Cruise report from R/V Svea week 6-7, 2021

Type: Report
Author: Lena Viktorsson


The surface water, which had been warmer than normal during the autumn and winter, had now cooled down considerably, especially in the Skagerrak and the Kattegat. There the temperature varied between 0.6–3.4 degrees, at most stations there the temperature was below normal in the surface water. However, in the Baltic Proper the surface water was warmer, at about 3-4 degrees. At most stations in the western and eastern Gotland Basin, the temperature was close to four degrees, which was warmer than normal for the season.

The salinity of the surface water in the Baltic Proper was still above normal for the season and varied between 6.9–8.7 psu. Normal salinities were observed in the Skagerrak and the Kattegat, from around 18 psu in the Kattegat to around 30 psu in the Skagerrak.

The concentration of nutrients had decreased in the Skagerrak and the Kattegat since the beginning of January, which indicates that phytoplankton activity has started. In these seas, chlorophyll fluorescence was also noted in the entire surface layer. The concentrations of dissolved inorganic nitrogen were about 4 µmol/l, phosphate around 0.5–0.6 µmol/l and dissolved silicon 3–8 µmol/l in the Skagerrak and 10–13 µmol/l in the Kattegat.

In the Baltic Proper, mapping of the winter pool of nutrients was performed and here the levels were as high as in January or higher. The mapping showed higher than normal levels of dissolved inorganic nitrogen (4–5 µmol/l) in the eastern Gotland basin and parts of the western Gotland basin, mostly normal levels of phosphate (0.7–0.8 µmol/l) and higher than normal levels of dissolved silicon (15–18 µmol/l). In the deep water in the western Gotland Basin, the concentration of dissolved inorganic nitrogen was far above normal.

The oxygen concentration in the bottom water was normal in the Skagerrak and the Kattegat with concentrations between 5–6 ml/l at all stations. Even in the Arkona basin in the Baltic Proper, the bottom water was well oxygenated (7-9 ml/l). In Hanö Bay and the Bornholm Basin, there was oxygen deficiency in the bottom water at the deepest stations, about 0.5 ml/l, the same oxygen concentration was in Gdansk Bay. However, new oxygen had reached the bottom water at the BCS III-10 station in the eastern Gotland Basin, where the concentration was 3.73 ml/l. After this the oxygen concentration decreased to 0.9 ml/l at the next station BY9 Klaipeda and at all stations north of it the bottom water was oxygen-free (hydrogen sulphide was measured), from 80–130 m, starting at the shallowest depths in the northern Baltic Proper.