Cruise report from R/V Svea week 10-11, 2025

Summary

During the expedition, which is part of the Swedish pelagic monitoring program, the Skagerrak, Kattegat, Öresund, and the Baltic Proper were visited. In addition to the regular sampling, several special tasks were carried out, such as the deployment of an ocean data buoy, a bottom rig, and various method tests and additional sampling.
In all marine areas, surface water temperatures were normal for the season.
The concentrations of nutrients in the surface water had decreased compared to the sampling in February in the Skagerrak and Kattegat, due to increased production commonly seen during the spring bloom. In the Baltic Proper, concentrations had also decreased, although the spring bloom had not fully started there.

The oxygen situation was generally good in the Skagerrak, Kattegat, and Öresund, with no signs of oxygen deficiency observed. In the Baltic Proper, good oxygen conditions in bottom waters were only found in the Arkona Basin. In the Hanö Bay and the Bornholm Basin, acute oxygen deficiency (< 2 ml/l) was measured from 70 meters depth, with bottom water levels ranging between 0.4–0.7 ml/l. In the Bornholm Basin, this represents a significant decrease compared to February, when levels were above 2 ml/l, compared to 0.7 ml/l measured in March.

In both Gotland Basins, acute oxygen deficiency was observed from 70 meters depth, and hydrogen sulfide was detected from 80 and 90 meters depth in the Western and Eastern Gotland Basins, respectively. At several stations, hydrogen sulfide concentrations in bottom water were higher than normal, and the long-term trend shows increasing levels of hydrogen sulfide at multiple stations.

The next expedition with R/V Svea is planned to start on April 7 in Kalmar and conclude in Lysekil around April 13.