Cruise report from R/V Svea week 32-33, 2025

Summary

During the expedition, which is part of the Swedish pelagic monitoring program, Skagerrak, Kattegat, the Öresund Strait, and the Baltic Proper were visited.

Surface water temperatures ranged from 16 to 19 °C in Skagerrak, around 18 °C in Kattegat and the Öresund, and between 18 and 19 °C in the Baltic Sea. The surface water salinity was 25 – 32 psu in Skagerrak, 16 – 26 in Kattegat, and 6 – 8 in the Baltic Sea. Windy weather along the west coast had mixed the warm surface water down and weakened the halocline in parts of Skagerrak and Kattegat.

Concentrations of dissolved inorganic nutrients (nitrogen, phosphate, and silicate) were generally low or below the detection limit throughout most of the surveyed area, except in the Baltic Sea, where elevated levels of silicate persist. Elevated phosphate levels combined with low surface water temperatures at the southern coast of Öland suggest upwelling in the area.

Oxygen levels in the bottom waters of the west coast were good (3.0 – 5.8 ml/l). In the Arkona Basin, oxygen levels were close to the limit for severe oxygen deficiency (2.0 ml/l), which is a worsening compared to July. In the Bornholm Basin, severe oxygen deficiency persists from 70 – 80 meters depth, while oxygen levels in the Hanö bay have increased since July and are now around 4 ml/l. In the Eastern Gotland Basin severe oxygen deficiency was observed from 70 meters and hydrogen sulphide was present from somewhere between 125 and 150 meters and deeper, unfortunately not detected with higher vertical resolution. In the Western Gotland Basin, severe oxygen deficiency was found from 60 – 70 meters, and hydrogen sulphide occurred from 80 meters.

The next expedition with R/V Svea is scheduled for September 24 – 29, starting in Lysekil and ending in Kalmar.