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F. Große et al.: Looking beyond stratification
Biogeosciences, 13, 2511-2535, 2016
www.biogeosciences.net/13/2511/2016/
3°E 6°E 9° E 12° E 3° VV
1
0.9
0.8
0.7 -Ö
Й
’ 3° Е 0° Е 9° Е 12° Е
Figure 7. Spatial distribution of oxygen deficiency index (ODI) according to Eq. (5 ) for the years 2002 (a) and 2010 (b).
of about 40 m, while the regions deeper than 90 m (C and D)
show a weaker decrease.
Net advection of organic matter, which is not taken into
account in the ODI, appears to be of minor importance for
subsurface O2 relative to the local surface production as the
net advective input of organic matter is significantly less than
the local production. The O2 concentrations at the beginning
of the stratified period were also not taken into account as
they show lower values in regions with higher minimum con
centrations and vice versa.
In summary, the ODI represents well the spatial and tem
poral variations of minimum bottom O2 concentrations de
spite the small set of controlling factors. This confirms that
a simple combination of only stratification duration, organic
matter production and bottom depth is sufficient to repro
duce the main spatial and temporal patterns of the minimum
bottom O2 in the seasonally stratified North Sea. Thus, the
findings from Table 1 can be applied to most parts of the
North Sea. In addition, the similarity of the ODI inside the
regions analysed in Sect. 3.3.1 and inside the regions selected
for the mass balance analyses (see Fig. 2, regions 2-4) and
their surrounding areas shows that these regions can be con
sidered as representative, allowing for a meaningful analysis
of the O2 dynamics in these regions.
3.4 Driving mechanisms and year-to-year variability of
sub-thermocline O2 dynamics
The previous analyses showed that stratification constitutes
a necessary condition for O2 deficiency, but year-to-year
variations especially in the biological factors mainly control
the O2 dynamics. For a better understanding of the processes
controlling sub-thermocline O2 a more detailed analysis is
provided by the mass balances in Fig. 8. As the bottom O2
dynamics are also influenced by the processes in the mid
water, Fig. 8a and b show the O2 mass balances for the sub-
MLD volume (V su b) in region 3 (see Fig. 2) for the years
2002 and 2010, respectively.
The stratification characteristics are similar for both years
with an average MLD of about 15 m and a stratification pe
riod (grey area) of 187 days in both years, only differing by
a later onset (and breakdown) of stratification in 2010. The
temporal evolution of the MLD (dash-dotted grey) is also
similar, with deeper MLDs at the beginning and end of the
stratified period and few events of enhanced mixing during
the summer months (May to August).
The sub-MLD O2 concentration (solid magenta) at
the beginning of the stratified period in 2002 is about
9.81 mgCLL -1 , being about 0.33 mgCLL -1 lower than
in 2010. At the end of stratification, the 2002 value of
6.85 mgCLL -1 is about 0.81 mg CLL -1 lower than in 2010.
In 2002, the clearly diverging temporal evolution of sim
ulated O2 and the corresponding saturation concentrations
(CL,sat. dash-dotted magenta) reveals that the different O2
evolution is caused not only by decreasing CL solubility.
Hence, other factors must play an important role for the CL
evolution below the MLD.
3.4.1 The influence of advection and mixing
The comparison of advection (ADVo,; including horizon
tal and vertical components; dashed light blue) and verti
cal mixing (MIXo,; turbulent diffusion; dashed dark blue)
for the years 2002 and 2010 shows strong variations be
tween the 2 years. ADVo, regularly changes its influence
on the sub-MLD CL concentrations during stratification in
both years. However, considering the temporally integrated
effect, ADVo, causes a net gain of about 25.8 gCLm -2 in
2002, whereas in 2010 it results in a slight net loss in CL.
Advection positively affects the CL concentrations dining the
last 2-3 weeks of the stratified period in both years and even
causes a net CL increase in 2002.
