We present models of the dynamics ofphytoplankton aggregates. We start with an individual-based modelin which aggregates can grow, divide, joint and move randomly.Passing to infinity with the number of individuals, we obtain amodel which describes the space-size distribution of aggregates.The density distribution function satisfies a non-linear transport equation, which contains terms responsible for thegrowth of phytoplankton aggregates, their fragmentation,coagulation, and diffusion.