Summary
During the expedition, which is part of the Swedish national marine monitoring program, Skagerrak, Kattegat, Öresund, the Baltic Proper, was visited. The weather was sunny and mostly calm. Several surface accumulations of cyanobacteria were observed in the Baltic Sea. During the expedition, live samples of phytoplankton were analysed, the results of these analyses are published here, https://www.smhi.se/publikationer/publikationer/algrapporter. The surface temperature was normal for the season and varied between 15-18 ° C. A thermocline existed at all stations, but the depth varied between 5-20 m. The nutrient concentrations were low, which is normal in the summer. In the Baltic Proper, dissolved inorganic nitrogen was depleted down to the halocline of between 40-60 m depth. However, phosphate remained in the surface water, although at low levels, the phosphate concentration started increasing again at about 20 m depth. In the Skagerrak and the Kattegat, the concentrations of both dissolved inorganic nitrogen and phosphate were very low from the surface down to 15-20 m. The deep water in the Eastern Gotland Basin was oxygenated from 80 m to about 130 m, just as in June. In the central part of the basin, oxygen concentration reached almost 2 ml/l in the deep water while the oxygen concentration in the northern and southern parts was lower. Hydrogen sulphide was measured from 140 m at the Gotland depth and 70 m in the Western Gotland Basin and it was then oxygen-free all the way down to the bottom. In the Bornholm Basin, oxygen deficiency (<2 ml/l) was observed from 70 m to the bottom and at one of the stations, hydrogen sulphide was measured at the bottom.