Recent observations have found that swimming phytoplankton species have a patchy spatial distribution, even down to millimeter scales, in oceans and seas.
This is rather surprising, because it goes against the intuitive expectation that the turbulent flow in such bodies of water should mix the microorganisms and spread them rather homogeneously. We study how the length scales and interactions in the system lead to the observation of patchiness. We have discovered that mild turbulence can lead to the formation of small dense patches of phytoplankton, when the typical scales of the turbulence roughly match the scales of the interaction among the microorganisms.
This work is a collaboration between Dr. Marco Mazza’s team at Loughborough University and the Max Planck Institute for Dynamics and Self-Organization in Göttingen, Germany
Phytoplankton often encounter turbulence in their habitat. As most toxic phytoplankton species can swim, resolving the interplay between their motility and turbulence has fundamental repercussions for our understanding of their ecology and of the entire ecosystems they inhabit.
The spatial distribution of swimming phytoplankton cells exhibits patchiness at distances of decimeter to millimeter scales, for numerous species with different motility strategies. The explanation of this general phenomenon remains challenging. Furthermore, hydrodynamic interactions between cells, which grow more relevant as the density in the patches increases, have been so far ignored. Our project combines particle simulations and continuum theory to study the emergence of patchiness in motile microorganisms in three dimensions. By addressing the combined effects of motility, cell–cell interaction, and turbulent flow conditions, we uncover a general mechanism: The coupling of cell–cell interactions to the turbulent dynamics favours the formation of dense patches. By identifying the important length and time scales, independent of the motility mode, we have been able to elucidate a general physical mechanism underpinning the emergence of patchiness. Our results shed light on the dynamical characteristics necessary for the formation of patchiness and complement current efforts to unravel planktonic ecological interactions.