Cruise report from R/V Svea week 46-47, 2020

Type: Report
Author: Karin Wesslander


The surface water (0–10 m) had only become slightly colder since October and was warmer than normal at almost all stations, the temperature was 10–12 degrees. The salinity of the surface water in the Baltic Proper was still above normal for the season and varied between 6.6–8.5 psu. In the Kattegat and Skagerrak, the salinity of the surface water was normal.

The concentrations of inorganic nutrients in the surface water had increased since October, which is normal for the autumn when phytoplankton activity decreases and the water mass mixes. The concentrations of phosphate, inorganic nitrogen and silicate were generally normal in the Skagerrak and the Kattegat but were higher than normal in parts of 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 lack of oxygen in the Skagerrak's bottom water, but at the coastal station Släggö, the oxygen concentration had decreased to 2.7 ml/l. The oxygen concentration in the Kattegat's bottom water was 4–4.5 ml/l. In the Sound, the oxygen concentration in the bottom water had increased from 2 ml/l, which was measured in October, to 3.8 ml/l this November.

At this visit, there was no oxygen deficiency in the Arkona Basin, which is an improvement from October, while the oxygen situation in the other basins was largely the same. In the Bornholm Basin, there was an acute lack of oxygen (<2 ml/l) from 70 m, but no hydrogen sulphide was noted. In the Eastern Gotland Basin at station BCSIII-10 there was also no anoxia but acute oxygen deficiency from 80 m. At BY10 and BY15 Gotland Deep there was acute oxygen deficiency from 80 m down to 125–150 m where hydrogen sulphide was measured down to the bottom. In the Northern and Western Gotland Basin, there was a sharper boundary at the transition to oxygen-free status; at station BY31Landsort Deep there was acute oxygen deficiency from 80 m and hydrogen sulphide was noted just below from 90 m. At station BY38 Karlsö Deep there was acute oxygen deficiency already from 60 m and hydrogen sulphide from 70 m.