Cruice report from R/V Svea week 28-29, 2026

This expedition, conducted within the Swedish pelagic monitoring programme, covered the Skagerrak, the Kattegat, the Sound and the Baltic Proper.

Sea surface temperatures were generally close to the seasonal norm throughout the surveyed area, except at a few stations in the Baltic Proper where temperatures were below normal. Surface temperatures ranged from 12.6 to 19.11°C. The highest values were recorded in the Kattegat. Surface salinity was above the seasonal norm across most of the surveyed area.

Concentrations of dissolved inorganic nitrogen (DIN) and phosphate in surface waters were generally low, which is typical for the season. Exceptions were observed at a few stations in the Sound and the southern Baltic Sea, where the phosphate concentrations were higher than normal. Surface-water silicate concentrations were low in the Skagerrak, as expected for the season, while elevated concentrations, above the seasonal norm, were observed throughout the Baltic Proper.

Oxygen conditions in the Baltic Proper remain very poor. Good oxygen conditions were found only in the southern areas, including the Arkona Basin, the Bornholm Basin, and at station BY39. Acute oxygen deficiency, defined as oxygen concentrations below 2 ml/l, was observed from depths of 70–90 m in both the Western and Eastern Gotland Basins, with the exception of the shallow station BY39. Hydrogen sulphide was detected from 70–90 m depth in the Western Gotland Basin and from 80–125 m depth in the Eastern Gotland Basin.

At several stations in the Skagerrak, pronounced chlorophyll fluorescence maxima occurred at approximately 30 m depth, which were dominated by dinoflagellates and diatoms. In the Kattegat, chlorophyll fluorescence maxima were observed at 20–30 m depth and were dominated by dinoflagellates. At station Anholt E, an additional deeper maximum at 60 m was dominated by chainforming diatoms. In the Baltic Proper, surface accumulations of cyanobacteria were observed, and both surface samples and the upper water column were dominated by cyanobacteria. The vessel's automated Imaging FlowCytobot (IFCB) recorded cyanobacteria throughout the Baltic Sea from the Sound onwards, as well as two species of cyanobacteria in the Kattegat during the return transit.

Several stations in the Baltic Proper exhibited pronounced chlorophyll fluorescence maxima between 15–20 m depth. In the Kattegat, elevated fluorescence peaks were observed between 20 and 30 m depth. Surface accumulations of cyanobacteria were observed east of Gotland. Further information on the
phytoplankton situation is available in the July Algaware report:

The July Algaware report

The next regular monitoring cruise is scheduled to commence begin in Lysekil on 6 August and conclude in Lysekil on 13 August.