A study into the photophysiology, growth and migration of Ceratium hirundinella in Chaffey Reservoir in subtropical northern New South Wales, Australia, revealed that a proportion of cells formed subsurface accumulations at depths that optimized light intensity (212–552 µmol photons m–2 s–1) for photosynthesis and cell growth. At high incident irradiance, Ceratium migrated downwards from the near-surface waters, avoiding high-light-induced, slow-recovering non-photochemical quenching of photosystem II. Overnight deepening of the surface mixed layer by convective cooling produced homogeneous distributions of Ceratium with a significant proportion of the population below the depth where light saturation of photosynthesis occurred. Ceratium migrated towards the surface from suboptimal light intensities, at a velocity of 1.6–2.7 x 10–4 m s–1. Subsurface accumulations occurred under a variety of turbulence intensities; however, accumulation was significantly reduced when the turbulent velocity scale in the mixed layer was >5 x 10–3 m s–1, beyond which turbulent diffusion dominated advection by swimming. The formation of subsurface accumulations with increased computed water column integral photosynthesis by 35% compared to a uniform cell distribution.