We discuss the speculated role of swimming organisms in mixing up the ocean waters - the so-called biogenic mixing contribution. In particular, the focus is on a recent proposal of large-scale ocean mixing via a Darwinian-drift mechanism. The suggested mechanism seemingly allows for the smallest aquatic organisms, primarily the zooplankton, to generate motion on length scales large enough for efficient mixing to occur even in a stably stratified system such as the abyssal ocean. This is because, rather counter-intuitively, the importance of the drift mechanism appears to increase with increasing viscosity, thereby favouring the smaller aquatic organisms in whose case the induced fluid motion is strongly influenced by viscosity. The proposal lends significant credence to the importance of a biogenic mixing contribution since the aquatic biomass is dominated by the small species. However, a careful examination shows that the importance of the drift-induced mixing mechanism is greatly overestimated. The error lies in equating the motion generated by a passive particle to that induced by an aquatic swimmer; the effects of any drift-induced mixing being far more modest in the latter case. The inevitable conclusion is then that a viscosity-enhanced biogenic mixing mechanism is a rather unlikely candidate in contributing to oceanic mixing on a global scale.
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