Relationships between dynamics of red tide‐causing raphidophycean flagellates and algicidal micro‐organisms in the coastal sea of Japan

SUMMARY Temporal fluctuations of algicidal micro‐organisms against the red tide causing raphidophycean flagellates Chattonella antiqua (Hada) Ono and Heterosigma akashiwo (Hada) Hada ex Hara et Chihara were investigated using the microplate most probable number (MPN) method in northern Hiroshima Bay and Harima‐Nada, the Seto Inland Sea, in 1992 and 1993. In Har‐ima‐Nada, both flagellates appeared at low levels (< 1 cell mL −1 ), and killer micro‐organisms against the two flagellates (C‐killer for C. antiqua and H‐killer for H. akashiwo ) also appeared at low densities (< 2 mL −1 ). In northern Hiroshima Bay, C. antiqua cells were scarce (< 1 cell mL −1 ), and C‐killers occurred at a low level (≤ 3.4 mL −1 ). Conversely, red tides of H. akashiwo occurred there in June of both years. The dynamics of H‐killers revealed a close relationship with that of H. akashiwo populations. H‐killers followed the increase of H. akashiwo cells, reached a maximum level after the beginning of decline of H. akashiwo , maintained a high level for at least 1 week after the crash of bloom, and then decreased. C‐killers consistently remained at low densities during the period of H. akashiwo red tides in both 1992 and 1993. Hence, algicidal micro‐organisms specifically associated with the occurrence and crash of H. akashiwo red tides, and presumably contributed to the rapid termination of the red tides in the coastal seas such as northern Hiroshima Bay.