How small is too small – the minimum size of a swimmer to generate biogenetic turbulence is not fixed

Biogenic mixing has long been recognized as a potentially significant contributor to oceanic material transport and energy redistribution. A prevailing belief in the field holds that there exists a universal minimum swimmer size below which organisms cannot generate sustainable biogenic turbulence. However, based on a first-principles analysis of the Navier–Stokes equations, we hypothesize that the minimum effective size for generating sustainable biogenic turbulence is not a fixed threshold but depends on the geometric alignment between the swimmer’s orientation and the local background shear as well as the background shear strength itself. We tested this hypothesis using an analog experiment with body size from 1 mm to 10 mm. Our results reveal that the direction and intensity of biogenic spectral energy flux are governed by the relative tensor geometry between the swimmer-induced stress and the background rate of strain tensor. This work redefines the criteria for sustainable biogenic turbulence and highlights a previously overlooked but fundamental mechanism of biogenic mixing with implications for oceanic transport, productivity, and the physical feedback of the biosphere to the environment.