Results of a numerical model simulating the behaviour of phytoplankton
cells are presented and discussed. The motion of the cells (or of their
aggregates) develops along the vertical according to a turbulent velocity
field simulating vertical dynamics within the mixed layer. In addition,
the model describes a number of biological processes depending on the
vertical position of the cells and on their individual physiological
history. In particular, the energy source for the cell growth, occurring
through photosynthesis, is represented by sun light, whose availability is
a function of time, of the vertical position of the growing cell as well
as of the other cells, because of the self-shading effect which is
explicitely accounted for. Photoadaptation processes are also included in
the model.  The results show the relative importance of the different
physico-biological factors affecting the phytoplankton cell behaviour.