Plankton sinking and turbulence

To overcome sinking, plankton have devised various types of strategies. One is swimming, that is, an active, energy-consuming self-propulsion than can oppose gravitational sinking. Other phytoplankters can modify their buoyancy, becoming, at least for some time, positively buoyant. Others can exploit the turbulence in the mixed layer, using it to stay suspended for longer times. This latter option was discussed at length by Margalef [5], who associated the level of turbulence in the surface layer with the type of organisms that are environmentally favoured. The terminal velocity of spherical impurities with the same density grows with the squared radius of the falling impurity. Thus, larger plankton cells tend to sink faster than smaller ones, provided they can be approximated by a somewhat spherical shape. When the turbulence in the euphotic layer is low, Margalef argued, smaller, possibly swimming phytoplankters such as flagellates are favoured as the larger species sink more rapidly out of the euphotic zone. On the other hand, when the level of turbulence is high, larger plankton can benefit from turbulent suspension and become favoured with respect to the small ones (which feel diffusion limitation of nutrients more severely).