  We find the mixing induced by a small swimming organism in a density
stratified fluid. We model the swimmer as a spherical squirmer and quantify
mixing through a mixing efficiency that is the ratio of rate of change of
potential energy of fluid to the total work done by the swimmer. Assessing
mixing in the near field and far field of the swimmer separately, we find that
the near-field mixing aligns with past work, but the overall mixing is much
larger than that caused by a point-sized swimmer (like force-dipole), although
still small in weak stratification. Equivalent results are also obtained for a
homogeneous dilute suspension of non-interacting swimmers. Our study highlights
the impact of swimmer size on mixing, revealing that small-sized swimmers
cannot induce substantial mixing. However, we propose few (unexplored) pathways
through which small swimmers could induce significant mixing, ultimately
contributing towards oceanic mixing.
