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  order ­
  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  period. ­
  Thus,  the  main  variations  between  the  2  years  must  be
related  to  biological  factors.
The  temporal  evolution  of  the  biological  consumption  processes ­
  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  consumption ­
  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,  respectively. ­
  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  decrease ­

  in  bottom  CL  in  2007  results  from  the  enhanced  supply ­
  of  organic  matter  and  the  subsequent  increased  degradation ­
  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  surrounding ­
  waters.  The  large  contribution  of  bacterial  remineralisation ­
  (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.  However, ­
  as  NIT  also  accounts  for  about  8  %  of  the  total  biological ­
  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  experiencing ­
  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  different ­
  characteristics  with  respect  to  stratification  and  O2  consumption. ­
  Observations  and  model  results  suggest  that  the
area  between  54-57°  N  and  4.5-7°  E  shows  the  highest  potential ­
  for  low  O2  conditions,  but  also  areas  around  the  Doggerbank
  experience  lowered  bottom  O2  concentrations.