Summary
During the cruise, which is part of the Swedish pelagic monitoring program, the Skagerrak, Kattegat, the Sound and the Baltic Proper were visited.
The warming of the surface water had started in all the visited sea areas, but for natural reasons the temperature increase is somewhat slower in the Baltic Sea compared to the Skagerrak and Kattegat. The salinity in large parts of the Baltic Sea's surface water was higher than normal.
The levels of nutrients in the surface water had generally decreased since the last visit in March and were low above the halocline where biological production consumes available nutrients. The silicate concentrations were more variable in the surface layer and levels that were both higher and lower than normal were noted in the Baltic Proper. The varying levels are probably due to varying biological activity in the surface water in the different areas and because certain plankton groups consume silicate during growth.
The oxygen situation was still very bad in most of the Baltic Proper, only in the Arkona Basin was the deep water well oxygenated. In Hanö Bay, Bornholm Basin and in the south eastern Baltic Proper, conditions were slightly better than in March; no hydrogen sulphide could be measured and the oxygen levels were just over 0 ml/l. The improved oxygen conditions near the bottom may be due to a smaller inflow that occurred in early April. This inflow will probably improve the oxygen conditions in Hanö Bay and the Bornholm Basin in the coming months.
In the Western and Eastern Gotland Basin, oxygen-free conditions were found from depths exceeding 80-90 meters and the hydrogen sulphide concentrations are still at record high levels.
Acute oxygen deficiency was found already from a depth of 60 meters in the southern part of the Western Gotland Basin and from a depth of 70-80 meters in other basins.
Chlorophyll fluorescence measurements with CTD showed that there was some plankton activity, in the form of fluorescence peaks at a depth of about 10-15 meters in the outer Skagerrak. In the Kattegat, high plankton activity was noted with fluorescence peaks that coincided with the stratification at a depth of 10-20 meters, and in the Baltic Proper, some activity was found above the halocline in the eastern parts, however, no strong fluorescence peaks were observed.