Vertical migration by the marine dinoflagellate Prorocentrum triestinum maximises photosynthetic yield.

Although vertical migration has been widely documented in many dinoflagellate species, the adaptive advantages of the strategy remain unclear. To investigate relationships between diurnal vertical migration and depth-related variation in photosynthetic yield in a marine dinoflagellate, Prorocentrum triestinum (Schiller), two experimental approaches were adopted. First, vertical distribution patterns of P. triestinum in a shallow estuary were investigated with respect to depth-related variation in photosynthetic rate. Second, changes in vertical positioning of P. triestinum in response to manipulations in incident light flux were investigated in an artificial water column. During the field experiment, changes in dissolved oxygen concentration in bottle cultures of P. triestinum indicated maximum rates of photosynthesis at 0.5 m depth. Cell densities of P. triestinum in the water column were also highest at 0.5 m depth and varied in accordance with photosynthetic yield. Fluorescence measurements indicated photoinhibition at super-saturating photosynthetic photon flux densities (PPFDs). Similar patterns were evident in the artificial water column. Depth distribution of P. triestinum varied significantly according to incident PPFD; the band of maximum cell density shifted downward in response to an increase in PPFD and upward in response to a PPFD decrease. Distributions of cells with respect to PPFD were very similar to those observed in the field experiment. Overall, vertical positioning of P. triestinum during the day is consistent with an active migratory mechanism for maximising photosynthetic production.