FIRST RECORD OF A BLOOM OF THALASSIOSIRA-DIPOROCYCLUS IN THE EASTERN SETO INLAND SEA

A bloom of a colony-forming diatom of the genus Thalassiosira suddenly occurred in almost all areas of the eastern Seto Inland Sea during th e period from late fall to winter of 1994-1995. In order to obtain bas ic information on the bloom and for species identification, we examine d the morphological characteristics of individual cells and colonies b y light and electron microscopes. We also investigated its distributio ns and the relationship between the bloom and environmental factors. S ince the observations by microscope revealed that the morphological ch aracteristics of the species agreed with previous descriptions, we ide ntified the species as Thalassiosira diporocyclus. In the eastern Seto Inland Sea, this bloom was first found around the southern Harima-nad a in late October of 1994, and it spread widely throughout the eastern Seto Inland Sea during November and December 1994. The density of col onies began to decline in early January of 1995, and was hardly observ ed in any of the areas in early February. The high abundance of this s pecies caused low concentrations of nutrients and nori production was economically damaged. This is the first record on the bloom of T. dipo rocyclus in the eastern Seto Inland Sea. Careful monitoring of this sp ecies is necessary to prevent recurrent damage to fisheries in this ar ea.