Letter

Nature 460, 624-626 (30 July 2009) | doi:10.1038/nature08207; Received 15 December 2008; Accepted 9 June 2009

A viscosity-enhanced mechanism for biogenic ocean mixing

Kakani Katija1 & John O. Dabiri1,2

  1. Bioengineering,
  2. Graduate Aeronautical Laboratories, California Institute of Technology, Pasadena, California 91125, USA

Correspondence to: Kakani Katija1John O. Dabiri1,2 Correspondence and requests for materials should be addressed to J.O.D. (Email: jodabiri@caltech.edu) or K.K. (Email: kakani@caltech.edu).

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 mixing2. Ongoing debate regarding biogenic mixing has focused on comparisons between animal wake turbulence and ocean turbulence3, 4. Here, we show that a second, previously neglected mechanism of fluid mixing—first described over 50 years ago by Charles Darwin5— 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 device6. 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.

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