F. Große et al.: Looking beyond stratification 
2523 
www.biogeosciences.net/13/2511/2016/ 
Biogeosciences, 13, 2511-2535, 2016 
The vertical mixing of O2, MIXo 2 , is highest within the 
coastal region A and adds up to 116.1 gC^m -2 , which is 
due to strong tidal mixing. The stratification period, i stra t, of 
151 days in region B is shorter than in region C (220 days) 
and does not cover the entire summer period. Thus, MIXo 2 
in region B is significantly larger than in regions C and D. 
The evolution of the O2 concentrations between the be 
ginning and end of the summer period reveals some interest 
ing aspects in relation to the previously mentioned parame 
ters. The O2 concentrations at 1 April show significant dif 
ferences between the regions ranging between 9.5 mg O2 L _1 
(regions C and D) and 10.1 mgC^L -1 (region A). The O2 
concentrations at the 30 September yield values between 
7.7mg02L _1 (region A) and 8.3 mgC^L -1 (region D). This 
implies a consistently decreasing O2 consumption during 
summer from region A to D. This spatial gradient in the O2 
consumption is opposite to that in f strat , which shows a steady 
increase from regions A to D. 
In order to give an impression of the impact of EXP org on 
the O2 dynamics of the water volume below the MLD, V su b, 
we link the amount of exported organic matter to the amount 
of O2 available within V su b assuming the organic matter is 
remineralised completely in the area of settlement. Based 
on the O2 concentration at the beginning of April, the total 
amount of O2 available is 1365kt for region B and 4590 kt 
for region C. The total amount of exported organic matter 
is calculated as the product of EXP org and the total area of 
the considered region. This calculation yields 130ktC and 
115ktC for the regions B and C, respectively. As O2 con 
sumption and C release occur with a molar ratio of 1 : 1 dur 
ing bacterial remineralisation (Neumann, 2000), we obtain 
the daily O2 consumption by dividing by the total duration 
of the considered 6-month period (= 183 days), yielding 0.71 
and 0.63 ktC>2 d _1 for regions B and C, respectively. 
The initial O2 mass is calculated as the product of the ini 
tial O2 concentration and V su b. Assuming the daily O2 con 
sumption to be constant for each region, division of this mass 
by the daily consumption rate calculated above provides an 
estimate of the amount of time required for the consumption 
of the entire amount of O2 available in V su b. This calculation 
yields a period of about 2 years for region B, whereas the 
corresponding value for region C is significantly higher with 
almost 12 years. This great difference between the resulting 
periods (factor 6), compared to the relatively small difference 
between the daily consumption rates (factor 1.1), illustrates 
clearly the large influence of the sub-MLD volume, V su b, on 
the temporal evolution of the O2 concentrations below the 
MLD. 
The same calculation based on the threshold of 
6mg02L _1 used by OSPAR (OSPAR-Commission, 2005) 
yields a consumption period of 283 days for region B, which 
indicates the relatively high potential for O2 deficiency in this 
region. 
This characteristic based on the four different North Sea 
regions demonstrated that the duration of stratification alone 
cannot explain the temporal evolution of sub-MLD O2 con 
centrations. It shows the great importance of the organic mat 
ter export which drives the biological O2 consumption. In 
addition, the volume below the MLD plays a key role as it 
governs the amount of O2 which is available throughout the 
stratified period, and in combination with the organic matter 
export defines whether O2 deficiency may occur or not. Thus, 
these three quantities can be considered as the key parame 
ters governing the O2 dynamics of the seasonally stratified 
North Sea. 
3.3.2 The oxygen deficiency index (ODI) 
The ODI resulting from the simulated stratification dura 
tion (fstrat). summer surface primary production (PP m id) and 
model topography for the years 2002 and 2010 is shown in 
Lig. 7a and b, respectively. It can be seen that the ODI tends 
to be higher in 2002 than in 2010 in the region where mini 
mum bottom O2 is lowest in both years (see Lig. 6). North of 
the Doggerbank, the ODI also shows slightly higher values 
than in the surrounding waters which corresponds to the low 
ered bottom O2 in this region. The variations of the minimum 
concentrations between the 2 years in this region are also 
well-reproduced by the ODI. Especially in 2002, the highest 
ODI coincides with the lowest concentrations in the entire 
domain. In 2010, the highest ODI is located a bit south of 
the minimum O2 concentrations, which is mainly caused by 
the high surface production in this region. Along the north 
ern British coast, the ODI also shows high values for both 
years which is in good agreement with the slightly lower 
minimum bottom O2 in this area. However, ODI values tend 
to be too high and do not represent the slightly lower mini 
mum O2 concentrations off the eastern Scottish coast around 
57-58 °N as the bottom depth in this area exceeds 90 m (i.e., 
/ D = 0). Directly northwest of Denmark, the ODI also yields 
high values for both years with higher values in 2002. This 
corresponds well to the simulated bottom O2 concentrations 
in this area, even though ODI values are too high, compared 
to ODI values in the central North Sea. 
With respect to the factors selected for the calculation of 
the ODI, Table 1 shows that stratification alone is not suffi 
cient to explain the North Sea O2 dynamics. While the re 
duction in the O2 concentrations is steadily decreasing from 
regions A to D, stratification duration is characterised by 
a steady increase from regions A (80 days) to D (226 days). 
Regarding the PP m id. the strongest O2 reduction occurs in 
the regions of highest productivity A and B. In the northern 
regions, the higher PP m id in region D does not correspond 
to stronger reduction in O2. As f stra t is also higher in this 
region, a further factor is needed to describe the basic O2 
dynamics. The reduced effect of surface production in the 
northernmost area is likely to result from the dilution effect 
due to the higher V su b. Considering the bottom depth D\, ot 
as a proxy for V su b, it is shown that the strongest decrease 
occurs in the shallower regions A and B with average depths