Diel vertical migration and cell division of bloom-forming dinoflagellate Akashiwo sanguinea in the Ariake Sea, Japan

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Diel vertical migration and cellular division processes of Akashiwo sanguinea were investigated in the Ariake Sea in August 2009. The investigation was carried out in the inner part of the Sea for 24 h from 8:00 on 14 August. Water samples were collected every 2 hours, and the temperature, salinity, and chlorophyll fluorescence were monitored hourly. To calculate the in situ growth rate, the duration of the cell division was investigated using incubation experiments in a laboratory. In the Ariake Sea, cells of A. sanguinea were abundant in the subsurface layer in the daytime and at a depth of 4 or 5 m at night. In contrast, cells were mostly absent in the deeper layer in the daytime and in the subsurface layer at night. Downward migration seemed to start at 14:00, indicating that dusk is not the cue for migration. A dense patch was formed at a depth of 5 m from 20:00 to 4:00. The highest frequency of dividing cells was recorded at dusk (18:00) in the Ariake Sea and at light-dark transition (circadian time 14) in the laboratory. The estimated duration of cell division was 1.29 h, and the estimated in situ growth rate was 0.45 d−1. The present study demonstrated that cells of A. sanguinea actively divided in the deeper layer (3.5 to 5 m) from 18:00. To predict red tide occurrence by vertically migrating species, the migration pattern and the growth rate in the natural environment should be further clarified for each species.