Seasonal relationships between cyst germination and vegetative population of Scrippsiella trochoidea (Dinophyceae)

Abstract

The seasonal occurrence of vegetative cells and cysts of the dinoflagellate Scrippsiella trochoidea in the water column was investigated in Yongil Bay (southeastern coast of Korea). To measure germination ratios of cysts, cysts were isolated monthly from natural sediment samples and incubated in the laboratory. Vegetative cell numbers peaked in June to July, when the surface water temperature increased to over 18C. Mass encystments were detected in the water column in August 1996 and July 1997, when the vegetative population flourished. Active germination was observed during the period of decreasing water temperature in September and October, when the vegetative population declined. Thus, there was an opposing pattern of seasonality in the potential germination of cysts and the proliferation of vegetative cells, whereby decreases in one paralleled increases in the other. The dormant period of cysts was ca 60 d, far longer than reported previously. Germination ratios increased in October 1996 and September 1997 after a 2 mo dormancy period following the mass encystments. Germination appears to be flexible with respect with length of dormancy and the age composition of cysts, both of which are based on the time and the scale of encystments as well as on conditions in the benthic environment.