The surface water temperature varied between 6-10 °C, which is normal for the season. At most stations in the Baltic Proper, the temperature in the entire depth profile was above normal. The cooling that normally occurs during winter was 2019/2020 very weak. The surface water had now started to warm up. The salinity in the Baltic Proper was above normal at all stations visited, both in the surface and in the deep water.
Dissolved inorganic nitrogen in the surface water was basically consumed in all sea areas, which is normal after the spring bloom. Phosphate levels, just as in March and April, were higher than normal in the Bornholm Basin, the southeastern Baltic Proper and in parts of the Eastern Gotland Basin. Phosphate levels were now also higher than normal in the Gulf of Hanö and in parts of Arkona. The silicate levels were above normal at most stations visited in the Kattegat and the Baltic Proper.
The oxygen situation in the Baltic Sea continues to be serious. Completely oxygen-free conditions, when toxic hydrogen sulphide is formed, were found in the Western, Northern and Eastern Gotland basins from 80-90 meters deep. Acute oxygen deficiency (<2 ml/l) was found from 70-80 meters depth throughout the area and even shallower from about 60 meters depth in the Hanö Bight. In Hanö Bight, Bornholm Basin and in the southeastern Baltic Proper, oxygen levels in the bottom water have continued to decline. In March they were above 2 ml / l, which is the limit for acute oxygen deficiency, in April between 1.4-1.7 ml/l and during this expedition in May 0.4-1.0 ml / l. In the Skagerrak and Kattegat, the oxygen situation was good.
The inflow that occurred in November/December 2019 had now reached the southeastern Baltic Proper. Oxygen levels of about 1.0-1.5 ml/l could be found close to the bottom at BCSIII-10 and at intermediate depth, 80-125 meters, at BY10.