2520 
F. Große et al.: Looking beyond stratification 
Biogeosciences, 13, 2511-2535, 2016 
www.biogeosciences.net/13/2511/2016/ 
I 
10.2 
9.8 
9.4 " 
9 i 
8.6 (N 
O 
8.2 E 
B 
7.8 O 
(U 
7.4 ra 
co 
7 I 
6.5 
6.2 
5.8 
Figure 4. Spatial distribution of observed and simulated average bottom Ot concentrations in late summer (a) 2001, (b) 2005 and (c) 2008, 
and (d) standard deviation in 2005. Colour scale of panel (b) applies to panels (a)—(c). Circles indicate sample sites, underlying colours show 
simulation results. Averages and standard deviation were calculated for the entire observation period (bottom-left comer of each panel). 
White circles in (d) mark model bottom grid cells with only one corresponding observed value (i.e., no standard deviation). 
due to the decreasing vertical resolution of the model in 
greater depth. 
In 2005, the observed minimum values are about 
0.3mgO7L _1 higher than in 2001. This relative increase 
compared to 2001 is reproduced well by the simulation show 
ing a similar increase by about 0.2mgO7L _1 . The obser 
vations show lowest bottom O2 a bit north and south of 
the simulated minimum, but still relatively low values less 
than 7.2 mgCL L _1 in the centre of the simulated minimum. 
Highest observed concentrations of 9.3mg07L _1 are lo 
cated in the very deep area at the eastern end of the Nor 
wegian Trench, and in the inner German Bight. In the Ger 
man Bight, the model indicates only slightly higher values 
compared to 2001, while in the northern North Sea, the sim 
ulation shows lower values compared to 2001, which can be 
seen in the observations as well. 
In 2008, the observations reveal significantly higher bot 
tom O2 concentrations in the area of the 2001 minimum, 
compared to the previous years. In contrast, observed con 
centrations in most other parts of the North Sea are lower 
than in 2001 and 2005. The overall minimum concentration 
in 2008 of 7.2 mg02 L _1 is reached close to the Dutch coast. 
In the southern North Sea, the simulation yields lower bot 
tom O2 concentrations, and significantly higher values in the 
2001 minimum area. For the western central and northern 
North Sea the picture is different. Here, the observations re 
sult in consistently lower values compared to 2001 and 2005, 
whereas the simulated O2 shows higher values in most areas, 
except for the eastern Norwegian Trench. 
The simulated SD in 2005 is mainly representative for 
the years 2001 and 2008 as well. It shows that in most ar 
eas of the North Sea the changes in bottom O2 dining Au 
gust/September are very low throughout a period of 3 to 4 
weeks, indicated by a SD of less than 0.1 mg02L _1 . The 
observations also show only small SDs in most areas. 
In general, the basin-wide distributions of simulated bot 
tom O2 represent the observed spatial patterns and their year- 
to-year variations quite well, even though absolute values 
are not always reflected. Both observed and simulated bot 
tom O2 concentrations show that the 50 m isobath (broadly 
along 54° N, 0° E to 57° N, 8° E; Thomas et al., 2005) marks 
the separation line between the northern regions unaffected