Small-scale hydrodynamics as a framework for plankton evolution

Abstract

Small-scale water movements in pelagic systems constitute the proximal physical framework in which plankton has evolved. The size and displacement capacities of plankton (by swimming, floating or sinking) overlap with qualitative discontinuities in the small-scale motions of water, and the effects of these motions on diffusion of substances and particles. It is argued in this paper that this overlapping could be the basis for understanding different ecological behaviour and adaptive pathways in the diverse phylogenetic groups of plankton. The scales of turbulence relative to organism body size determine the 'hydrodynamic weather' to which organisms are exposed. The especially small organisms undergo laminar shear, larger ones can swim between different eddy sizes depending on their displacement capacities. This latter fact confers to some organisms great opportunities for behavioural adaptations, which increases the prey encounter taking advantage of the small-scale structure in the water column. Better understanding of the small-scale hydrodynamic environment, considering intermittency, buoyancy-conditioned mixing, and coherent structures, and its description with statistics that are directly applicable to interactions between plankton organisms, may provide new clues, in the form of 'hydrodynamic climates', for understanding the apparently paradoxically high diversity of plankton.