Cruise report from R/V Svea week 45, 2021

Type: Report
Author: Örjan Bäck


During the cruise, which is part of the Swedish national marine monitoring programme, the Skagerrak, the Kattegat, the Sound and the Baltic Proper were visited.

The water in the Skagerrak was well mixed which resulted in weak stratification and higher salinity in the surface water. Temperatures higher than normal for the month where observed at several places in the Skagerrak and at some places in the Kattegat. The water in the Baltic Proper was well mixed down to 30 - 45 metres, shallowest mixed layer depth was found in the southwest in the Arkona Basin. Below the mixed layer there was a thermocline with decreasing temperature and a small increase in salinity, the salinity then increased more at the permanent halocline at around

60 - 80 metres. The temperature and the salinity in the deep water in the basins around Gotland were somewhat higher than normal.

The nutrient concentrations were generally at low levels in the upper parts of the water body in all areas. Concentrations increase below the halocline, low to normal values where observed on the west coast. In the Baltic Proper low to normal values of DIP where found below the halocline and with ammonium missing (due to technical issues with the instrument) DIN haven’t been possible to evaluate, DIN concentrations far over monthly mean was observed in October.

The oxygen situation was good in the Skagerrak, an improvement since October which probably depends on the mixing of the water column which was seen in temperature and salinity. In the Kattegat the oxygen concentrations were lower than normal below the halocline which was located at 20 metres. At Anholt E the oxygen concentration was just over 2 ml/l at the bottom which is close to the limit of acute hypoxia (<2 ml/l).

The oxygen conditions were still very poor in large parts of the Baltic Proper. At BY15 in the Eastern Gotland Basin, BCSIII-10 in the southeast, as well as in the Eastern, Northern and Western Gotland Basins, hydrogen sulphide concentrations were found much over of which is normal, several stations are now closing in on record high values of hydrogen sulphide. A small increase in oxygen concentrations in the bottom water could be seen in the Arkona Basin, the Bight of Hanö and at BY4 in the Bornholm Basin which led to that the oxygen levels there where on normal values for the month and somewhat higher than what was observed in October.

Plankton activity measured by fluorescence from the CTD was generally low in all areas. No peaks in chlorophyll fluorescence were noted. The fluorescence was low and evenly distributed throughout the well mixed surface layer, then rapidly decreasing below the thermocline.

SMHIs next regular monitoring cruise with R/V Svea is planned to 7th to 18th December, starting in Kalmar and ending in Lysekil. During the December cruise mapping of nutrients will be made in the Bothnian Sea and the Bothnian Bay. This year in addition a coordination and intercalibration with Umeå University which are responsible for the regular national monitoring the year around in the Gulf of Bothnia.