Oxygen levels in some parts of the Baltic Sea have improved, particularly in the Gulf of Finland. However, in other areas, such as the Baltic Sea’s main basin and the Bothnian Sea, oxygen conditions remain poor.
The annual research cruise of the marine research vessel Aranda passed through the Gulf of Finland, the northern Baltic Sea and the Gulf of Bothnia. The voyage is carried out within the framework of the EU Marine Strategy Framework Directive and the monitoring program developed by the Baltic Marine Environment Protection Commission. Data collected during the expedition included oxygen, salinity and temperature stratification of the sea, and nutrient levels.
The results show that oxygen levels in the Gulf of Finland, from bottom to surface, are the best in five years. Conditions were good from the eastern part of the Gulf all the way to the Hanko headland. Oxygen-free water with hydrogen sulphide was only detected on the north-western coast of Estonia.
Oxygen levels were also high in the Archipelago Sea. Storm Jari and unusually high temperatures helped to mix the water from the surface to the bottom.
Worse in some places
Scientists generally believe that the improvement was not due to human intervention, but to the weather. The winters of 2020 and 2025 had similar weather patterns, with prolonged westerly winds pushing new water into the Gulf of Finland.
The latest data is not entirely positive either, with a deterioration in oxygen levels in the deep areas of the Bothnian Sea.
At the same time, the situation in the main basin of the Baltic Sea remains unchanged. Oxygen-free water is found from 80-90 meters depth, and the oxygen-poor area extends all the way from the Bornholm Deep to the northern parts of the main basin.
– The oxygen-free area in the main basin now measures nearly 50,000 square kilometers,, said researcher Pekka Kotilainen in a press release.