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

A study into the photophysiology, growth and migration of Ceratium hirundin ella in Chaffey Reservoir in subtropical northern New South Wales, Australi a, revealed that a proportion of cells formed subsurface accumulations at d epths that optimized light intensity (212-552 mu mol photons m(-2) s(-1)) f or photosynthesis and cell growth. At high incident irradiance, Ceratium mi grated downwards from the near-surface waters, avoiding high-light-induced, slow-recovering non-photochemical quenching of photosystem II. Overnight d eepening of the surface mixed layer by convective cooling produced homogene ous distributions of Ceratium with a significant proportion of the populati on below the depth where light saturation of photosynthesis occurred. Cerat ium migrated towards the surface from suboptimal light intensities, at a ve locity of 1.6-2.7 x 10(-4) m s(-1). Subsurface accumulations occurred under a variety of turbulence intensities; however, accumulation was significant ly 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 swimm ing. The formation of subsurface accumulations with increased computed wate r column integral photosynthesis by 35% compared to a uniform cell distribu tion.