Cruise report from R/V Svea week 15, 2026

Summary

During the expedition, which is part of Sweden’s marine pelagic monitoring program, the Skagerrak, Kattegat, Öresund, and the Baltic Proper were surveyed. Surface water temperatures were within normal ranges across all sea areas, varying from just below 3°C to 6.5°C. Salinity in surface waters was generally higher than normal in all regions. On the west coast, wind-driven mixing resulted in no clear halocline at all or a weaker deeper-than-usual halocline.

Phosphate concentrations in surface waters were above normal in all areas except around Bornholm, where levels were typical for the month. Dissolved inorganic nitrogen (DIN) was elevated in the Skagerrak at the Å-stations, while concentrations in other regions were low but still within the normal range for April. In the Kattegat and the southern Baltic Proper, DIN levels were below detection limit, indicating complete consumption. Silicate concentrations were generally higher than normal in surface waters across all areas.

Oxygen conditions were good at all stations in the Skagerrak, Kattegat, and Öresund, with no signs of oxygen deficiency. In the Bornholm Basin, oxygen-rich water had entered the system. In March, this was only visible near the seabed, but April measurements showed increased oxygen levels from around 60 m depth at station BY4 and from 65 m at BY5. At BY10, a slight increase was observed above and below 100 m, while small amounts of oxygen were detected around 100 m at BY15. However, concentrations remained low, and no oxygen was present below 125 m. Overall, this suggests that the inflow is limited and will only temporarily improve oxygen conditions in the Hanö Bay and Bornholm Basin.

In deeper parts of the Baltic Proper, oxygen deficiency or completely anoxic conditions persisted, with hydrogen sulphide present from depths of approximately 80–100 m. At BY15, hydrogen sulphide began at 125 m, and at BY10 only near the seabed.

Chlorophyll fluorescence, an indicator of plankton activity, was relatively high in surface waters of the Kattegat down to the halocline at about 20 m depth. This suggests that mixing has returned nutrients to the surface, fueling phytoplankton growth. In the Baltic Sea, the pattern was reversed compared to March, with the lowest fluorescence observed in the Arkona Basin. In the Skagerrak, fluorescence was low at offshore stations but extremely high at the coastal station Släggö.

SMHI’s next regular expedition with R/V Svea is scheduled for May 4–11, starting in Lysekil and ending in Kalmar.