Cruise report from R/V Svea week 42, 2020

Type: Report
Author: Lena Viktorsson


The surface water (0–10 m) had cooled down by 3–4 degrees since September and the temperature was now 12–13°C, which is normal for the month. The salinity in the surface water in the Baltic Proper was still above normal for the season and varied between 7-8 psu. In the Skagerrak and the Kattegat, the observed surface salinity was normal at most stations.

The concentrations of inorganic nutrients in the surface water had increased since September, which is normal for the autumn when phytoplankton activity decreases. The concentrations of phosphate, inorganic nitrogen and silicon were generally normal in the Skagerrak and the Kattegat but were higher than normal in the Baltic Proper. In the deep water in the western Gotland Basin, the concentration of dissolved inorganic nitrogen was far above normal.

There was no oxygen deficiency in the bottom water was in the Skagerrak, the concentration was above 4 ml/l at all stations, including the coastal station Släggö. In the Kattegat, the lowest oxygen concentration was measured at the station Anholt E where the concentration was just below 4 ml/l, which is the limit for oxygen deficiency. In the Sound, there was hypoxic conditions (<2ml/l) from 25 m to the bottom. In the Baltic Proper, the oxygen situation has remained largely unchanged since September. In Arkona, the station BY1 was well oxygenated while there was hypoxia in the bottom water at the station BY2. In the Bornholm basin and the Hanö bight there was hypoxia from 70 m. At the station BY10 the oxygen concentration was zero or close to zero between 90–125 m and hydrogen sulphide was measured in the bottom water, at a depth of 145 m. At the station BCSIII-10 hypoxia prevailed from 80 m. As in September, hydrogen sulphide was first measured at 150 m at the BY15 station. At 80 m, the concentration of both hydrogen sulphide and oxygen was below the detection limit, but between 100–125 m there were low concentrations of oxygen. At the two stations in the western Gotland Basin and at the station BY20, hydrogen sulphide was measured from a depth of 80 m.