Cruise report from R/V Svea week 11, 2021

Type: Report
Author: Johan Kronsell


During the expedition, which is part of the Swedish national marine monitoring programme, the Skagerrak, the Kattegat, the Sound and the Baltic Proper were visited. During the first part of the survey the winds were mostly from the north, and the air temperature was 0-2℃, but the wind direction changed to west and the temperature rose to 4-5℃ during the later parts of the cruise.

The cold surface water that was present in the Skagerrak and the Kattegat during the previous survey had been mixed with deeper water, and the temperature was now 3.5-5℃. For the stations in the Baltic Proper the sea surface temperature was basically unchanged since the previous survey, and were at most stations 2.5-4℃. Except for the station in the southeastern Baltic Proper, the sea surface temperature was normal for the season at all stations.

The sea surface salinity in the Baltic Proper was normal to above normal for the season, and varied from 6.6 psu in the northern parts up to 8.4 psu in the Arkona Basin. In the Kattegat and the Skagerrak mostly normal salinity in the surface, with 23-25 psu in the Kattegat and 29-32 psu in the Skagerrak. The Sound also had a for the season normal average surface salinity, but a shallow stratification with significant difference in salinity in the upper and lower water mass was present.

Concentrations of nutrients in the Skagerrak and the Kattegat were normal for the season, with the exception of phosphorus at two stations in the Skagerrak, where levels were above normal. For the stations in the Baltic Proper it was obvious at which stations the spring bloom had started, since at these there was a lower concentration of nutrients in the surface water compared to the levels during the previous expedition. Measurements of chlorophyll fluorescence also confirmed that the spring bloom was ongoing in the Arkona Basin, the Bight of Hanö, and in the Bornholm Basin, and also at the coastal station RefM1V1 in Kalmarsund. In the other basins, the spring bloom had not yet started, but there was a slightly increased level of chlorophyll fluorescence in the surface of the Western Gotland Basin.

Oxygen concentration in the bottom water in the Skagerrak, the Kattegat and in the Sound was for the most parts normal for the season, with concentrations of 7 ml/l in the offshore stations in the Skagerrak, and just below 5 ml/l at Släggö, and between 5-6 ml/l in the Kattegat and the Sound. Also, the Arkona Basin in the Baltic Proper had good levels of oxygen in the bottom water, with concentrations of 7 ml/l. In the Bight of Hanö and the Bornholm Basin oxygen deficiency was found in the bottom water, and the levels were slightly lower compared to the previous expedition, and were now only 0.2-0.4 ml/l. At BCSIII-10 in the southeastern Baltic Proper the concentration was lower compared to the last visit, and was now below the level of oxygen deficiency with only 1.7 ml/l. At the deeper stations in the Eastern and Western Gotland Basin, oxygen deficiency was found from 70-80 meters depth, and anoxic conditions with hydrogen sulfide present were found from 90-100 meters depth.