Abstract
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 � 10 -4 m s -1 . Subsurface accumu- lations occurred under a variety of turbulence intensities; however, accumulation was significantly reduced when the turbulent velocity scale in the mixed layer was >5 � 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.
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