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Abstract

It has long been suggested that biological activity, such as the swimming of fish, may induce turbulence and mixing of an intensity that is comparable to that induced by physical processes, such as internal wave breaking. A recent survey using a MicroRider turbulence package mounted on a Slocum glider, in the Sargasso Sea down to depths of 600 m, provides clear evidence of turbulence induced by biological activity. Microstructure shear and temperature gradient measurements show extremely short and intense patches of turbulence. The patches are detected equally by both shear-probes and FP07 thermistors, but seldom simultaneously. The spectra of shear roll up more steeply than the classic Nasmyth spectrum and the variance of shear suggests dissipation rates as large as 10^-5 W/kg. However, the buoyancy flux deduced from the variance of the gradient of temperature, and the model of Osborn and Cox, is typically about 10^-10 W/kg, making the mixing efficiency ~10^-4 at night and ~10^-2 during daylight. Although the turbulence induced by biological activity is extremely intense, it contributes little to vertical mixing.