Phytoplankton cells have the ability of forming aggregates which are dispersed 
in the water column as a result of currents and turbulence, leading to a patchy 
distribution of phytoplankton. Phytoplankton is the first level of food accessible 
to animals. It is in particular the main food available to the early larval stages 
of many fish species, including the anchovy. At such stages, larvae are passive 
and can only eat the prey passing in a very close vicinity. The best situation is
when the larva is near a phytoplankton aggregate, while on the other hand larvae 
which stay far from aggregates are not likely to survive. Thus, being able to describe 
the distribution in numbers of phytoplankton aggregates of different sizes as well as 
locating them in the space turn out to be of  up-most importance in connection 
with the study of fish recrutment.
        I am going to present some models of the phytoplankton dynamics.
First, we present models given by Adler [1] and by N. El Saadi, O. Arino [2]. 
In these models locations of phytoplankton's cells are described by superprocesses 
which are solutions of some stochastic partial differential equations.
The next model is presented in paper [3] where individuals are phytoplankton aggregates 
which consists of phytoplankton cells. Then we go to an Eulerian model in which
the distribution of the size and the positions of the phytoplankton
aggregates are described by a non-linear transport equation which contains
terms responsible for the growth of phytoplankton aggregates, their fragmentation, 
coagulation and diffusion. We study asymptotic behaviour of moments of the solutions
and we explain why phytoplankton tends to create large aggregates [4].

[1] R. Adler, Superprocesses and plankton dynamics,
in: Monte Carlo Simulation in Oceanography, Proc.
of the 'Aha Huliko'a Hawaiian Winter Workshop,
University of Hawai at Manoa (1997), 121-128.

[2]. N. El Saadi, O. Arino, A superprocess with spatial interactions
for modelling the aggregation behavior in phytoplankton, submitted
somewhere.

[3] R. Rudnicki, R. Wieczorek, Fragmentation - coagulation models 
of phytoplankton, submitted to Phys. Rev. E

[4] O. Arino, R. Rudnicki, Phytoplankton dynamics, 
Comptes Rendus Biologies, 327 (11) (2004), 961-969.