Cruise report from R/V Svea week 41, 2021

Type: Report
Author: Martin Hansson


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 cooling of the surface water had now started in all sea areas. In general, only normal surface water temperatures were noted. In the deep water, temperatures above normal were found in some areas. The salinity of the surface water was still higher than normal in parts of the Baltic Proper but normal in other areas. In deep water, the salinity was lower than normal in the Skagerrak and higher than normal in the Kattegat.

The nutrients in the surface layer had generally increased slightly or were at the same level as the measurements in September. The concentration of silicate remained above normal in parts of the Baltic Proper. In the oxygen-free deep water of the Baltic Proper, the levels of dissolved inorganic nitrogen were well above normal.

The oxygen situation in parts of the Skagerrak was worse than normal with levels below 4 ml/l, which is the limit when the first signs of oxygen deficiency can be seen. The oxygen levels in the Kattegat's deep water had decreased further since the last measurements in September and were now below the limit for oxygen deficiency at the bottom at all stations.

In the Baltic Proper, the oxygen situation remained very poor. At many stations, hydrogen sulphide levels were noted, which were much higher than normal in the deep water. No hydrogen sulphide was measured in Hanö Bight, but the oxygen concentration was close to zero. In the Bornholm Basin, hydrogen sulphide was measured from a depth of 80 meters. In the Eastern and Western Gotland Basins, completely oxygen-free conditions were measured from a depth of about 70 meters. Acute oxygen deficiency was generally noted from a depth of 60-70 meters, but at BY39 at Ölands Södra Udde already from a depth of 50 meters.

The plankton activity measured by fluorescence from CTD was generally low in all sea areas. No peaks in chlorophyll fluorescence were noted. The fluorescence was low and evenly distributed throughout the well-mixed surface layer and then rapidly decreased below the thermocline.