Cruise report from R/V Svea week 19-20, 2025

Summary

During the expedition, which is part of the Swedish pelagic monitoring program, the Skagerrak, Kattegat, Öresund, and the Baltic Proper were visited. Stockholm University conducted sediment and benthic fauna sampling in the Baltic Sea.

Surface water temperatures were generally normal for the month, ranging from 9 – 11 °C in the Skagerrak and Kattegat and 6 – 10 °C in the Baltic Proper. In the Skagerrak and Kattegat, a weak thermocline had begun to form with warmer surface waters, while the Baltic Proper was more well-mixed and colder down to the halocline.

Surface salinity was higher than normal for the month in large parts of the Baltic Proper. In the Skagerrak and Kattegat, salinity levels were normal or above normal.

Concentrations of dissolved inorganic nutrients in the surface waters of the Skagerrak and Kattegat were low or below detection limits, which is typical for this time of the year. In the Baltic Proper, phosphate and silicate concentrations were higher than normal, while DIN was below detection levels at several stations. In the southern parts of the Baltic, levels were below normal.

Oxygen levels in bottom waters were normal for the month in the Skagerrak and Kattegat (4.7 – 6.4 ml/l). In the Arkona Basin, no severe oxygen deficiency was observed, but the oxygen situation had worsened since April. In the Bornholm Basin and Hanö Bay, severe oxygen deficiency (<2 ml/l) prevailed from 70 – 80 meters, with oxygen concentrations ranging from 0.4 to 1.7 ml/l. In the Eastern Gotland Basin, hydrogen sulphide was detected from 125 meters, with severe oxygen deficiency already from 70 meters. In the Western Gotland Basin, hydrogen sulphide was measured from 80 – 90 meters, and severe oxygen deficiency was present from 70 – 80 meters.

There will be no expedition in June due to budget cuts. The next expedition with R/V Svea is planned for July 14 – 20, starting and ending in Lysekil.