A primary study has been done on the ecology of 3 species of Ceratium, which are common in the Seto Inland Sea area and well known as causing or composing organisms of red tide phenomena, on the ground of materials obtained from stations near Sensuijima island, Tomo, Fukuyama, during the observational period of May, 1975 to February, 1976.
As the result of this study, the followings have become clear;
1. There are inverse correlations between the values of surface and 5 m layers as to the whole of cell number of 3 species and total phytoplankton represented by chlorophyll a content when examined on the basis of deviations from average values of water column, though the distributional patterns of these organisms are markedly different from that of chlorophyll a.
2. These organisms tend to show diurnal vertical migration bipolarly between surface and 5 m layers despite that total phytoplankton throng and disperse periodically centering around the surface.
3. Although total phytoplankton throng around 6:00, these organisms thrive 0:00 to 3:00 in surface layer.
4. Five meter layer refered above is in acord with the daily compensation depth estimated from transparency of sea water of the continual observational period.
5. According to the whole year observation, C. fusus and C. furca are abundant May to June and September to October when water temperature ranges 15 to 20°C, though C. tripos does not increase in number in autumn.
6. The most optimum temperatures and optimum temperature ranges of these organisms calculated from the data of the whole year observation thought to be reasonable are shown in Table 6.