Diel observations on populations of Dinophysis and Alexandrium spp. (Dinophyceae) from the NW Adriatic Sea

Quanti-qualitative changes in Dinophysis and Alexatidrium populations from the Gulf of Venice NW Adriatic Sea - were studied on a short time scale (44 h cycles). Under hydrographic conditions of marked thermohaline stratification (July 1995), D. sacculus populations assembled in the surface waters (12 24 yt isopycnals). Division processes took place mostly in the early hours of the morning and at 14:00 h, 4 to 10 % of the population consisted of morphotypes of reduced size (possibly gametes resulting from successive cell divisions). Similar percentages were also observed in October for D. caudata, with a maximum division between 06:00 h and 10:00 h. During this month, coexisting populations of Alexandrium cf. fundyense and A. cf. tamarense displayed rhythmic changes in cell density that slightly differed from those exhibited in July by A. pseudogonyaulax, suggesting a circadian control of the cell cycle rather than a response to hydrographic changes