The horizontal dynamics of a single plankton `patch' in an ocean is considered theoretically in terms of semi-empirical turbulent diffusion equation taking into account scale dependence of turbulent diffusivity. Both one-dimensional and two-dimensional cases are considered, for a one-species 1-D model exact formula for the size of the patch vs. time is obtained. For a 2-D case, an expression for the critical radius of plankton patch is derived. Validity of turbulent diffusion equation and comparison with experimental results are discussed.