The oxygen situation in the Baltic Sea 2024

Report - The oxygen situation in the Baltic Sea 2024

In 2011, SMHI published Report Oceanography No. 42, a climatological atlas detailing the oxygen status in the deep waters of the Baltic Sea. Since then, annual updates have been released as new data are reported to the International Council for the Exploration of the Sea (ICES) data center. This report provides an update for 2023 and presents the preliminary results for 2024. The oxygen data for 2024 were
collected from various sources, including ICES coordinated trawl surveys, national monitoring programmes, and research projects involving Poland, Estonia, Latvia, Denmark, Sweden, and Finland.

For the autumn period, each profile in the dataset was analyzed for the occurrence of hypoxia (oxygen deficiency) and anoxia (total absence of oxygen). The depths of onset of hypoxia and anoxia were then interpolated between sampling stations to produce two surfaces that represent the depths at which hypoxic and anoxic conditions are present, respectively. The volume and area of hypoxia and anoxia were then calculated and the results transferred to maps and diagrams to visualize the annual autumn oxygen situation during the analyzed period.

The updated results for 2023 and the preliminary results for 2024 show that the severe oxygen conditions in the Baltic Proper after the regime shift in 1999 continues. In 2023 anoxia was found at 19% of the bottom areas and 33% suffered from hypoxia including anoxic areas. Preliminary results for 2024 show
that anoxia affected 18% of the bottom areas and 34% suffered from hypoxia (including anoxic areas). The concentration of hydrogen sulphide is record high in all the basins around Gotland. In the Eastern and Western Gotland Basin hydrogen sulphide in the bottom water has reached levels not recorded before. The inflows that occurred during 2023 had a short-term positive effect that could be seen in the Southern Baltic Proper but not further into the Baltic Proper than to the Eastern Gotland basin at intermediate depths. Unfortunately, these pulses of oxygenated water are too small to have any major positive impact on the overall oxygen situation. In late 2024 two inflows occurred that might improve the oxygen
situation in 2025 in the southern Baltic Proper.