Fig. 4. Time evolution of the cells' distribution by two-dimensional projections of velocities onto x–z plane. Contour plots of the probability density function (see Equation 5) at six time instants. The difference between colour levels is 0.00015 s² µm^–2. In the 20 min plot, where the number of motile cells that had reached the observation area is small, the peak corresponding to suspended dust particles is seen. On later images, where the number of active swimmers is large enough, this peak vanishes.