CMG Collaborative Research: Interactions of Phytoplankton with Dissipative Vortices

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Intellectual merit: The aim of this project is to develop coordinated laboratory experiments and computational models to address a fundamental question in oceanography concerning magnitudes and mechanisms of turbulence effects on phytoplankton and other particles at the spatial scale of individual organisms. The importance of external energy in the form of turbulence in determining relative success of different kinds of phytoplankton dates to the seminal analysis of Munk and Riley (1952) and Margalef (1978). Margalef's "mandala" asserts that high nutrient concentrations and turbulence intensities favor dominance by diatoms, whereas low values favor non-red-tide dinoflagellates. Subsequent work has revealed a wide spectrum of turbulence effects among species of dinoflagellates, including growth stimulation. The physicochemical mechanisms that govern these effects largely remain to be determined, however.Through iteration between innovative numerical models and experiments, the investigators will close a growing gap between textbook understanding of turbulent flows and understanding of consequences for suspended organisms and particles. Models and experiments have used one-dimensional shear to assess turbulence effects at the level of single cells and chains. Effects of fluid straining on concentration fields and cell rotation have been predicted, and effects on cell growth and motion, documented. Current understanding of turbulence, however, places greater emphasis on vorticity, gradients in vorticity and vortices at dissipation scales experienced by individual phytoplankton cells. We propose to develop a framework for both numerical and analog evaluation of effects that cells experience from being in and near viscous-scale vortices, that capture effects of vorticity as well as fluid deformation, evolution of concentration fields, and fluid-structure interactions. Roles of vorticity and gradients in vorticity in determining cell motions and thereby shaping concentration fields have been underappreciated, partly because a signature feature of turbulence, i.e., vortex stretching, is impossible in the two-dimensional flows that so far have been used as theoretical models and the primary basis of analog devices.Numerical approaches will use two simplified models of small-scale vortex structure and evolution, the Burgers vortex and the Lundgren stretched-spiral vortex, giving particular attention to diffusion of vorticity within and away from both. Both decaying and equilibrium vortices will be explored. Models of cells and chains of cells will be based on shapes and flexural stiffnesses of actual cells and chains. Each will be placed successively at a range of positions within and near a vortex and will be fully coupled mechanically to the fluid. Behaviors of interest are cell and chain translation, rotation and deformation and their feedbacks to local velocity and vorticity fields that could be used by grazers to locate a cell. Also to be modeled is the diffusion of scalars (nutrients with cell as sink or metabolites with cell as source), allowing calculation of diffusive fluxes for nutrient acquisition and prediction of chemical fields used by grazers. The investigators will further take advantage of their existing models of flow around flagella to include motile dinoflagellates in the modeling and measurement scheme.Analog experiments will exploit the fact that flows near Kolmogorov scale are dominated by viscosity, just as in earlier Couette experiments, but will incorporate realistic, 3D time variation. Borrowing from a burgeoning variety of geometries used in microfluidics, the investigators will construct a variety of small devices that utilize shed vortex streets, mild jets and cavity flows to match deformation rates, vorticities and gradients in them that produce interesting effects on phytoplankton in their numerical models of vortices. These analogs will be used to test the model predictions and to pose new questions of the models.Broader impacts: Results for phytoplankton extend easily to other important phenomena such as diffusion of attractants from eggs spawned in a turbulent environment (e.g., by abalone and other benthic invertebrates) and corresponding sperm swimming capabilities. They have implications for other important encounter processes such as particle coagulation and sedimentation, hydrosol filtration, and predator-prey interactions. This new approach provides both a natural bridge from larger-scale, direct numerical simulation (DNS) models of turbulence to these individual-scale effects of turbulence and a logical path to parameterizing these effects in larger-scale fluid dynamic models.Turbulence intensity is one of the parameters most likely to be influenced by climate change, and the investigators will work closely with the Center for Ocean Sciences Education Excellence Ocean Systems (COSEE-OS) that has chosen oceans under climate change as its major focus. They will also build on their history of providing teaching and outreach materials in biomechanics at low Reynolds numbers for graduate students, undergraduates and high-school teachers. They will complement both of these efforts with professionally produced, evocative visual animations of the important phenomena that they identify for incorporation into the COSEE-OS website.