Cruise report from R/V Svea week 37-38, 2021

Type: Report
Author: Lena Viktorsson


The temperature in the surface water in all sea areas was normal and varied between 14–18 degrees, coldest in the western Gotland Basin. In all sea areas, there was a well-mixed surface layer separated from the underlying water by a thermocline at 10–20 meters deep. The salinity of the surface water was above normal in the eastern part of the Baltic Proper and below normal in the Bornholm Basin. In parts of the deep waters of the Baltic Proper, the salinity was also above normal.

The concentrations of nutrients in the surface water remained low and had only increased slightly since August. The concentration of silicate was above normal in large parts of the Baltic Proper. Otherwise, the concentration of nutrients in the surface water was normal. Below the halocline in the western Gotland Basin, ammonium levels were still well above normal.

The oxygen situation was largely unchanged since August with oxygen deficiency or anoxia in the bottom water at all stations in the Baltic Proper. No hydrogen sulphide was measured in Hanö Bight, but the oxygen concentration was very close to zero. In the Bornholm Basin, hydrogen sulphide was measured from of 80–90 meters depth. In the eastern Gotland Basin, completely oxygen-free conditions were measured from a depth of 90 meters, in the western Gotland Basin from a depth of 70 meters at the station BY32 and from 80 m at the station BY38. Acute oxygen deficiency was noted from 60-70 meters.

The biological activity had decreased and only at one station in the Skagerrak was a small chlorophyll fluorescence peak noted.