Behavior of Peridinium bipes (Dinophyceae) resting cysts in the Asahi Reservoir

The role of resting cysts includes short- and long-term survival under extreme conditions, bloom initiation, species dispersal, reproduction, and preservation of genetic variation. Accordingly, it is important to understand their behavior in a water environment, especially in lakes and reservoirs where dinoflagellate blooms are observed. In this study, we estimated the behavior of the Peridinium bipes cysts in the Asahi Reservoir using laboratory experiments and field surveys. It was observed that the amount of light strongly influenced excystment, and few cysts germinated under dark conditions in the laboratory experiment. The minimum temperature on excystment was inferred from the laboratory experiment and field surveys to be about 5°C. Although the frequency of excystment did not depend on the water temperature from 10° to 20°C, the average preparation period for excystment decreased with the increase of water temperature. In the Asahi Reservoir, the excystment was estimated to occur during the months of March and April. If excystment did not occur in spring, the dinoflagellate bloom was not be observed until about July, although the bloom often began to appear in about May in the Asahi Reservoir. Consequently, the blooming season in the Asahi Reservoir is affected by the biomass of the germinated cysts in spring. Received: August 31, 2000 / Accepted: March 1, 2001