Cruise report from R/V Svea week 38, 2025

Summary

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

Surface water temperatures ranged from 15–17 °C in the Skagerrak, 15–16 °C in the Kattegat and the Sound, and 13–17 °C in the Baltic Proper. The surface water salinity was 29–32 psu in the Skagerrak, 20–23 psu in the Kattegat, 12 psu in the Sound, and 6–8 psu in the Baltic Proper.

Concentrations of dissolved inorganic nutrients in the surface water showed nitrogen levels typical for the season, which were low or below the detection limit throughout the study area. Phosphate concentrations were also generally low across the area, although slightly higher than normal, particularly in the southern Baltic Proper. Silicate concentrations were seasonally low in the waters along the west coast but higher than normal throughout the Baltic Proper.

Oxygen levels in the bottom waters of the west coast were good (3.4–5.8 ml/l). In the Arkona Basin, oxygen levels have increased since August, with concentrations just at the threshold of severe oxygen deficiency (2.0 ml/l) at station BY2, but well above this level at station BY1. This indicates that an inflow of oxygen-rich bottom water through the Sound into the Arkona Basin has occurred since August. In the Bornholm Basin, severe oxygen deficiency still prevails from depths of 70–80 metres, including in the Hanö Bay, where oxygen concentrations have decreased since August. In the Eastern Gotland Basin, severe oxygen deficiency was observed from depths of 70–80 metres, and hydrogen sulphide was detected between 80–90 m in the northeastern part and 140–150 m in the southwestern part. In the Western Gotland Basin, severe oxygen deficiency was observed from 70 m, and hydrogen sulphide was present from 80 m.

The next expedition with R/V Svea is planned for 20–27 October, starting in Kalmar and ending in Lysekil.