Cruise report from R/V Svea week 50-51, 2025

Summary

During the cruise, which is part of the Swedish pelagic monitoring programme, the Skagerrak, Kattegat, the Sound, the Baltic Proper and the Gulf of Bothnia were visited. In the Gulf of Bothnia, SMHI and the Umeå Marine Research Center (UMF) carried out a joint survey of, among other things, nutrients.

The cooling of surface waters continued in December, but surface water temperatures were above normal at the vast majority of stations in all sea areas. Surface water temperatures were 8–9°C in the North Sea area, 7–8°C in the Baltic Proper, and 5–6°C in the Gulf of Bothnia.

Nutrient concentrations in surface waters had increased in both the Kattegat and the Baltic Proper since October. Concentrations of dissolved inorganic nitrogen were normal, while phosphate concentrations were above normal at some stations in the Baltic Proper. Concentrations of silicate were above normal throughout the Baltic Proper.

In the Gulf of Bothnia, a north–south gradient in nutrient concentrations was observed. Silicate and dissolved inorganic nitrogen decreased southward, while phosphate concentrations increased. Silicate concentrations were above normal throughout the area, except at a couple of stations in the Quark. Phosphate concentrations were above normal at all but three stations in the southern parts of the Bothnian Sea, where they were normal. Dissolved inorganic nitrogen concentrations were below normal in the Bothnian Bay.

Bottom water oxygen concentrations were normal in the Skagerrak, Kattegat (5–6 ml/l) and The Sound (4 ml/l). In the Arkona Basin, bottom water oxygen conditions were good at one of two stations, while the other showed oxygen deficiency. In the Bornholm Basin and the southern parts of the Eastern Gotland Basin, acute oxygen deficiency prevails, with oxygen concentrations close to zero. In the remainder of the Baltic Proper, conditions are completely anoxic below 80–90 m depth, with very high concentrations of hydrogen sulphide in the bottom waters.

The next expedition with R/V Svea is planned to start on 7 January in Gothenburg, when a nutrient survey of the Kattegat will also be conducted.