Cyst formation: an important mechanism for the termination of Scrippsiella trochoidea (Dinophyceae) bloom

A sediment trap study was conducted at Daya Bay, South ChinaSea, to investigate the relationships between encystment andpopulation dynamics of Scrippsiella trochoidea from December1999 to January 2001. A dense bloom of S. trochoidea occurredduring the study period from August to September 2000, withthe maximum cell number of 3.18 x 10⁴ cells mL^–1.Two morphotypes of cysts, one with a thick calcareous wall (calcifiedcyst) and another without the obvious calcareous cover (non-calcifiedcyst), were observed during this investigation. The morphologicaland excystment characteristics of these two cyst types werestudied as well. Mass encystments of S. trochoidea, with themaximum of 3.05 x 10⁵ cysts m^–2 d^–1for calcified cyst, and 1.54 x 10⁷ cysts m^–2 d^–1for non-calcified cyst, coincided with the maximum abundanceof the vegetative cells. Encystment caused the transfer of atotal of 2.24–4.49 x 10⁸ cells m^–2 vegetativecells from the water column to the sea bottom during the bloomand resulted in a considerable loss of the bloom population.High assemblages of cysts of S. trochoidea were detected inthe surface sediments as well. This rich ‘seed bed’in the surface sediments caused by the high efficiency of encystmentafter blooms acting as a benthic reservoir for future vegetativepopulation, together with the short dormant period (15–26days) and high germination rate (50–90%), may explainthe repeated occurrence of S. trochoidea blooms in Daya Bay.