Sexuality and cyst formation of the spring-bloom dinoflagellate Scrippsiella hangoei in the coastal northern Baltic Sea

The temporal sequence and the magnitude of the sexual reproduction and subs equent cyst deposition of the common spring-bloom dinoflagellate Scrippsiel la hangoei (Schiller) Larsen was studied during spring 1996 on the SW coast of Finland, Baltic Sea. The abundances of the different size of fractions of S. hangoei (14 to 18 mu m, 18 to 22 mu m and >22 mu m) were monitored in the water column, and the deposition of resting cysts was measured using m oored sediment traps. Cyst sedimentation rates were measured throughout the seasonal cycle in order to estimate cyst resuspension rates for the quanti tative assessment of the fraction of population undergoing encystment. The onset of sexual reproduction, indicated by a significant increase of the sm all cells (14 to 18 mu m) representing gametes, occurred in a nutrient repl ete environment well before the exponential growth phase and peak abundance s of vegetative cells. Gamete formation was followed by high abundances of large cells (>22 mu m) representing planozygotes, and subsequent sedimentat ion of resting cysts. Approximately 60% of the asexually growing bloom popu lation was estimated to form planozygotes, suggesting that encystment was a n important factor in bloom termination and possibly plays a role in the re gulation of the magnitude of the bloom. Finally encystment accounted for 40 % of the entire S. hangoei population, resulting in a considerable loss of the bloom population and an input of the vernal phytoplankton biomass to th e benthos.