Cruise report from R/V Svea week 4-5, 2026

Summary

The phytoplankton spring bloom had commenced in both the Skagerrak and the Kattegat, as indicated by declining nitrate concentrations in surface waters and by distinct chlorophyll fluorescence maxima above the thermocline.

In the Skagerrak, surface water temperatures were low and below the monthly mean in coastal areas east of the tip of Skagen towards the Southern North Sea, while waters originating from the Northern North Sea exhibited temperatures above normal. Salinity followed a similar pattern, with lower values in coastal areas and higher values toward the North Sea. In the coastal regions where surface waters had cooled, a thermocline was present at approximately 15–20 m depth. Phosphate and silicate concentrations were close to winter maxima throughout the Skagerrak, whereas nitrate concentrations along the Swedish coast were lower than normal for the month. At several stations in the northern Skagerrak, pronounced chlorophyll fluorescence maxima, indicating elevated plankton activity, were observed at depths of around 10 m, just above the thermocline. No oxygen deficiency was detected in bottom waters.

In the Kattegat, surface water temperatures were below normal at several stations, particularly during the latter part of the expedition, and surface salinity was lower than normal in the southern parts of the area. A shallow pycnocline was present at approximately 5 m depth, above which relatively high chlorophyll fluorescence was observed. Phosphate concentrations were within normal ranges, while nitrate concentrations had begun to decline and were below normal. The low nitrate concentrations, together with elevated chlorophyll fluorescence in the surface layer, indicate ongoing or recently initiated phytoplankton production, similar to conditions observed in the Skagerrak. No oxygen deficiency was observed in bottom waters, although oxygen concentrations were locally close to the threshold for oxygen deficiency.