Cruise report from R/V Svea week 11-12, 2022

Type: Report
Author: Örjan Bäck


During the cruise, which is part of the Swedish national marine monitoring programme, the Skagerrak, the Kattegat, the Sound and the Baltic Proper were visited.

The water in the Skagerrak was well mixed with weak stratification at the outer stations, low salinities in the surface with an underlying halocline at the more coastal stations. The concentrations of the nutrients DIN and DIP (dissolved inorganic nitrogen and dissolved inorganic phosphorus) correlated strongly with salinity, with high values in the surface at Å17 and very low values of DIN above the halocline at the other stations. The concentrations of silicate were higher than normal everywhere except below the halocline at Å17.

Strong haloclines were observed at every station in the Kattegat with lower than normal salinity in the surface layer. The concentrations of nutrient in the form of DIN and DIP were low but normal both above and below the halocline in the Kattegat. The concentrations of silicate were generally just above what is normal in the area for the season. All stations on the west coast were well oxygenated, with concentrations well above the limit for oxygen deficiency (4 ml/l).

In the Baltic proper there was a well-mixed upper layer with cold and low saline water above the permanent halocline. The salinity at the stations around Bornholm were at normal levels in the entire water mass, while the salinity in the rest of the Baltic Proper were higher than normal, both above and below the halocline. The concentrations of nutrients in the form of DIN were low in the well-mixed upper layer, in particular in the shallowest 20 metres. The concentrations of DIP were normal above the halocline in the entire Baltic Proper. Below the halocline higher values than normal were present at every area except for in the Arkona Basin. High concentrations of silicate were measured in the entire area below the halocline. The oxygen conditions were still very poor in large parts of the Baltic Proper, only in the Arkona Basin the water was well oxygenated. In the Eastern, Northern and Western Gotland Basin hydrogen sulphide concentrations higher than normal were measured closest to the bottom, several stations are now closing in on record high values of hydrogen sulphide.

Plankton activity measured by fluorescence from the CTD was generally low in all areas. No peaks in chlorophyll fluorescence were noted. The fluorescence was low and evenly distributed throughout the well mixed surface layer, then rapidly decreasing below the thermocline. Low nutrient concentrations indicated that a phytoplankton bloom had happened in the southern Baltic Proper but it was nothing that could be seen in the fluorescence data.