Turbulence-plankton interactions : a new cartoon

Abstract

Climate change will alter turbulence intensity, motivating greater attention to mechanisms of turbulence effects on organisms. Many analytic and analog models used to simulate and assess effects of turbulence on plankton rely on a one-dimensional simplification of the dissipative scales of turbulence, i.e., simple, steady, uniaxial shears, as produced in Couette vessels. There shear rates are constant and spatially uniform, and hence so is vorticity. Studies in such Couette flows have greatly informed, spotlighting stable orientations of nonspherical particles and predictable, periodic, rotational motions of steadily sheared particles in Jeffery orbits that steepen concentration gradients around nutrient-absorbing phytoplankton and other chemically (re)active particles. Over the last decade, however, turbulence research within fluid dynamics has focused on the structure of dissipative vortices in space and time and on spatially and temporally varying 2 vorticity fields in particular. Because steadily and spatially uniformly sheared flows are exceptional, so therefore are stable orientations for particles in turbulent flows. Vorticity gradients, finite net diffusion of vorticity and small radii of curvature of streamlines are ubiquitous features of turbulent vortices at dissipation scales that are explicitly excluded from simple, steady Couette flows. All of these flow components contribute instabilities that cause rotational motions of particles and so are important to simulate in future laboratory devices designed to assess effects of turbulence on nutrient uptake, particle coagulation and predatorprey encounter in the plankton. The Burgers vortex retains these signature features of turbulence and provides a simplified “cartoon” of vortex structure and dynamics that nevertheless obeys the Navier-Stokes equations. Moreover, this idealization closely resembles many dissipative vortices observed in both the laboratory and the field as well as in direct numerical simulations of turbulence. It is simple enough to allow both simulation in numerical models and fabrication of analog devices that selectively reproduce its features. Exercise of such numerical and analog models promises additional insights into mechanisms of turbulence effects on passive trajectories and local accumulations of both living and nonliving particles, into solute exchange with living and nonliving particles and into more subtle influences on sensory processes and swimming trajectories of plankton, including demersal organisms and settling larvae in turbulent bottom boundary layers. The literature on biological consequences of vortical turbulence has focused primarily on the smallest, Kolmogorov-scale vortices of length scale η. Theoretical dissipation spectra and direct numerical simulation, however, indicate that typical dissipative vortices with radii of 7η to 8η, peak azimuthal speeds of order 1 cm s-1 and lifetimes of order 10 s as a minimum (and much longer for moderate pelagic turbulence intensities) deserve new attention in studies of biological effects of turbulence.