Cruise report from R/V Svea week 28, 2021

Type: Report
Author: Martin Hansson


The warm weather during June and July increased the surface water temperature in all sea areas. All sea areas, except the Skagerrak, had higher temperatures than normal. Salinity remained higher than normal in the eastern part of the Baltic Proper and in parts of its deep water.

Dissolved inorganic nitrogen and phosphorus in the surface water were in principle consumed in all sea areas and generally showed normal levels for the season. The silicate levels in the surface water remained higher than normal in the Baltic Proper. In the Kattegat, where silicate levels in June were higher than normal, levels were now back to normal for the season.

The oxygen situation is still very bad in the Baltic Proper. In Hanö Bight, oxygen-free conditions were observed from depths exceeding 70 meters and the hydrogen sulphide content had increased significantly since the last visit in June. This could be clearly seen in the levels of dissolved inorganic nitrogen and phosphorus as well as silicate as the levels had in principle doubled in the deep water from high levels to very high levels.

In the Bornholm Basin, oxygen-free conditions were noted from a depth of 80 meters. Further east in the southern part of the Eastern Gotland Basin, the bottom water was oxygenated but the levels were very close to zero. In the Eastern Gotland Basin, completely oxygen-free conditions were measured from a depth of 80 meters, and in the Western Gotland Basin, no oxygen conditions were found from depth exceeding 70 meters. The oxygen situation in the bottom water was worse than the monthly average at all stations where hydrogen sulphide was observed.

Acute oxygen deficiency was also noted shallower than usual. In the Arkona Basin, which normally has better oxygen conditions in the bottom water, oxygen levels were measured on the limit of acute oxygen deficiency. In Hanö Bight and in the Bornholm Basin, acute oxygen deficiency was noted from a depth of 65 meters and in the Eastern Gotland Basin from a depth of 70 meters. In the Western Gotland Basin, acute oxygen deficiency was noted already at a depth of 55-60 meters.

Surface accumulations of cyanobacteria were visible on the sea surface in Arkona and in the Bornholm Basin. In the other basins in the Baltic Proper, areas with large amounts of flocks of cyanobacteria were spotted in the upper body of water. More information about the algal situation can be found in the AlgAware report for July;