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Published July 30, 2009 | Supplemental Material
Journal Article Open

A viscosity-enhanced mechanism for biogenic ocean mixing


Recent observations of biologically generated turbulence in the ocean have led to conflicting conclusions regarding the significance of the contribution of animal swimming to ocean mixing. Measurements indicate elevated turbulent dissipation—comparable with levels caused by winds and tides—in the vicinity of large populations of planktonic animals swimming together1. However, it has also been noted that elevated turbulent dissipation is by itself insufficient proof of substantial biogenic mixing, because much of the turbulent kinetic energy of small animals is injected below the Ozmidov buoyancy length scale, where it is primarily dissipated as heat by the fluid viscosity before it can affect ocean mixing. Ongoing debate regarding biogenic mixing has focused on comparisons between animal wake turbulence and ocean turbulence. Here, we show that a second, previously neglected mechanism of fluid mixing—first described over 50 years ago by Charles Darwin — is the dominant mechanism of mixing by swimming animals. The efficiency of mixing by Darwin's mechanism is dependent on animal shape rather than fluid length scale and, unlike turbulent wake mixing, is enhanced by fluid viscosity. Therefore, it provides a means of biogenic mixing that can be equally effective in small zooplankton and large mammals. A theoretical model for the relative contributions of Darwinian mixing and turbulent wake mixing is created and validated by in situ field measurements of swimming jellyfish using a newly developed scuba-based laser velocimetry device. Extrapolation of these results to other animals is straightforward given knowledge of the animal shape and orientation during vertical migration. On the basis of calculations of a broad range of aquatic animal species, we conclude that biogenic mixing via Darwin's mechanism can be a significant contributor to ocean mixing and nutrient transport.

Additional Information

© 2009 Nature Publishing Group. Received 15 December 2008; Accepted 9 June 2009. We acknowledge W. M. Graham, J. H. Costello and H. Swift for assistance in field measurements and M. Schaadt for preparatory dive assistance. Additional logistical support was provided by the Coral Reef Research Foundation. Field work in Palau was supported by the National Science Foundation Biological Oceanography Program (to M. N. Dawson and J.O.D.). Additional support (to J.O.D.) from NSF Biological Oceanography, Ocean Technology, Fluid Dynamics, and Energy for Sustainability and from the Office of Naval Research (K.-H. Kim) is acknowledged, as are NSF and NDSEG fellowships (to K.K.) and the Charles Lee Powell Foundation. Author Contributions K.K. and J.O.D. designed the study, analysed the data and wrote the manuscript. K.K. performed the experiments and viscous flow simulations. J.O.D. performed the inviscid flow simulations and derivation of equation (1).

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