Cruise report from R/V Svea week 47-48, 2025

Summary

SMHI conducted measurements of oxygen and nutrient conditions during SLU Aqua’s fisheries survey, the Baltic International Trawl Survey (BITS-Q4), which covers parts of the Baltic Proper. SMHI’s participation aimed to compensate for the cancellation of the regular November cruise due to insufficient funding. During the survey, SMHI carried out CTD measurements in connection with each trawl haul and collected water samples for analyses of nutrients, oxygen, total pH, and chlorophyll. Measurements were conducted at four of SMHI’s standard monitoring stations (BY38, BY39, BY20, and Hanö Bay), as well as at the survey station BY13.

Weather conditions during the cruise were generally calm. Wind speeds reached a maximum of approximately 20 m/s in gusts during the middle of the survey. Weather conditions were sunny at the beginning of the cruise and gradually transitioned to mostly cloudy toward the end.

Surface water temperatures were higher than normal in the waters around Gotland, while no such anomaly was observed in Hanö Bay. Concentrations of inorganic nutrients had increased since October, and phosphate concentrations were above normal in Hanö Bay. Oxygen deficiency was observed from depths as shallow as 50 m, with acute oxygen deficiency occurring from approximately 60 m. At the deeper stations, bottom waters were completely anoxic below depths of approximately 70–80 m.

The next cruise with R/V Svea is planned for 4–16 December, starting in Kalmar and ending in Lysekil. During the December expedition, a nutrient survey of the Gulf of Bothnia will be conducted.