Factors regulating germination of resting cysts of the spring bloom dinoflagellate Scrippsiella hangoei from the northern Baltic Sea

The role of cyst germination as a factor in bloom initiation was investigat ed for the dinoflagellate Scrippsiella hangoei (Schiller) Larsen from the n orthern Baltic Sea. This species blooms in very cold, often ice-covered wat ers, and is responsible for a significant fraction of the production in tha t region. Dormancy, temperature, oxygen and light were studied as factors p otentially controlling the germination of S. hangoei resting cysts. Laborat ory-stored and field-collected cysts began to germinate in December followi ng a mandatory dormancy period lasting 6 months. Germination after this mat uration interval was maximal when temperatures were within a 0-9 degrees C 'window'. Mandatory dormancy is therefore the primary factor regulating the timing of germination in this species, as temperatures in the natural envi ronment normally fall within this range at the time when S. hangoei cysts d eposited the preceding year have matured. Non-optimal temperatures, darknes s and low oxygen conditions all maintain a state of quiescence in the cysts . Cysts could germinate in darkness, but the rate of excystment was signifi cantly higher in the light. Likewise, excystment was completely inhibited i n anoxic conditions and was reduced under severe hypoxia, with normal germi nation under moderate hypoxic concentrations. Temporary exposure to high su lfide concentrations permanently reduced germination potential, indicating that S. hangoei cysts have low resistance to oxygen deficiency. Prolonged p eriods of anoxia at the sediment surface, as frequently occurs in the study area, might reduce the size of the viable cyst pool and thus, alter the ma gnitude of the inoculum for S. hangoei bloom initiation. Together, these in ternal and external regulatory factors play important roles in the bloom dy namics of this important dinoflagellate.