Features of the Annual Appearance of Gymnodinium mikimotoi and Mechanism of Expansion of its Distribution in the Western Seto Inland Sea, Japan

Abstract

The dinoflagellate Gymnodinium mikimotoi Miyake et Kominami ex ODA is responsible for prodigious red tides in western Japan. To clarify the mechanism by which the distribution of this organism expands and to examine associated environmental features, weekly observations were made at 37 stations in the Suo-Nada and the Iyo-Nada, in the western Seto Inland Sea, during May through August from 1989 to 1991. The initial appearance of G. mikimotoi was observed in the southwestern region of the Suo-Nada. This region was characterized by high water temperature and low salinity in comparison with the surrounding areas. In 1991, high cell densities of G. mikimotoi were observed in this region following the occurrence of low oxygen concentrations in the bottom layer in July. This increased growth of G. mikimotoi may have been due to the increase in dissolved inorganic nitrogen concentrations which accompanied the decrease in oxygen concentrations. The motile cells of G. mikimotoi in the southwestern region of Suo-Nada were introduced to other areas through diffusion of the upper water mass as a consequence of the breakdown of the stratification in this region, resulting in the expansion of the distribution of G. mikimotoi in the western Seto Inland Sea. It was also determined that the northern coast of the Suo-Nada is also a possible area for the initial appearance of this species. The cells that first appear in the Iyo-Nada may originate from the Suo-Nada, the eastern coast of Yamaguchi prefecture, and/or the southwestern area of Hiroshima Bay.