However, the stratification, forced by solar radiation, was well developed (see sections in 
appendix 4). All sections show a pronounced thermocline at about 30 m depth and a well- 
mixed layer on top, which became weaker along the UK coast due to strong tidal mixing. 
Even the shallow sections along 54°N and 55°N display a thermocline in central parts. In the 
Norwegian Trench (section 58°N) the thermocline had deepened to almost 100 m. 
The temperature difference between surface and bottom (appendix 3, figure 3.1, bottom) was 
around 8 K at the deep Skagerrak stations and about 6 K in the central North Sea. 
1} 
https://www.bsh.de/DE/DA TEN/Meerestemperaturen/Meeresoberflaechentemperaturen/ 
meeresoberflaechentemperaturen_node.html 
Salinity: 
At the surface only in the northwestern part of the North Sea an inflow of Atlantic Water (S > 
35) (appendix 3, Figure 3.2, top) was observed reaching down to 58°N. In the bottom layer a 
oroad inflow over the entire northern sections down to 57°N was observed. The sections 
(appendix 4) reveal that it is reaching up to 40m. An intrusion of Atlantic Water from the 
southwest through the English Channel was not observed. In the Skagerrak the outflow of 
low-salinity water from the Baltic is visible at the surface. Salinity differences between 
surface and bottom of up to 3 (appendix 3, Figure 3.2, bottom) mark the spreading of the 
Baltic outflow along the Norwegian southeast coast. 
Altogether the salinity situation is similar to those in summer 2019 and 2018. 
Oxygen: 
During the survey oxygen measurements were made with the CTD and oxygen was 
analysed from a large number of water samples by titration (see „instruments and methods‘) 
Appendix 3, figure 3.3 and 3.4 show the comparison for the surface and bottom layer. 
Please note that all data are preliminary results (further steps of quality assurance will 
follow). 
Fig. 3.3 (top, left) shows the distribution of dissolved oxygen in the surface layer measured 
Dy Winkler-titration. The concentration ranges from 5 mL/L in the southeast North Sea to 6.5 
mL/L in the northwest North Sea. 
Fig. 3.3 (top, right) shows the same distribution measured by CTD. 
The difference between the concentration of dissolved oxygen in the surface, measured by 
titration on one and by a CTD sensor on the other hand (Fig. 3.3, bottom, left) is on average 
9.129 mML/L. 
Fig. 3.4 (top, left) shows the distribution of dissolved oxygen in the bottom layer measured by 
Winkler-titration. The concentrations of dissolved oxygen ranges between 5 mL/I and 6 
mL/L. They are generally lower than the surface-values. For example at GN037 (57°N, 5°E), 
the concentration of dissolved oxygen is 4.99 mL/L in the bottom layer compared to 5.86 
mML/L in the surface layer. This could be due to remineralisation processes in the bottom 
layer, resulting in oxygen consumption. 
Fig. 3.4 (right) shows the same distribution as on the left site but measured by CTD. 
The difference between the concentration values of dissolved oxygen in the bottom layer, 
measured by titration on one and by a CTD sensor on the other hand (Fig. 3.4, bottom) is on 
average 0.138 mL/L. 
Overall the difference between the two different measurement methods (roughly estimated: 
2.5 %) is lower than the expanded measurement uncertainty of the Winkler titration, witch is 
around 4.4 %, which means they are in good accordance. 
This has to be verified with data from further cruises. 
17