Growth of Ceratium hirundinella in a subtropical Australian reservoir:: the role of vertical migration

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 mu mol 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.