Movement of red water plankton in the year of no red water occurrence in the case of Omura Bay in 1966

摘要

In spite of no occurrence of red water in 1966, we observed many microalgae totaling 129 species of Cyanophyceae, Heterocontae, Chrysophyceae, Bacillariophyceae, Desmocontae, Dinoflagellata and Chlorophyceae which occurred in the southern part of Omura Bay. Among them, ten species including Dictyocha fibula, Gonyaulax polygramma, Gymnodinium type-'65 (causative organisms of the red water of this bay in the past), Gymnodinium nelsoni, unknown species of Gymnodinium (specimen No. 57 in Plate II), and Ceratium fusus were selected as important organisms and their movement was pursued in the present study. Many of these species occurred concentratedly during the period of early and middle August and some species increased in density of population to some extent though not enough to cause red water phenomena. It is presumed that this period was most important for the occurrence of red water phenomena being provided with most environmental requirements in view of several occurrences of localized discoloration though slight in intensity. The increases might have been the preceding biological phenomena of red water lacking one or more unknown decisive requirements.