Importance of cysts in the population dynamics of the red tide flagellate Heterosigma akashiwo (Raphidophyceae)

To elucidate roles of cysts in occurrences of Heterosigma akashiwo blooms, cyst dynamics were studied in northern Hiroshima Bay, the Seto Inland Sea o f Japan, where H. akashiwo regularly forms red tide in June. Monthly measur ements of seasonal changes in the densities of vegetative cells of H. akash iwo and their germinable cysts in surface sediments (top 1-cm layer) were m ade for 2 years at three stations. Vegetative cells of H. akashiwo could be detected from April through December throughout the water column, and the existence of vegetative cells was confirmed in surface waters even in winte r after incubation of sampled seawater in culture medium. Germinable cysts, enumerated by the extinction dilution method, existed in sediments in all seasons, even before and after the seasonal bloom. The effects of incubatio n temperature on the germination of natural cysts of H. akashiwo in sedimen ts were examined. Germination was not observed at 5 degrees C, was low at 1 0 degrees C, while it increased at 15 degrees C, and maintained a high leve l to 25 degrees C. The bottom water temperature reached 15 degrees C (suita ble for the germination of cysts) and the surface about 18 degrees C or mor e (suitable for the growth of vegetative cells) 2 to 3 weeks before the blo oms. The dark survival of H. akashiwo cysts was tested, and it was found th at the cysts were viable for at least 650 d at 11 degrees C, and for 165 d at 25 degrees C, indicating a significant role of cysts in the survival dur ing winter and summer seasons. The cysts presumably also play an important role in seeding primary populations into water columns when the bottom wate r reaches a suitable temperature (around 15 degrees C); thereafter the popu lations develop with great annual regularity to bloom in June. These result s suggest that initiation of H. akashiwo red tides in the Seto Inland Sea c ould be triggered by bottom water temperature.