Cruise report from R/V Svea week 43, 2025

Summary

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

Strong winds and rough seas prevailed during the cruise, which made sampling difficult and resulted in one sampling and two buoy missions not being carried out.

The surface water had begun to cool somewhat. In both the Skagerrak and Kattegat, warmer water masses were observed below the surface, while the Baltic Proper showed slightly higher deep-water temperatures than normal.

Nutrient concentrations in the surface water had increased since September, particularly phosphate and silicate, which were above normal levels in almost all sea areas. Elevated nutrient concentrations were also measured in the deep water, especially in the Baltic Proper, where high levels of hydrogen sulphide were also recorded.

The oxygen situation remains very problematic in the Baltic Proper. Acute oxygen deficiency was observed from depths of 60–70 metres, and hydrogen sulphide from 80–90 metres. The most critical levels were measured in the western and eastern Gotland Basins, where hydrogen sulphide concentrations were very high—at some depths among the highest ever recorded.

Plankton activity was generally low, reflecting a season in which productivity has begun to decline.

The next cruise with R/V Svea is scheduled for 5–16 December, starting in Kalmar and ending in Lysekil. During the December cruise, nutrient mapping will be carried out in the Gulf of Bothnia.