charges  from  reprocessing  plants.  The  main  source  is  reactivated  radionuclides  from
highly  polluted  sediments  in  the  Irish  Sea.
Consequently,  measures  to  reduce  external  pollutant  and  nutrient  inputs  may  not  immediately ­
  improve  the  chemical  state  of  the  North  Sea.  Natural  processes,  among
them  storm  frequencies,  precipitation  rates,  discharge  volumes  of  rivers,  and  current
anomalies,  by  reinforcing  or  counteracting  such  measures,  exert  considerable  influence ­
  not  only  on  source  strengths,  but  also  on  the  dispersion  and  distribution  of  substances ­
  and  compounds  in  the  marine  environment.  Pollution  variability  thus  has  natural ­
  and  anthropogenic  components  whose  quantification  often  poses  problems,
rendering  an  assessment  of  the  effectiveness  and  efficiency  of  ecological  measures
difficult.  Therefore,  it  is  increasingly  necessary  to  take  into  account  »physical  anomalies« ­
  of  the  North  Sea  system  when  interpreting  and  evaluating  its  chemical  state.
The  most  important  monitoring  results  as  to  nutrients,  organic  substances,  metals,  and
radionuclides  have  been  compiled  in  the  following.
Nutrients
Spatial  distribution  patterns  of  phosphate  and  silicate  levels  as  well  as  the  summary
concentration  of  nitrate  and  nitrite  in  German  Bight  sea  water  in  winter  closely  resemble ­
  those  of  salinity  because  the  dominant  process  during  this  season  of  low  biological
activity  is  hydrodynamic  mixing.
Nutrient  levels  representative  of  coastal  water  (salinity  30)  and  sea  water  in  the  German ­
  Bight  proper  (S  =  33)  were  derived  by  regression  analysis  from  salinity  and  nutrient ­
  measurements.  In  the  winter  of  2004,  these  levels  did  not  differ  much  from  those
of  the  preceding  winter.  Phosphate  levels  in  the  German  Bight  were  slightly  above,  in
coastal  waters  clearly  above,  reference  values  from  1936.  High  concentrations  of  nitrogen ­
  compounds  indicated  that  major  nutrient  loads  are  still  present  in  these  waters.
Silicate  levels,  which  are  hardly  influenced  by  industrial  and  agricultural  activities,  were
within  the  limits  of  natural  fluctuations.
In  summer  2004,  the  North  Sea  was  stably  stratified  in  the  vertical.  The  surface  layer
was  characterized  by  a  dearth  of  nutrients  and  algal  growth  was  limited  here,  especially ­
  by  nitrate.  Chlorophyll  maxima,  therefore,  were  found  mostly  in  the  depth  range  of
the  thermocline,  where  light  conditions  and  nutrients  from  the  bottom  layer  were  still
sufficient  for  algal  growth.  Nutrient  limitation  was  also  reflected  in  the  composition  of
particulate  matter  (C:N  ratios).  Nutrient  consumption  in  summer  is  largely  compensated ­
  by  the  production  of  dissolved  organic  nitrogen  and  phosphorus  compounds,  which
consequently  are  dominant  among  the  compounds  present  in  the  water  column.
The  distribution  of  oxygen  saturation  levels  in  the  bottom  water  was  well-balanced
compared  to  conditions  in  summer  2003.  Strongest  oxygen  deficiencies  occurred  in
the  eastern  North  Sea,  at  around  85  %,  and  dropped  but  locally  below  the  75  %
threshold  separating  good  from  moderate  oxygen  supply.
On  the  whole,  monthly  levels  of  most  nutrients  at  Helgoland  Roads  in  2004  corresponded ­
  to  the  mean  values  of  the  period  2000  -  2003  or  even  fell  short  of  them.  Only
phosphate  was  measured  at  concentrations,  which  not  only  exceeded  these  reference
values  throughout  the  year,  but  were  even  slightly  above  the  mean  values  of  the  1990s.