gulf of Finland?

Although the saline pulse is generally considered to be a good thing for the Baltic Sea, Lips said its effect tends to be short-lived and some regions experience negative consequences. 

"During the 2014 inflow, the higher salinity water pushed the oxygen-depleted water northwards, where it reached Estonian waters and the Gulf of Finland. The opposite situation occurred, with the water in this area becoming much more oxygen-poor," the professor said. 

In 2014, it took nine months for the impact to reach the Gulf of Finland.

Lips said there are even worse long-term effects. Although the oxygen conditions in the central part of the Baltic Sea temporarily improve, in the long term the stratification of the water deepens and the mixing between the upper and lower water layers decreases. This means that oxygen moves much less between the different layers of water.

"In the long run, it is still the case that not much oxygen gets down there. The new oxygen that comes in runs out relatively quickly. And because the water is generally more stratified, storms and other processes do not get as much oxygen into the lower layers as they used to with less stratification. In many areas, such as the Gulf of Finland, it will still generally get worse for a while," Lips said.

More frequent inflows could help

TalTech marine researcher Taavi Liblik said the inflow's effect is generally positive until the Gotland Trench. 

"Measurements over the past decade have shown that the short-term – up to one year – effect from the south to the Gotland Trench is positive: oxygen depletion is reduced and phosphorus release from sediments is slowed down," Liblik said.

He said even a short-term positive effect was not visible in the Gulf of Finland in 2014.

More frequent pulses could alleviate the lack of oxygen, the scientist believes. "If there were several successive inflows over several years, the result could be positive," he told ERR.

Liblik said the role of humans cannot be excluded.

"High oxygen uptake in the bottom layers is linked to decades of excess nutrient discharges into the sea and the resulting excess primary production in the upper layers of the sea," he emphasized.

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