jats:titleAbstract</jats:title>jats:pHigh‐resolution large‐eddy simulations are used to study the influence of submesoscale mixed layer instability and small‐scale turbulence on phytoplankton growth in light‐limited conditions. Four simulations are considered with small‐scale turbulence driven by varying levels of surface cooling. Significant small‐scale turbulence is seen even without surface forcing, and the downward mixing of phytoplankton is sufficient to briefly delay the developing bloom. Moderate and strong values of the constant surface heat flux (jats:italicQ</jats:italic> =− 10,−100 W/mjats:sup2</jats:sup>) are sufficient to prevent a bloom. In contrast to the critical depth hypothesis, the growth rate for phytoplankton does not appear to be controlled by the mixed layer depth. Instead, a comparison between the turbulent diffusivity above the compensation depth and a critical value predicted by the critical turbulence hypothesis closely matches the timing and magnitude of phytoplankton growth.</jats:p>