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F. Große et al.: Looking beyond stratification
Biogeosciences, 13, 2511-2535, 2016
www.biogeosciences.net/13/2511/2016/
sidering the biological sink processes, pelagic remineralisa
tion of organic matter (REM pe i; dashed light green) has the
strongest effect on the sub-MLD O2, accounting for 50%
of the overall biological consumption. Benthic reminerali
sation (REM S ed; dashed yellow) accounts for 18.8%, while
zooplankton respiration (RES zoo ; dashed dark green) and ni
trification (NIT; dashed red) contribute 22 and 8.8 %, re
spectively. This order in the relative importance is consistent
throughout the entire period 2000-2012 (not shown).
In 2002, REMpei is strongest among all years yield
ing —103.1 g02 m -2 , while 2010 represents the year of
weakest REM pe i. For RES zoo , 2002 yields a value of
-45.1 g0 2 m- 2 (1.8-fold of 2010 value). The 2002 and
2010 values constitute the highest and lowest among all
years, respectively. The same applies to REM se d with a 2002
value of —28.5g02m -2 (1.2-fold of 2010 value). NIT is
also strongest in 2002 resulting in —18.1 g 02 m -2 , while
in 2010 it is about 13 % below the average value of
—13.0 ±2.5 g0 2 m -2 .
The integrated effect of all biological sink pro
cesses (REMpei, REM S ed, RES zoo and NIT) adds up to
—204.8 g 02 m -2 in 2002 and to —136.4 g 02 m -2 and 2010,
i.e., the biological O2 consumption in 2002 is 1.5 times
higher than in 2010 and 1.2 times higher than the 2000-2012
average of 169.9 ± 21.2 g02 m -2 . The relative contribution
of the individual processes to the biological O2 consump
tion shows only minor variations during the analysed period.
REMpei contributes to 53.6 ± 1.7 %, while REM se d accounts
for 17.2 ± 0.9 %. For RES Z oo and NIT the average contribu
tions result in 21.6 ±1.5 and 7.6 ± 0.8 %, respectively. The
EXPorg below 25 m depth (not presented; calculation analo
gous to Table 1) in 2002 is nearly 1.6 times larger than in
2010, which is in good agreement with the differences in the
integrated effect of the biological O2 sinks.
In late April and late June 2002 (Fig. 8a) two events of
enhanced mixing reveal direct and indirect effects on the bi
ological processes. The renewal of the nutrient pool causes
short-term increases in PP around the MLD which in turn en
hances RES zoo an d REMp e i. Consequently, only the stronger
event in late June causes a net increase in O2. It should be
noted that the shown strengthening in the different biological
effects is also influenced by the change in the MLD (i.e., in
tegration depth), however, it is also visible when considering
a constant MLD (not shown).
PP shows the strongest effects on sub-MLD O2 when the
MLD is shallowest which indicates the existence of a deep
chlorophyll maximum (DCM). This explains the negative in
fluence of MIXo 2 during these periods as O2 concentrations
are highest within the DCM due to high PP. The only minor
positive or even negative effect of MIXo 2 during most of the
stratified period emphasises the importance of stratification
for the sub-MLD O2 dynamics as it efficiently limits the O2
supply.
The good agreement between the variations in EXP org and
the integrated effect of the biological O2 sinks between the 2
years confirms that the supply of detrital matter to the deep
layers is the driving force of sub-MLD O2 consumption. The
strong influence of pelagic remineralisation demonstrates its
crucial role for the bottom O2 concentrations as it directly
affects the potential O2 supply from the mid-water into the
bottom layer.
3.5 Bottom layer dynamics of the North Sea O2
minimum zone
Even though the dynamics in the mid-water affect the bot
tom O2 levels, lowest concentrations occur in the bottom
layer. In order to show which processes are the main con
tributors to the O2 dynamics in this layer, Fig. 8c and d show
the mass balances for the bottom layer in region 3 for 2002
and 2010. The average bottom depth in this region is 47.75 m,
and the model bottom layer encompasses a volume of about
14.4km 3 .
The O2 concentrations at the beginning of the stratified
period, 9.79 and 10.12mgO2L _1 for 2002 and 2010, re
spectively, are similar to those in V su b. The concentrations
at the end of stratification, 6.76mg02L _1 in 2002 and
7.55mg02L _1 in 2010, show larger differences to those for
Esuh-
The effect of the physical factors, ADVo 2 and MIXo 2 , on
the bottom O2 is different to that for V su b. While in 2002
ADVo 2 shows a similar effect on O2 as for the sub-MLD O2,
its effect in 2010 is opposite to that for V su b, resulting in a mi
nor increase of about 1.1 g02m -2 . During the last 3 weeks
of stratification in 2002, the same positive effect of ADVo 2
as in the sub-MLD mass balance is shown, initiating the re
covery of the bottom O2 before MIXo 2 intensifies. MIXo 2
has a consistently positive effect on the bottom O2 in both
years. Its integrated effect is increased relative to V su b by the
factor 1.7 and 1.4 in 2002 and 2010, respectively.
The relative contribution of the biological O2 sinks in the
bottom layer is also different to the sub-MLD volume. The
2000-2012 averages reveal that in the bottom layer REM se d
accounts for 50.1 ± 1.2 % of the total biological O2 con
sumption, while REMpei contributes to 32.2 ± 1.4%. Thus,
aerobic remineralisation consistently adds up to more than
80 % of the biological O2 consumption in the bottom layer.
This shift results from the different volumes considered, and
the fact that REM se d only has a direct effect on the deepest
pelagic layer. Average O2 consumption due to REM se d re
sults in values between 3.9 and 6.5 mmol02 m -2 d _1 .
For RES ZO o> the influence on the bottom O2 concentrations
is lower than in V su b (11.3 ± 1.2 % during 2000-2012). This
relates to the fact that zooplankton tends to stay in the up
per part of the water column where phytoplankton concen
trations are higher. NIT represents the weakest sink for bot
tom O2 with an average contribution of 6.4 ±0.6% during
these years. PP as a potential source for O2 is negligible in
the bottom layer due to light limitation.
