Laboratory models of diel vertical migration in the dinoflagellate Ceratium hirundinella

SUMMARY. Diel vertical migrations of a dinoflagellate, Ceratium hirundinella, were induced in a laboratory tube (1.63 × 0.15 m) under a light‐dark cycle. The timing of vertical migrations differed between cultures in the exponential and stationary phases of growth; the latter showed a greater coincidence with the light regime. Migration of cells into the surface layers occurred at low values of surface irradiance (<550 μeinsteins m−2 s−1). At irradiances more closely approaching summer sunshine (> 1300 μE m−2 s−1) there was a marked avoidance of surface waters, and population maxima were found at depths associated with a relative irradiance level of 10% or c. 150 μE m−2 s−1). Thermal stratification restricted downward movement of cells into the cooler layers. The combination of high surface irradiance and thermal stratification resulted in large, stable, sub‐surface maxima of Ceratium, similar to those observed in natural waters under comparable environmental conditions.