Turbulence-plankton interactions : a new cartoon
Turbulence-plankton interactions : a new cartoon
Date
2009-02-11
Authors
Jumars, Peter A.
Trowbridge, John H.
Boss, Emmanuel S.
Karp-Boss, Lee
Trowbridge, John H.
Boss, Emmanuel S.
Karp-Boss, Lee
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Keywords
Plankton
Shear
Turbulence
Vortex
Vorticity
Shear
Turbulence
Vortex
Vorticity
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.
Description
Author Posting. © John Wiley & Sons, 2009. This is the author's version of the work. It is posted here by permission of John Wiley & Sons for personal use, not for redistribution. The definitive version was published in Marine Ecology 30 (2009): 133-150, doi:10.1111/j.1439-0485.2009.00288.x.