Turbulent velocity profiles for dissipation rate of 1e-6 W/kg

Fundamental marine ecosystem dynamics such as mating, predation, and infection require individual plankton to move relative to one another. Ambient turbulence is often invoked as a mechanism to facilitate such interactions. The local intensity of turbulence is quantified as the dissipation rate of turbulent kinetic energy. While the dissipation rate is central to understanding large-scale fluxes of heat, salt, and nutrients in the ocean, we show that it can be a poor descriptor of the turbulent environment experienced by individual plankton. A dissipation rate is a single integrated quantity representing all the complex motions in a turbulent region; the instantaneous turbulent environment of plankton may bear little resemblance to that predicted by the dissipation rate or quantities derived from it. Most importantly, the statistics (probabilities) of the relative motions of plankton in turbulence cannot be recovered from the dissipation rate or its spectrum: the probabilities of the plankton experiencing any given turbulent shear are lost in the calculation. This presents a fundamental barrier to our understanding of the effects of ambient turbulence on planktonic ecosystem dynamics in the ocean. Rather than relying on dissipation rates, we show that quantifying the probability distributions of the microscale turbulent motions can provide much richer insights into the turbulent environment of individual plankton. Expanding such statistical analyses, and improving our understanding of the Lagrangian properties of ocean turbulence as experienced by plankton in the ocean will lead to significant increases in our ability to understand and quantify the effects of turbulence on plankton.

Methods
Data were acquired with the Modular Microstructure Profiler (MMP, MacKinnon and Gregg, 2003; Mickett et al., 2004) deployed during the FLEAT (FLow Encountering Abrupt Topography) experiment around Palau (Johnston et al., 2019; MacKinnon et al., 2019). The 440 vertical velocity profiles gave data from ~20 m to as deep as 500 m, with ~1.7 mm spatial resolution; turbulent velocities were low-pass filtered to remove noise created by the instrument vibrations.