2528 
F. Große et al.: Looking beyond stratification 
Biogeosciences, 13, 2511-2535, 2016 
www.biogeosciences.net/13/2511/2016/ 
Region no. 04:20 02 
— Oxygen cone. 
■ 1 Oxygen sat. 
Vert, mixing 
Advection 
" ■ Primary prod. 
Zoopl. respir. 
Pelagic remln. 
Benthic remin. 
Nitrification 
I strat. period: 
28/03-06/09 
thickness: 
bot. depth: 
: area: 
: voliume: 
b.2p m 
45.00 m 
1.91g+09 
1.19E+1Q 
m 2 
m 3 
(a) 
"O c? ^9 *0 V V 
-70 
lajeo 
50 
40 
30 
20 
10 
0 
-10 
-20 
-30 
-40 
: strat. period: 
30/03-26/10 
thickness: 
böt. depth: 
area: 
; voliume: 
9.2b m 
99.2Î3 m 
1.77E+09 
1.63ÎE+10 
m 2 
m 3 
(b) 
-20 
-30 
-40 
'is 6- Up 'o 
Figure 9. Mass balances of simulated bottom Ot in regions 4 (a) and 5 (b); (see Fig. 2) during stratification (grey shaded) in 2002. Same 
legend for (a) and (b). Black y axes apply to processes, magenta y axes apply to Ot (saturation) concentrations. Changes in Ot due to 
different processes are cumulative. 
stratified period, but revealed a faster decrease in bottom O2 
during 2007 compared to 2008. As the simulation showed the 
same tendency with respect to the differences between the 2 
years, the mass balances for station North Dogger for 2007 
and 2008 are presented in Fig. 10a and b, respectively, in or 
der to interpret the observed temporal evolution of bottom 
O2. 
The simulation yields an average rate of O2 reduction 
during the stratified period of about 0.009 mg CFL -1 d -1 in 
2007, and a rate of about 0.007 mg CL L _1 d _1 in 2008. The 
integrated effect of the physical factors, ADVo, and MIXo,, 
is quite similar for both years providing a gross increase in 
O2 of about 16.3 g 02 m -2 integrated over the stratified pe 
riod. Thus, the main variations between the 2 years must be 
related to biological factors. 
The temporal evolution of the biological consumption pro 
cesses is also similar in both years, with higher rates in 2007. 
The integrated effect of all biological sink processes results 
in —40.6 g02 m -2 , which corresponds to an average con 
sumption rate of 0.18 g O2 m -2 d _ 1 . For 2008, the simulation 
yields a 6.8g02m -2 lower biological consumption due to 
a 0.02 gCL m -2 d -1 lower consumption rate. This constitutes 
a relative difference of 13 % between the 2 years. For EXP org 
below 25 m depth during summer (calculation analogous to 
Table 1), the same relative difference is found. 
The relative contribution of the different processes to the 
overall bottom CL consumption shows only minor changes 
between the 2 years. In both years REM se d accounts for about 
58 %, while REMpei accounts for about 31 %. RES zoo and 
NIT contribute to about 3 and 8 % in both years, respec 
tively. This shows that the variations in the CL dynamics 
between 2007 and 2008 at North Dogger are mainly driven 
by differences in the organic matter export. The steeper de 
crease in bottom CL in 2007 results from the enhanced sup 
ply of organic matter and the subsequent increased degra 
dation by bacteria. The enhanced release of ammonium due 
to pelagic and benthic remineralisation consequently triggers 
an increase in nitrification. 
In contrast to the findings by Greenwood et al. (2010), who 
argued that the relatively strong advection at North Dogger 
may ventilate the bottom layers in terms of CL, our results 
suggest that advection only has a minor positive effect due to 
the only slightly higher CL concentrations in the surround 
ing waters. The large contribution of bacterial remineralisa 
tion (REM S ed and REMpei) accounting for almost 90% of 
the overall biological consumption at station North Dogger 
confirms that the estimates for C remineralisation rates made 
by Greenwood et al. (2010) provide reasonable results. How 
ever, as NIT also accounts for about 8 % of the total biolog 
ical O2 consumption, this process should be considered to 
obtain more precise estimates for C remineralisation rates. 
4 Conclusions and perspectives 
The North Sea is one of the shelf regions regularly experi 
encing seasonal O2 deficiency in the bottom water (Diaz and 
Rosenberg, 2008; Emeis et al., 2015; Rabalais et al., 2010). 
However, not all areas of the North Sea are similarly affected 
by low O2 conditions (e.g., Queste et al., 2013) due to differ 
ent characteristics with respect to stratification and O2 con 
sumption. Observations and model results suggest that the 
area between 54-57° N and 4.5-7° E shows the highest po 
tential for low O2 conditions, but also areas around the Dog- 
gerbank experience lowered bottom O2 concentrations.