Warming of the surface water (0-10m) at all sea basins continued in May. At several stations, the increase was about 5 ℃ compared with previous month. The salinity of the surface water at the outer stations in the Skagerrak were slightly above normal. In the Kattegat, which had salinity above normal in the surface during the last expeditions, more normal levels in May. In the Baltic Sea, the salinity of the surface water was generally normal for the season, but deeper down in the water column the salinity was above normal at many of the deeper stations.
Dissolved inorganic nitrogen in the surface water was in general consumed by the spring bloom at all stations except in the Sound. Concentration of dissolved inorganic phosphorus in the surface water was normal for the season at all stations. Dissolved silicate concentrations in surface water remained higher than normal in large parts of the Baltic Proper and in the Sound. In the Kattegat in general normal concentrations and in the Skagerrak concentrations below normal for the season.
In the southern basins of the Baltic Proper, a significant fluorescence peak was found in the water column. Also at BY20 in the Eastern Gotland Basin and BY31 and BY32 in the Western Gotland Basin there were distinct peaks in the measurement of chlorophyll fluorescence. At the other stations in the Baltic Proper a more even activity in the surface layer. A distinct peak of chlorophyll fluorescence was also found in the Sound and at the outer stations in the Skagerrak.
The oxygen situation is still poor in large parts of the Baltic Proper. In the Bornholm Basin, hydrogen sulfide was found in the bottom water and acute oxygen deficiency (<2 ml/l) was found from a depth of 70 meters. Also in the Bight of Hanö the oxygen concentration at a depth of 70 meters was significantly below the limit for acute oxygen deficiency. And at BY1 in the Arkona Basin - which is just below 45 meters deep - acute oxygen deficiency was found in the bottom water. At BCSIII-10 in the southeastern Baltic Proper, the oxygen concentration had decreased since April, and was now only 0.8 ml/l.
In the Eastern Gotland Basin, oxygen-free conditions were found at 80 meters depth, and from a depth of 125 meters and down, hydrogen sulfide was found at all stations. In the Western Gotland Basin, acute oxygen deficiency was found from a depth of just below 60 meters, and in the water sample at 70 meters, the oxygen concentration was below reporting limit, and hydrogen sulfide was measured at a depth of 80 meters. At the shallow station in Kalmarsund and at BY39 south of Öland, which is also a relatively shallow station, oxygen conditions were good from surface down to bottom.