Cruise report from R/V Svea week 15, 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 surface temperature increased in all monitored basins in April. In the Kattegat the temperature increased the most by 2℃ since the visit in March. In the Skagerrak and Kattegat, the surface salinity was above normal at the stations closest to the coast. In large parts of the Baltic Proper, the salinity both surface and deep water, was also higher than normal.

In general, the dissolved nutrients had decreased since the March cruise. The levels of dissolved inorganic nitrogen in the surface water had decreased and were in principle consumed at all stations except the stations near the coast in the Skagerrak. Phosphate levels were normal in the Baltic Proper and the Kattegat, but just as for the dissolved inorganic nitrogen, higher than normal closer to the coast in the Skagerrak. The silicate levels in the surface water were still higher than normal in large parts of the Baltic Proper and at some stations in the central Skagerrak.

In the Baltic Proper, it was obvious at which stations the spring flowering had passed and where it continued, as at these there was a lower concentration of nutrients in the surface water compared with the levels during the previous cruise. In the Arkona and Bornholm Basin, the chlorophyll fluorometer on the CTD showed relatively low plankton activity in the surface layer down to the halocline. At stations in the Eastern Gotland Basin, a fluorescence peak was found directly above the halocline, at a depth of 50 - 60 meters, probably a remnant of the spring bloom. In the Western Gotland Basin, the plankton activity was larger and here from the surface down to a depth of 30 - 50 meters. The secchi disk depth was large, 11 - 14 meters, in the Hanö Bight, the Arkona Basin and in the Bornholm Basin, where the spring bloom was over. In the other Baltic areas, the visibility depth was around 8 meters.

The oxygen situation remains poor in large parts of the Baltic Proper. In the Bornholm Basin and in Hanö Bight, oxygen-free conditions were found at the bottom or from depths exceeding 80 meters. Acute oxygen deficiency (<2 ml/l oxygen) was found from a depth of 70 meters. In the eastern Bornholm Basin and further east up to and including BCSIII-10, the deep water was not completely oxygen-free with levels ranging from 0.6 ml/l up to 2 ml/l.

In the Eastern Gotland Basin, the oxygen content varied just over zero from a depth of 75 - 80 meters, at a depth of about 90 meters slightly higher oxygen levels were noted and from a depth of 100 - 125 meters the oxygen levels were again just above zero and from 125 meters depth and downwards hydrogen sulphide was found. Acute oxygen deficiency was noted at a depth of 65 meters. In the Western Gotland Basin, completely oxygen-free conditions were found from depths exceeding 80 - 90 meters. Acute oxygen deficiency from 65 - 75 meters depth. At the relatively shallow stations in the Baltic Proper, such as in the Arkona Basin, the Kalmarsund and at Öland's southern cape, good oxygen conditions were noted throughout the profile.