Cruise report from R/V Svea week 29, 2025

Summary

During the expedition, which is part of the Swedish pelagic monitoring program, Skagerrak, Kattegat, the Sound, and the Baltic Proper were visited.

Surface water temperatures in all sea areas were around normal, ranging between 16.3 – 19.8 °C.

Dissolved inorganic nitrogen and phosphorus in the surface water showed low concentrations, which is normal for the season. Silicate concentrations in the surface water were ranging between slightly above to above normal at all stations in the Baltic Proper and in the Sound.

In the Baltic Proper, the oxygen situation remains very poor. Good oxygen conditions were only found in the Arkona Basin and at station BY39. Acute oxygen deficiency, meaning oxygen levels below 2 ml/l, was noted from 60 to 80 meters depth in the Western and Eastern Gotland Basins, except at the shallow station BY39. Hydrogen sulphide was measured from 70 to 80 meters depth in the Western Gotland Basin and from 80 to 145 meters depth in the Eastern Gotland Basin. In the Bornholm Basin and the Hanö Bay, acute oxygen deficiency was present from 70 meters depth, but no hydrogen sulphide was detected in the bottom water.

At several stations in the Baltic Proper, high peaks of chlorophyll fluorescence occurred from the surface down to 20 meters depth. In the Kattegat, large peaks were observed between 20 and 25 meters depth. Surface accumulations of cyanobacteria were noted throughout the Baltic Proper, with prominent surface blooms at the southern tip of Öland and in the Hanö Bay. It was evident that the cyanobacterial bloom was widespread throughout the Baltic Sea.

More information about the algae situation can be found in the Algaware report for July: https://www.smhi.se/publikationer/publikationer/algrapporter External link..

The next cruise is planned to start on August 9 in Lysekil and end in Lysekil on August 15.