Cruise report from R/V Svea week 6-7, 2025

Summary

During the expedition, which is part of Sweden's pelagic monitoring program, the Skagerrak, Kattegat, the Sound, and the Baltic Proper were visited. Nutrient mapping was conducted in the Baltic Proper.

In all sea areas, surface water temperatures were above normal at most stations. In Skagerrak, temperatures ranged between 4.0–7.0°C, while in Kattegat and the Sound, they were around 4°C. In the Baltic Proper, the average temperature was 3.3–4.8°C, with the largest deviations in the southern basins. Salinity in the surface water was above normal at several stations in the Baltic Proper, while in Skagerrak and Kattegat, it varied without major anomalies.

In Skagerrak, a very thin, cold, and fresh surface layer was observed near the coast, similar to the conditions in January. In Kattegat and the Sound, a well-mixed surface layer was present, with a thermocline at approximately 15 meters depth. In the Baltic Proper, the halocline was found at varying depths: around 30 meters in the Arkona Basin, 60 meters in the Western Gotland Basin, and 70 meters in the Eastern Gotland Basin. Below the halocline, temperatures remained high at many stations, and in the Bornholm Basin, signs of a minor inflow were observed.

The concentrations of nutrients in the surface water varied between sea areas. In Skagerrak, silica concentrations were above normal at most stations, while phosphate and nitrogen concentrations were slightly lower at the outermost stations. In Kattegat and the Sound, nutrient levels showed little variation, but silica concentrations were consistently above normal. In the Baltic Proper, nitrogen levels were within the normal range at most stations, while phosphate levels were above normal in the southern basins. Silica concentrations were higher than normal at almost all stations, particularly in the southern parts.

The oxygen situation was generally good in Skagerrak, Kattegat, and the Sound, with no signs of oxygen deficiency observed. In the Baltic Proper, the situation was more varied. Oxygen concentrations in the Arkona Basin remained good, but in Hanö Bay, bottom oxygen levels had decreased since January and were now close to zero. In the Bornholm Basin, acute oxygen deficiency was recorded, and in the Baltic Proper, waters below 80–90 meters depth were completely anoxic. At station BY38, which had been oxygenated following storm Charly in January, the water was once again oxygen-free.

The next expedition with R/V Svea is planned to start on March 7 in Kalmar and end in Gothenburg March 13.