Cruise report from R/V Aranda week 14-15, 2019

Type: Report
Author: Karin Wesslander


The sea surface temperature was between 2.9–6.5 ºC, coldest in the northern parts of the Baltic Proper and warmest in Skagerrak and Kattegat. The sea surface salinity was as highest 30.5 in the open Skagerrak and as lowest 6.8 in the Baltic Proper and the salinity was above normal at some stations in the Baltic Proper. In Skagerrak, the halocline was observed between 10–20 meters and in Kattegat between 5–15 meters. In the Baltic Proper, the halocline started at 30 meters in the Arkona Basin and at 65 meters in the Eastern Gotland Basin. The dissolved inorganic nitrogen in the surface layer was consumed at almost all stations, which is normal for April. There were very small amounts of phosphate in the Skagerrak and Kattegat surface layer (<0.1 μmol/l) but in the Baltic Proper there were plenty of phosphate left in the surface (0.5 μmol/l). The concentration of silicate was above normal in the surface layer in Skagerrak, Kattegat and most of the Baltic Proper. Normal levels of silicate were only found in the Arkona and Bornholm Basins. It was generally low fluorescence (plankton activity) in Skagerrak and Kattegat but higher in the Baltic Proper.

Acute oxygen deficiency (<2 ml/l) was found from 65–75 meters depth in the Baltic Proper. Hydrogen sulphide was observed in the Eastern, Western and Northern Gotland Basin. Anoxic conditions, when hydrogen sulphide forms, were observed in the Eastern Gotland Basin from 130 meters in the Gotland Deep (BY15) and from 90 meters in the Fårö Deep (BY20). In the Western and Northern Gotland Basin anoxic condition was observed from 80–90 meters. The deep water in the Arkona Basin, Skagerrak and Kattegat was well  oxygenated.