i 6

calculated. Large computer systems are so efficient, in the meantime,
 that thereby three-dimensional. models with numerous layers and
ä&a horizontal solution of several kilometres can be computed, and
thereby permit the simulation of the large-scale circulation.

In spite of the state of numerical modelling attained today, models
can only represent - each in accordance with the points in question
— partlal aspects of the facts in reality.

Thus, in the models presented here, for example, no mesoscale eddies
are resolved which is important for the transport of substances with
regard to short periodic variations and mixing processes. Also the -
in principle possible - modelling of the surface waves caused by the
wind, which not only influence the vertical but also the horizontal
mixing, is also not included.
From the general knowledge about the North Sea circulation one can
already deduce that the substances introduced into the North Sea are
not evenly distributed over the entire area. It would be a improper
simplification to assume that the substances introduced in- to the
North Sea would behave like milk in a well-stirred cup of coffee.
Balances which place the quantity of substances introduced in ratio
to the entire volume of the North Sea (43x1012 m3), could lead
to local concentrations which are much too low and are without
strength of statement.

The North Sea, at the present time, is endangered by the indroduction
 of many different. types of pollutants. The direst” introduction
at sea (dumping and incineration, burdens owing to the general shipping
 traffic - such as oil and ship’s garbage) are conspicuous; of
far greater significance are the run-offs of pollutants via the rivers
 and via the atmosphere.

Beside the community sewage, which is still predominantly released
via the rivers, above all it is industrial discharges and also residues
 of agricultural production (e.g. nutrients, insecticides) which
enter the North Sea via the rivers. A further significant, but so
far difficult to quantify burden takes place over the atmosphere.
Ihe transport and the spreading of pollutants in the sea is determined,
 in the first instance, by the currents, However, physical,
chemical, and biological. processes to which the substances are subject
 in the surrounding waters and with the sediments, have decisive
 influence upon the distribution of the pollutants. The knowledge
 about these complex processes, at the present time, is not yet
sufficient by far in order to take all these factors quantitatively
 into consideration in numerical models. The temporal changes
of the pollutant concentration at one place can be described by the
transport equation.

This equation is the fundamental for the modelling of the pollutant
spreading:

temporal. change of
the concefitration
at a fixed place

= advection + diffusion + reaction

(chemical) 4 Sources
(biological}  sinks