We observed populations of phytoplankton species causing red tides in the inner part of the Ariake Sea on July 29th and August 6th of 2002, and August 16th and 23rd of 2004. Hourly vertical profiles of water temperature, salinity, and dissolved oxygen and chlorophyll-a levels have been collected from June 2002 until the present. The diurnal vertical movements of Ceratium furca (Dinophyceae) blooms in 2002 and those of Chattonella antiqua and C. marina (Raphidophyceae) blooms in 2004 were estimated. The average velocities of upward and downward movements of Ceratium were 1.7±0.3 and 0.4-1.0±0.3 m · h-1, and those of Chattonella were 1.5-2.0±0.3 and 1.7±0.2 m · h-1, respectively. The Ceratium population moved downward in synchrony with changes in pycnocline layer level , but the Chattonella population moved upward across the pycnocline layer, independently of pycnocline layer level. It is considered that the discrepancy between the movements of the organisms is related to ecological strategies for obtaining nutrients for growth. Moreover, we think that these organisms that cause red tides could bloom rapidly by utilizing nutrients in oxygen deficient layers below the pycnocline layer.
