i 6 
calculated. Large computer systems are so efficient, in the mean- 
time, 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 indroduc- 
tion of many different. types of pollutants. The direst” introduction 
at sea (dumping and incineration, burdens owing to the general ship- 
ping traffic - such as oil and ship’s garbage) are conspicuous; of 
far greater significance are the run-offs of pollutants via the riv- 
ers and via the atmosphere. 
Beside the community sewage, which is still predominantly released 
via the rivers, above all it is industrial discharges and also resi- 
dues 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 deter- 
mined, in the first instance, by the currents, However, physical, 
chemical, and biological. processes to which the substances are sub- 
ject in the surrounding waters and with the sediments, have deci- 
sive influence upon the distribution of the pollutants. The knowl- 
edge about these complex processes, at the present time, is not yet 
sufficient by far in order to take all these factors quantita- 
tively 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