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Phytoplankton's patchiness has profound effects on the ecology of the oceans. It plays a fundamental role in microorganisms population’s composition and it also modulates the encounter rate, the predation and the reproduction.
Clustering can occur over very different scales, from planetary to the microscale; in this work we are interested in the small scale clustering. It is due mainly to the combined effect of turbulence and phytoplankton motility. Indeed it has been found that motile microorganisms are more patchy that non-motile.
For these reasons the current study focuses on the effect of polydispersity (or variation) of the swimming parameter in a motile phytoplankton population trasported by a turbulent flow.
By means of extensive numerical simulations, we find that the variety of the population introduces a characteristic scale R∗ in its spatial distribution, that depends on the dispersion of the population. At scales smaller than R∗ the swimmers are homogeneously distributed, while at larger scales an inhomogeneous distribution is observed with a fractal dimension. Our numerical results, which extend recent findings for a monodisperse population, indicate that in principle it is possible to observe small scale, fractal clustering in a experiment with gyrotactic cells.
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