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 im 
mediately 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 influ 
ence not only on source strengths, but also on the dispersion and distribution of sub 
stances and compounds in the marine environment. Pollution variability thus has nat 
ural 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 anoma 
lies« 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 resem 
ble 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 Ger 
man Bight proper (S = 33) were derived by regression analysis from salinity and nutri 
ent 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 ni 
trogen 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, especial 
ly 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 compensat 
ed 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 corre 
sponded 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.