Development and collapse of a Gymnodinium mikimotoi red tide in the Seto Inland Sea

Abstract

A red tide due to Gymnodinium mikimotoi (Dinophyceae) occurred at Harima-nada, the Seto Inland Sea (Japan), in summer 1995. Throughout the development and collapse of the red tide, abundance of G. mikimotoi and its potential predators (heterotrophic dinoflagellates and ciliates) was monitored daily together with environmental variables. While a nitrate and phosphate cline was present at around 15 m, G. mikimotoi concentrations increased rapidly for about a week in the subsurface layer (5 to 15 m) . Then the population suddenly accumulated at the surface layer in the daytime and formed a red tide (ca 3 × 103 ml-1). Following the red tide formation, the abundance of naked heterotiophic dinoflagellates (h-dinos) started to increase. Subsequently, the abundance of the tintinnid ciliate Favella ehrenbergii increased (ca 10 ml-1) and apparently consumed G mikimotoi and h-dinos completely within a few days. On the basis of these results, the ecological differences between G. mikimotoi and Chattonella antiqua (Raphidophyceae; another representative red tide forming species in the Seto Inland Sea) are discussed, as well as the importance of microzooplankton grazing for the collapse of red tides.