Under advection, one understands here the transport by the mean cur- 
rent, whereas as diffusion the spreading by the turbulent variations 
of the current is signified. 
At the Institut für Meereskunde der Universität Hamburg and at the 
Deutsches Hydrographisches Institut numerical models have been de- 
veloped which are particularly occupied with the question of how the 
substances runoff over the rivers spread in the North Sea. 
The models have the same thing in common, that they only scrutinize 
one particular type of substances. Only the dispersion of water- 
soluble substances are investigated on the assumption that the sub- 
stances do not influence the current, Chemical, biological, and 
sedimentological processes are left out of consideration, because 
the knowledge about these processes is either too fragmentary or too 
complex in order to include them in the simulation by the models. 
The two models differ from one another in an important prerequi- 
site! 
Whereas the DHI model computes the dispersion of the pollution using 
a mean eirculation, the IfMB model examines the timedependent ecireu- 
lation for a period from 1969 to 1982. Furthermore the numerical 
procedures, on which the models fundamentally base, are different. 
} 
Problems of the Simulation of transport and spreading of 
substances 
The distribution of a substance in the sea, in the first instance, 
is determined by the mean current field — the ecirculation. The eir- 
culation transports substances in water over large distances 
(advection), fluctuations of the current and small-scale turbulen- 
ces cause a disorderly distribution over a smaller space (diffu- 
sion). 
Nowadays, the advective transport can be relatively well described, 
not only temporally but also spatially, with numerical current mod 
els (circulation models). On the other hand, the distribution by 
turbulence can only be simulated with exceptional numerical methods; 
for example, by the Monte Carlo method. 
The transport and distribution in the sea, however, also depend upon 
a diversity of physical, chemical, an biological processes. Thus, 
for example, a suspended substance particle, the density of which is 
slightly greater than water, in addition to the horizontal advective 
movement, is forced to make a vertical downwards movement. The beha- 
viour of a suspended substance particle is dependent upon the densi- 
ty of the surrounding water, and thereby upon the spatial and tempo- 
ral changes of the temperature and the salinity. On the basis of 
chemical and/or geochemical reactions with the seawater or with oth- 
er substances, substances can change their properties and thereby 
influence their transport routes. Chemical reactions, even today, 
can only be taken into account - with difficulty - in dispersion 
models because, on the basis of their complexity, they seldom let