Cruise report from R/V Svea week 49-50, 2021

Type: Report
Author: Martin Hansson, Lena Viktorsson


During the cruise, which is part of the Swedish pelagic surveillance program, the Skagerrak, the Kattegat, the Sound, the Baltic Proper and the Gulf of Bothnia were visited.

The surface water (0–10 m) had cooled down about 4-5 degrees since November, but the temperature was normal for the season at most stations in the Skagerrak, the Kattegat, the Baltic Proper and the Gulf of Bothnia. In the deeper parts of the Bothnian Sea and the Baltic Proper, water with temperatures above or well above normal was found. The salinity of the surface water remained higher than normal in parts of the central Baltic Proper, while it was lower than normal at some stations in the southern Baltic Proper, as well as in the Gulf of Bothnia and the eastern and north-eastern Bothnian Sea.

The concentrations of inorganic nutrients in the surface water had increased since November and were now close to the maximal winter pool, which is normal for the autumn when phytoplankton activity decreases and the water mass mixes. The concentration of inorganic dissolved nitrogen and phosphorus in the surface water was slightly lower than normal in parts of the Baltic Proper. The concentration of silicate was above normal at several stations in the Baltic Proper. The mapping of nutrients in the Gulf of Bothnia showed that the concentrations of phosphate were higher than normal in both the Bothnia Bay and the Bothnian Sea. Concentrations of silicate were higher than normal in the eastern part of the Gulf of Bothnia. The concentrations of dissolved inorganic nitrogen (DIN) were normal throughout the Gulf of Bothnia except for two stations in the north-eastern part of the Bothnian Sea where higher than normal levels were measured.

The oxygen situation in the Baltic Proper remains problematic. In the Bornholm Basin, the deep water was oxygen-free and hydrogen sulphide was measured. In Hanö Bight, low but measurable concentrations of oxygen from 70 m were still found. With the exception of BCSIII-10 and BY15, hydrogen sulphide from 70–80 m depth was measured in the remaining basins in the Baltic Proper. At BY15 it was oxygen-free or very low oxygen concentration from 80 m and hydrogen sulphide was first measured from 125 m. At BCSIII-10 it was almost oxygen-free from 70–80 m depth while in the groundwater there was acute oxygen deficiency.

In the Kattegat, no oxygen deficiency was noted and the oxygen levels were normal. In the Sound, on the other hand, oxygen deficiency was measured already at a depth of 15 m with concentrations around 3 ml / l from 15 m to the bottom. There was no oxygen deficiency in the Skagerrak, at the Släggö station where oxygen deficiency was measured in November, the oxygen concentration had now increased and the levels were above the limit for oxygen deficiency. In the Gulf of Bothnia, no oxygen deficiency was noted, which is normal.