Cruise report from R/V Svea week 49-50, 2023

Type: Report
Author: Helena Björnberg, Lena Viktorsson, Martin Hansson


During the expedition, as part of the Swedish pelagic monitoring program, the Skagerrak, the Kattegat, the Sound, the Baltic Proper, and the Bothnian Bay were visited. In the Bothnian Bay, SMHI and Umeå Marine Research Centre (UMF) conducted joint mapping, including nutrients.

The surface water temperature had continued to decline in December and was lower than normal in many areas.

Normally, the concentrations of dissolved inorganic nutrients increase during winter, but in December, the concentrations of both phosphate and dissolved inorganic nitrogen had decreased at several stations in Skagerrak. However, in Kattegat, the concentrations of inorganic nutrients had increased since November, which is normal. In the Baltic Proper, the concentration of nutrients in the surface layer had increased since November. The levels were within the normal range for the season, except in the Northern Baltic Proper where the levels were above normal.

The levels of dissolved inorganic nitrogen in the surface waters of the Åland Sea were at normal levels. In the Bothnian Sea, the levels were slightly higher than normal in the north-west and at some stations in the south-east. The levels increased with depth, and at these stations, levels above normal were also noted in the bottom water. In the central Bothnian Sea, the levels were higher than normal and around the normal range.

Phosphate levels in surface waters were higher than normal at all stations except coastal stations along the Swedish coast. Phosphate levels increased with depth, and the levels were well above normal in deep waters at some stations. Silicate levels were above normal at the majority of stations in the Bothnian Sea. Here too, the levels increased with depth, and at many stations, levels above normal were noted in deep waters.

In the Bothnian Bay, the concentration of dissolved inorganic nitrogen was around normal. In deep waters, nitrogen levels increased and showed both lower and higher levels than normal. Phosphate levels in surface waters were very low, which is normal for the season, except in the southern Bothnian Bay where levels were higher than normal. Levels increased with depth, and higher-than-normal levels were found in deep waters. Silicate levels in surface waters are naturally high due to the large runoff into the Bothnian Bay. Levels above normal were noted, and in the deep waters, the levels increased further, also exceeding normal levels.

The oxygen situation was good at all stations in Skagerrak and Kattegat; no oxygen deficiency was noted. In the Sound, oxygen deficiency was measured near the bottom.

In the main basins of the Baltic Proper, oxygen-free conditions occurred from 60 to 80 meters, which coincided with measured levels of hydrogen sulphide. Similar conditions could be observed in the Bornholm Basin and in the Hanö Bay. The oxygen situation in the bottom waters of the Arkona Basin were good.

The oxygen levels in the bottom water of the Bothnian Sea varied around 5,5 ml/l in the deeper areas and slightly higher in the bottom water of the shallower regions. Oxygen deficiency was observed in Gaviksfjärden. No oxygen deficiency was noted in the Bothnian Bay or Northern Quark, which is normal.

SMHI’s next regular expedition with R/V Svea is scheduled for January 11th to 17th, starting in Gothenburg and terminating in Lysekil.