The average water level in the Baltic Sea in February this year was 67 cm below the norm. According to scientists, this was caused by prolonged easterly winds, which pushed water out through the Danish Straits. This water loss and the subsequent change in wind patterns allowed more oxygenated and saltier water from the North Sea to flow into the Baltic, which has a positive impact on the ecosystem. Experts stress, however, that this is not enough to solve the Baltic Sea’s long-term problems, including cyanobacterial blooms.
In February this year, the Institute of Oceanology of the Polish Academy of Sciences reported that the water level in the Baltic Sea had fallen by 67 cm compared with the norm. It was therefore the lowest level since 1886, when measurements began. Around 275 km³ of water flowed out of the Baltic, although scientists point to the very positive impact of this phenomenon.
“This year, we experienced a very rare phenomenon that is beneficial for the Baltic Sea. The level of the Baltic fell by 67 cm, which in itself was the result of very strong south-easterly winds blowing huge masses of water out of the Baltic through the very narrow Danish Straits into the North Sea and the Atlantic,” Marcin Kotlarek, President of the Race for the Baltic Foundation, told Newseria. “When such a large outflow occurs in the opposite direction, it is followed by what is known as a major Atlantic inflow, which compensates for the low oxygen levels in the Baltic and brings in fresh, saltier and more oxygenated water. This is a very positive phenomenon, occurring once every few years, and it will help the Baltic Sea.”
The Baltic Sea needs an inflow of saltier and more oxygenated water. Its low salinity results from the large inflow of freshwater from rivers — around 250 of them — combined with minimal exchange of saltwater with the North Sea. According to the Race for the Baltic Foundation, a full exchange of water in the basin takes around 30 years on average, meaning that pollutants remain there for decades.
“The Baltic is a specific sea because it is shallow. Its average depth is only 55 metres. By comparison, the Mediterranean Sea is 1,500 metres deep. The Baltic is also very enclosed,” Marcin Kotlarek noted.
The waters of the Baltic Sea are divided into three layers. The bottom layer is the saltiest, but it is poor in oxygen and in some areas completely deprived of it. The intermediate layer separates it from the surface layer, which is well oxygenated but less salty. Limited exchange with the North Sea means that oxygen-rich water only occasionally reaches the depths of the Baltic. When deep layers lose oxygen, so-called dead zones form, where marine life has no chance of survival. This is why the inflow of oxygenated water is important for the entire ecosystem.
“On the one hand, we have the positive effect of this inflow: more oxygen and more fresh water. On the other hand, we have climate change. This summer started very warm. The Baltic is the second-fastest-warming sea in the world, with a very significant increase in temperature over the past decade. Rising temperatures are something cyanobacteria benefit from greatly — they need warm water in order to bloom,” said the President of the Race for the Baltic Foundation. “I would like to say that there will be fewer cyanobacteria this year, but I fear that this inflow may not help.”
The first bathing areas on the Baltic coast were closed due to cyanobacterial blooms even before the peak holiday season began. The basin faces many environmental challenges, one of which is eutrophication. This is an excess of nutrients in the water, such as nitrogen and phosphorus, which promotes algal blooms, biodiversity loss and may further reduce oxygen levels in the sea. According to HELCOM, the Baltic Marine Environment Protection Commission, 94 percent of the Baltic Sea area is affected by eutrophication.
“When there is a constant inflow of pollutants, phosphorus and nitrogen, and the Baltic is additionally heated by warm water, this unfortunately favours cyanobacterial blooms. Therefore, the positive aspect of the inflow, which has already largely compensated for the water loss from the winter months, will unfortunately be offset by a larger bloom caused by warm water,” said Marcin Kotlarek.
In January this year, the German Federal Maritime and Hydrographic Agency in Hamburg reported that the average annual surface temperature of the Baltic Sea in 2025 was 9.7°C, an increase of 1.1°C compared with the long-term average for 1997–2021. Last year was therefore the second-warmest year since data collection began in 1990. Only 2020 was warmer.
