Summary
During the cruise, which is part of the Swedish pelagic monitoring program, the Skagerrak, the Kattegat, the Sound and the Baltic Proper were visited.
The surface water temperature was normal for the month and varied from 2–4 °C in the Baltic Sea to 3–4 °C in the North Sea. In the North Sea, the salinity of the surface water was below normal for all stations.
In the Skagerrak, the concentration of dissolved inorganic nutrients in the surface water was normal for the month and had decreased since February, indicating that the spring bloom has started and been ongoing since the last cruise. Also, in the Kattegat, a reduction in dissolved inorganic nutrients was visible compared to February and the levels were within normal range.
In the Baltic Sea, the concentration of dissolved inorganic nutrients was still at high winter levels, but a slight decrease in dissolved inorganic nitrogen (DIN) could be seen in comparison to February, which may indicate that the spring bloom is about to start. Silicate levels were slightly above normal while DIN showed normal levels for the month. Phosphate levels were higher than normal in the Baltic Sea Proper and the Western Gotland Basin.
The oxygen situation in the bottom water was good (>4 ml/l) at all stations in the Skagerrak, the Kattegat and the Sound, no oxygen deficiency was noted.
In the Arkona basin, the oxygen situation was still good in the bottom water. In the Bornholm basin, the oxygen concentration had increased further since February as a result of the inflow that took place in December 2023, it was especially clear at station BY5 closest to the bottom. However, there was an intermediate layer with less oxygen present above this oxygenated layer. At stations BY4 and Hanöbukten, where the inflow was only initially visible in January, the concentration of oxygen has now in March returned to low levels. In the Eastern Gotland Basin, severe oxygen deficiency (<2 ml/l) occurred from 60 m and hydrogen sulphide was measured from 70 m. The highest hydrogen sulphide levels occurred in the Eastern Gotland Basin, while they are somewhat lower in the Baltic Sea Proper, here oxygen deficiency occurs from 70 m and hydrogen sulphide from 80 m (BY20). In the Western Gotland Basin, severe oxygen deficiency was present from 60 and 90 m, respectively, and hydrogen sulphide occurred from 80 and 100 m, respectively, at stations BY38 and BY31.
The next cruise with R/V Svea is planned for 9th–15th April, starting in Kalmar and ending in Lysekil.