Cruise report from R/V Svea week 32-33, 2026

Summary

During the cruise, which is part of the Swedish pelagic monitoring programme, the Skagerrak, Kattegat, the Sound, and the Baltic Proper were visited.

The surface water temperature was generally normal in all sea areas and had increased slightly since the July expedition. The salinity in the surface water was generally within the normal range in all areas.

Surface water phosphate concentrations were normal for the month at most stations and had decreased since July, in both the Western Sea and the Baltic Proper. Dissolved inorganic nitrogen was low (at the limit of quantification) in all sea areas. Silicate concentrations were normal for the month except in the Baltic Proper where they were higher than normal with the exception of the Arkona Basin.

The oxygen situation in the Skagerrak was good, no oxygen deficiency was measured. In the Kattegat and the Sound, the oxygen concentration was lower but no oxygen deficiency was measured (< 4 ml/l). In Arkona, the Bornholm Basin and Hanö Bight, there was oxygen deficiency in the bottom water at all stations, below 4 ml/l. In the Baltic Proper, hydrogen sulfide was measured in the bottom water at all stations except BSC-III 10 and the southern tip of Öland. In the Eastern Gotland deep there is an intermediate layer where the water is oxygen-free but hydrogen sulphide can be measured. The thickness of this layer has increased and now extends between 90-140 m. At other stations north and west of the deep hole of the Eastern Gotland Basin, hydrogen sulphide is found from about 70-80 m down to the bottom and oxygen deficiency from 60-70 m depth.

Chlorophyll fluorescence was relatively high in the surface water and there were sharp and high peaks at several stations in connection with the stratification. In the Arkona basin, a strong bloom of cyanobacteria was underway at the time of sampling the stations there.

SMHI’s next regular cruise with R/V Svea is planned for 8–13 September, starting in Lysekil and ending in Kalmar. Before that SMHI will participate in the IBTS Q3 fisheries expedition 15-28 August, run by the department of Aquatic Resources at the Swedish University of Agricultural Sciences (SLU Aqua).