We examine how different regimes of stable thermal stratification affect the motion of microswimmers (modelled as gyrotactic self-propelling cells) in free-surface turbulent channel flow. This archetypal setup mimics an environmentally-plausible situation that can be found in lakes and oceans. Results from direct numerical simulations of turbulence coupled with Lagrangian tracking reveal that rising of bottom-heavy swimmers depends strongly on the strength of stratification, especially in regions of high temperature and velocity gradients (thermocline): Here hydrodynamic shear may disrupt directional cell motility and hamper near-surface accumulation. For all gyrotactic re-orientation times considered in this study (spanning two orders of magnitude), we observe a reduction of the cell rising speed and temporary confinement under the thermocline: If re-orientation is not fast enough, long-term confinement is observed because cells align in the streamwise direction and their vertical swimming is practically annihilated.
