2530

F.  Große  et  al.:  Looking  beyond  stratification

Biogeosciences,  13,  2511-2535,  2016

www.biogeosciences.net/13/2511/2016/

This  study  demonstrated  that  ecosystem  models  are  capable ­
  of  describing  the  key  features  of  the  O2  dynamics  as  an
integral  part  of  the  North  Sea  ecosystem.  This,  in  combination ­
  with  the  provision  of  a  spatially  and  temporally  consistent ­
  picture  is  useful  for  the  detection  of  regions  susceptible
to  low  O2  conditions  which  therefore  require  enhanced  management. ­
  Additionally,  this  is  of  importance  for  monitoring
authorities  as  our  model  showed  that  bottom  O2  measurements ­
  taken  in  late  summer  provide  a  synoptic  picture  of  the
North  Sea  O2  conditions.
This  study  provides  a  general  characterisation  and
process-based  analysis  of  the  North  Sea  O2  dynamics  in  its
present  state,  including  the  actual  eutrophication  status  in  the
“continental  coastal  region”  as  defined  for  the  OSPAR  assessment ­
  (Claussen  et  ah,  2009).  The  question  on  the  anthropogenic ­
  contribution  to  the  O2  deficiency  problem  in  relation ­
  to  elevated  nutrient  supply  is  beyond  the  scope  of  this
study.  However,  the  capability  of  three-dimensional  models
to  describe  the  O2  dynamics  in  the  context  of  the  natural
variability  of  the  ecosystem  can  be  related  to  changes  in  anthropogenic ­
  drivers,  such  as  increased  atmospheric  deposition ­
  (Troost  et  ah,  2013)  or  riverine  nutrient  input  (Lenhart
et  al.,  2010).
Similar  model  studies  are  essential  for  the  assessment
within  the  Water  Framework  Directive  (WFD),  in  which  dissolved ­
  O2  is  used  as  a  key  parameter  (Best  et  ah,  2007).  As
the  WFD  assessment  depends  strongly  on  the  description  of
pristine  conditions,  related  to  natural  nutrient  levels  (Topcu
et  al.,  2009),  ecosystem  models  can  provide  a  consistent  picture ­
  of  the  North  Sea  O2  dynamics  under  these  pristine  conditions ­
  and  of  the  effects  of  WFD  reductions  (Schemewski
et  al.,  2015).  As  river  load  reductions  within  the  WFD  regulation ­
  affect  the  entire  North  Sea  ecosystem,  also  in  terms
of  bottom  O2  conditions,  these  scenarios  can  also  be  interpreted ­
  within  the  frame  of  the  Marine  Framework  Directive
(MSFD),  which  involves  O2  as  one  of  the  main  descriptors
for  the  definition  of  the  “Good  Environmental  Status”.
Recent  observational  studies  explicitly  highlight  the  importance ­
  of  organic  nutrient  loads  on  the  O2  dynamics  in
the  context  of  nutrient  reductions  (Kemp  et  al.,  2009;  Topcu
and  Brockmann,  2015).  Thus,  future  modelling  studies  on
the  effects  of  nutrient  reductions  on  the  marine  environment
should  differentiate  between  the  effects  of  organic  and  inorganic ­
  nutrient  inputs  in  order  to  optimise  measures  in  the
catchment  area  with  respect  to  cost  efficiency.

Diaz  and  Rosenberg  (2008)  report  an  exponential  expansion ­
  of  global  O2  deficiency  (and  hypoxia)  since  the  1960s
and  argue  that  future  changes  in  O2  conditions  will  strongly
depend  on  the  effects  of  climate  change  on  stratification  and
riverine  nutrient  supply.  For  the  North  Sea,  several  model
studies  predict  a  rise  in  water  temperature  (e.g.,  Lowe  et  al.,
2009;  Meire  et  al.,  2013;  Mathis  and  Pohlmann,  2014)  which
will  reduce  the  O2  solubility  (Weston  et  al.,  2008).  Stratification ­
  intensity  may  either  increase  (Lowe  et  al.,  2009;  Meire
et  al.,  2013)  or  even  decrease  (Mathis  and  Pohlmann,  2014),
implying  opposed  effects  on  bottom  O2.  Primary  production
could  increase  due  to  enhanced  nutrient  supply  caused  by
changes  in  weather  conditions  (Rabalais  et  al.,  2010)  or  due
to  a  temperature-driven  increase  in  metabolic  rates  (van  der
Molen  et  al.,  2013),  which  could  eventually  aggravate  the
O2  conditions  (Justic  et  al.,  2003).  In  contrast,  Groger  et  al.
(2013)  predicted  a  North  Sea  wide  reduction  in  primary  production ­
  by  about  30  %  due  to  reduced  winter  nutrient  import
from  the  Atlantic.  As  these  potential  changes  in  the  O2  conditions ­
  will  also  affect  the  biocoenosis  of  the  North  Sea  (Emeis
et  al.,  2015),  it  is  important  to  foster  the  analysis  of  potential
impacts  of  climate  change  and  changes  in  nutrient  loads  on
the  O2  dynamics.
Data  availability
The  time  series  data  from  the  Cefas  station  North  Dogger  can
be  accessed  via  the  Cefas  Data  Hub  (https://www.cefas.co.
uk/cefas-data-hub/)  according  to  Greenwood  et  al.  (2016).
The  time  series  data  of  MARNET  station  Ems  can  be  retrieved ­
  from  the  Deutsches  Ozeanographisches  Datenzentrum
  (DOD  Data  Centre)  via  email  query  to  dod@bsh.de.
The  spatially  resolved  data  from  the  North  Sea  cruises  in
2001  and  2005  have  been  released  in  the  framework  of  the
EU-FP6  project  CARBOOCEAN.  These  data  can  be  accessed ­
  via  the  CARBOOCEAN  data  portal  (http://dataportal.
carboocean.org/)  according  to  Thomas  et  al.  (2012)  and
Thomas  and  Borges  (2012),  respectively.  The  data  of  the
North  Sea  cruise  2008  (R/V  Pelagia  64PE294;  Zemmelink,
2008)  have  not  been  published,  yet,  but  can  be  requested  via
the  CODIS  data  portal  (http://www.nioz.nl/portals-en;  registration ­
  required).