We examined the effects of salinity on diel vertical migration (DVM) of two coastal flagellates, Chattonella antiqua and Karenia mikimotoi, in 90-cm-high columnar aquariums. Experiments were performed with surface salinities from 5 to 32 and bottom salinity constant at 32. Cells of each flagellate were injected into the bottom of the aquarium at night and the vertical distribution of cells was monitored every 4 h for 36 h in one series of experiments, or twice daily (day and night) for 5 days

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... another. Ascent and descent started at approximately the same time in all water columns, indicating that difference in surface salinity does not substantially affect DVM rhythm in the two flagellates. During daytime, C. antiqua and K. mikimotoi transited haloclines with surface salinities ≥ 15 and accumulated at the surface, although K. mikimotoi required 2 days to transit the halocline with surface salinity of 20 and 4 days for surface salinity of 15. However, neither flagellate could transit haloclines with surface salinities of 5 or 10; instead they accumulated in the halocline during daytime. At night, most cells of both species accumulated at the bottom in all water columns, although the distribution of K. mikimotoi gradually expanded to the upper layers on successive days when surface salinity was 5 or 10. We demonstrated that low salinity in the surface layer blocks upward migration in two flagellate species and delays surface accumulation and weakens population synchrony of DVM at night in K. mikimotoi.