Cruise report from R/V Svea week 33, 2020

Type: Report
Author: Lena Viktorsson


The surface water was, unlike in June, warmer than normal at most stations in the Baltic Proper, but mostly normal in the Kattegat and the Skagerrak. The surface water temperature varied between 18-21 degrees, of which the highest temperatures were measured in the Baltic Proper. The salinity of the surface water was higher than normal in the Baltic Proper, between 7 and 8, and at several stations the salinity was above normal all the way down to the bottom. There was a thermocline around 10 m at most stations and some stations a pycnocline at about 20 m. At the stations around Gotland, the halocline was around 70 m and in the Bornholm Basin around 50-60 m.

The concentrations of nitrogen and phosphorus in the surface water (0-10 m) were generally low. In the Baltic Proper lower than normal for dissolved inorganic nitrogen, within the normal for phosphate and above normal for silicon. The concentrations of dissolved inorganic nitrogen was close to or at the detection limit at most stations in all sea areas, while there were measurable but low concentrations of phosphate at all stations. In the deep water, the concentration of nutrients was normal, except in the Western Gotland Basin where there were unusually high levels of dissolved inorganic nitrogen.

In the Skagerrak and the kattegat, the oxygen concentration in the bottom water was generally good, the lowest concentrations were found at Släggö, Anholt E, N14 Falkenberg and in the Sound where the oxygen concentrations were below 4 ml/l closest to the bottom. In Arkona, Bornholm and in the south-eastern part of the Gotland Basin, there was an acute oxygen deficiency (<2 ml/l) at the bottom. Completely oxygen-free conditions, when toxic hydrogen sulphide was formed, were found from a depth of 70-80 meters and acute oxygen deficiency (<2 ml/l) from a depth of 60-70 m in the eastern and western Gotland Basin.

During the expedition, there was an ongoing cyanobacteria bloom in the Baltic Sea and surface accumulations were visible both east and west of Gotland, but were most extensive west of Gotland. It also appeared that cyanobacteria were present in the surface water of the Kattegat. At several stations there were peaks in chlorophyll fluorescence around 10-15 m.