First Record of a Bloom of <i>Thalassiosira diporocyclus</i> in the Eastern Seto Inland Sea

Abstract

A bloom of a colony-forming diatom of the genus Thalassiosira suddenly occurred in almost all areas of the eastern Seto Inland Sea during the period from late fall to winter of 1994-1995. In order to obtain basic information on the bloom and for species identification, we examined the morphological characteristics of individual cells and colonies by light and electron microscopes. We also investigated its distributions and the relationship between the bloom and environmental factors. Since the observations by microscope revealed that the morphological characteristics of the species agreed with previous descriptions, we identified the species as Thalassiosira diporocyclus. In the eastern Seto Inland Sea, this bloom was first found around the southern Harima-nada in late October of 1994, andit spread widely throughout the eastern Seto Inland Sea during November and December 1994. The density of colonies began to decline in early January of 1995, and was hardly observed in any of the areas in early February. The high abundanceof this species caused low concentrations of nutrients and nori production was economically damaged. This is the first record on the bloom of T. diporocyclus inthe eastern Seto Inland Sea. Careful monitoring of this species is necessary toprevent recurrent damage to fisheries in this area.