73

nodel also permit the recognition of vertical concentration differances.


4,2 Variability of the substance spreading (IfMH)

In this Chapter the spatial-temporal variability of the concentration
 distribution is examined; the question is now: which variations
of the pollutant concentrations in the North Sea could be caused by
the variability of the eurrents which determine the transport.

Concentrations variations which were caused by temporally variable
sources (which certainly in reality, more than likely is in accordance
 with the normal. state) are not considered in this study. Here,
this aspect is left out of cöonsideration for the present, in order
to exclusively quantify the influence of the variability of the currents;
 this signifies that here only a partial aspect of the reality
can be represented,

For illustration of the temporal and spatial variations of substances
 in the North Sea, in the following time series the concentration
for different regions of the North Sea are represented and interpreted-


 division of the North Sea into a series of individual regions in
accordance with the widely known box structure suggested by ICES.
(Note: The division of a dynamic, temporally variable regime into
spatially fixed boxes is controversial. The ICES box division of the
North Sea, therefore, was chosen in the first instance because its
internationally well-known.) The time-dependent current. field of the
IfMH model, which is available for the period from 1969 to 1982,
permits the determination of concentration variations in the individual
 boxes, These are given in Fig, 11 for three boxes of the Belgian,
 Netherlands, German, and Danish coastal regions. The variations
 presented clearly show that the fluctuations ö6f the current
Field can lead to considerable deviations of the concentration from
the mean« Owing to the strong variations of the concentration, the
significance of a mean value is of less importance, Although the
mean load (cöncentration of one or more substances) of a sea area
has influence for long-term effects in marine organisms for ecological
 questions, the maximum concentration is of by far greater significance.

The pollution concentration of the North Sea - in accordance with
these model results — is more likely to be characterized by deviations
 from an (arithmetical) mean than by this mean value. The variations
 calculated can deviate around more than 100 % from the mean.

In spite of the strong irregularities. of the variations shown in
Figs. 11b to d, some structures clearly stand out. This is especially
 the change of the concentrations with seasons, which - as already
 mentioned - in _ this study is not produced by varying sources
or by varying biological productivity. In the whole of the southern
North Sea and in the German Bight, in general _ the slightest concentration
 occurs during the winter months, in summer the maximum values
 are attained.