Characteristics of a Red Tide due to the Harmful DinoflagellateKarenia mikimotoi Occurred in Usuki Bay, Oita Prefecture, in 2003

Abstract

The dinoflagellate Karenia mikimotoi (Miyake et Kominami ex Oda) Hansen is responsible for extensive red tides in western Japan. In order to clarify the mechanism of occurrence of K. mikimotoi red tides in Usuki Bay which is located in the north-western part of Bungo Channel, Japan, field surveys were made in Usuki Bay and adjacent areas during May through September 2003. The initial appearance of K. mikimotoi red tide was observed in Suo-Nada, then the red tide was observed Iyo-Nada, Beppu Bay, and Usuki Bay. The distribution of it extended southward gradually. From result of analysis of the surface water temperature by satellite, the distribution of the red tide area corresponded with the warmer water mass (>25°C). Red tide outbreak in Usuki Bay, accompanied the warmer water mass intrusion into Usuki Bay from Beppu Bay and Iyo-Nada. Furthermore, the red tide of K. mikimotoi in Usuki Bay in August 2003 may have originated from Suo-Nada and warmer, water mass with the red tide intrusion in Usuki Bay.