Cruise report from R/V Svea week 32-33, 2021

Type: Report
Author: Lena Viktorsson


The slightly milder and windier weather during August resulted in a decrease in the temperature in all sea areas and was now normal at most stations. In all sea areas, there was a well-mixed surface layer separated from the underlying water of a thermocline between 15–20 m depth. At some stations in the Baltic Proper, very low surface water temperatures were noted for the season due to up-welling caused by the wind. Salinity remained higher than normal in the eastern part of the Baltic Proper and in parts of its deep water.

The levels of nutrients in the surface water remained low, although the levels of phosphate had increased slightly in the Baltic Sea since July and were above normal in the eastern Gotland Basin. The concentration of silicate remained above normal in large parts of the Baltic Proper. Otherwise, the concentration of nutrients in the surface water was normal. Below the halocline in the western Gotland Basin, ammonium levels were still well above normal.

The oxygen situation was largely unchanged since July with oxygen deficiency or anoxi in the bottom water at all stations. At Hanö Bight, where hydrogen sulphide was measured in July there was now a very low concentration of oxygen. In the eastern Gotland Basin hydrogen sulphide was noted from 80 m and in the western Gotland Basin from 60–70. The shallowest measurement of hydrogen sulphide at 60 m was noted in the southern part of the western Gotland Basin, station BY38. Acute oxygen deficiency was noted from 60–70 m.

In the Skagerrak and the Kattegat, several peaks in chlorophyll fluorescence were noted, where extra phytoplankton samples were taken for later analysis. In the Baltic Proper, the chlorophyll fluorescence was uniform throughout the well-mixed surface layer down to the thermocline. In Arkona, large amounts of organic material were noted in the deep water on the camera mounted on the CTD. More information about the algal situation can be found in the AlgAware report for August;