Cruise report from R/V Svea week 23, 2021

Type: Report
Author: Örjan Bäck


Heating of the surface waters were observed in all visited areas with surface temperatures over 15 ℃ and with a thermocline as result, which is normal for June. The salinity varied more, about 7 psu in the Baltic Proper, between 12-18 psu in the Sound and the Kattegat. This is higher than normal in the surface in the Sound and lower than normal in the Kattegat. In the Skagerrak salinity values over 32 psu were observed in the surface waters at the stations furthest from the coast, which is higher than normal, at the coastal stations normal values for salinity were observed, around 20-23 psu.

Dissolved organic nitrogen in the surface water was in general consumed by the spring bloom at all visited stations in the Sound and at P2 and Släggö in the Skagerrak. Concentrations of dissolved inorganic phosphorus in the surface water were normal for the season in all ocean areas. Dissolved silicate concentrations in the surface water remained higher than normal in the Baltic Proper and in the Kattegat.

The oxygen situation is still bad in large parts of the Baltic Proper. In the Bight of Hanö and the Bornholm Basin, which are about 80 and 90 metres deep, acute hypoxia (< 2ml O2/l) occurred from 60 metre and 70 metre respectively. In the Eastern and Western Gotland Basin, at BCSIII-10 in the south and at BY20 in the north acute hypoxia was measured from 70-80 metres depth. Hydrogen sulphide (forms in oxygen free conditions) was observed at the bottom in the Bornholm Basin and in the Bight of Hanö, and from 125 metres depth in the Eastern Gotland Basin. At BY29 hydrogen sulphide was observed from 90 metres depth and in the Western Gotland Basin at the stations BY32 and B38 from 80 and 70 metres depth respectively.

In the southern basins in the Baltic Proper large peaks in fluorescens were observed at just above 20 metres depth, also in the southeast large fluorescence peaks were found. Microscopic analysis of water samples taken in these peaks showed large amounts of phytoplankton from the group Prymnesiales.