Cruise report from R/V Svea week 6-7, 2020

Type: Report
Author: Lena Viktorsson


During the expedition, which is part of the Swedish national marine monitoring programme, the Skagerrak, the Kattegat, the Sound and the Baltic Proper were visited. Extra stations were sampled in the Baltic Proper for winter nutrient mapping. The winds were very hard during parts of the cruise and some stations could therefore not be sampled.

The surface water temperature varied between 5-6 °C which is about 2-3 °C above normal for the season. Due to the unusually warm surface water, the concentration of oxygen in the surface water was lower than normal in all sea areas, as warm water dissolves less oxygen than cold. In the Skagerrak there was a halocline at 30-40 m, in the Kattegat at 5-10 m and in the Baltic Proper of 50-80 m. The salinity in the surface water was higher than normal at all stations in the Skagerrak and at several stations in both the Kattegat and the Baltic Proper.

The mapping of nutrients in the Southern Baltic Proper showed that the concentration of phosphate and inorganic nitrogen in the surface water were normal, with the exception of some stations in the southern Baltic Proper where they were slightly below normal. Silicate levels were above normal in all areas except Arkona. The levels in the deep water were also normal for the season, except in the Western Gotland Basin, where the concentration of dissolved inorganic nitrogen (~12 μmol l) and silicate (~60 μmol/l) were significantly higher than normal below the halocline at 60 m.

The oxygen conditions in the bottom water in Arkona and the Bornholm Basin were good. Oxygen in the Bornholm Basin had decreased slightly since January, when the inflow that occurred in December had oxygenated the bottom water. Instead, oxygen deficiency was now found around 50 m at station BY5 as a result of the earlier bottom water being lifted higher in the water column. The oxygen concentration under the halocline decreased further into the Baltic Proper. In the Stolpe channel and the Eastern Gotland Basin and the Northern Baltic Proper, it was oxygen free from 70-80 m and in the Western Gotland Basin from 60 m. A layer with low oxygen levels was found between 80-125 m at BY15 but the levels were now lower than before, <0.5 ml/l. At the two stations in the Gulf of Gdansk, it was oxygen free from depth exceeding 80 m. At station BY39 Ölands Södra udde oxygen deficiency was observed already from 30 m depth.