„ 
Yet for various studies the use of numerical models might 
be unnecessary. When one has to decide whether or not 
certain pollutants are potential hazards, in how far 
certain chemical or biological processes are predominant 
over others, or what the best sampling strategy would be 
for a certain study, it can be useful to have other tools 
to analyse the situation or to decide whether further, 
more detailed studies (e,g. with mathematical numerical 
models) are required, 
In this context time scales play a role (e,‚g. Goldberg, 
1976). Especially the time scale found by dividing the 
volume of a sea area by the flux of water entering should 
be mentioned, 
This time scale is often called residence time, transit 
time ©r flushing time. As these terms, when properly de- 
fined, are in many cases different from the simple quotient 
of volume over flux, the more neutral term "turn-over time" 
(t5) is recommended, So, 
x volume 
o * mean Tlux entering 
This turn-over time under stationary conditions is equal to 
the mean transit time, that is the time water elements 
entering the area remain there. It is also equal to the 
mean age, that is the mean time the water elements pre- 
sent in the area have been there since they entered for 
the last time, if the sea area is "well mixed", that is 
if the properties of the water are the same everywhere in 
the area, 
Under the same conditions the turn-over time is equal to 
the mean flushing time, that is the mean time the elements 
in the area will stay there before £ihey leave. 
Because of these equalities the turn-over time is often 
used in estimating possible concentrations that may arise 
from a source of dissolved substance in £fhe area. The 
idea is that the total quantity of a substäance that is