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

Abstract

The role of cyst germination as a factor in bloom initiation was investigated for the dinoflagellate Scrippsiella hangoei (Schiller) Larsen from the northern Baltic Sea. This species blooms in very cold, often ice-covered waters, and is responsible for a significant fraction of the production in that region. Dormancy, temperature, oxygen and light were studied as factors potentially controlling the germination of S. hangoei resting cysts. Laboratory-stored and field-collected cysts began to germinate in December following a mandatory dormancy period lasting 6 months. Germination after this maturation interval was maximal when temperatures were within a 0-9°C 'window'. Mandatory dormancy is therefore the primary factor regulating the timing of germination in this species, as temperatures in the natural environment normally fall within this range at the time when S. hangoei cysts deposited the preceding year have matured. Non-optimal temperatures, darkness and low oxygen conditions all maintain a state of quiescence in the cysts. Cysts could germinate in darkness, but the rate of excystment was significantly higher in the light. Likewise, excystment was completely inhibited in anoxic conditions and was reduced under severe hypoxia, with normal germination under moderate hypoxic concentrations. Temporary exposure to high sulfide concentrations permanently reduced germination potential, indicating that S. hangoei cysts have low resistance to oxygen deficiency. Prolonged periods 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 magnitude of the inoculum for S. hangoei bloom initiation. Together, these internal and external regulatory factors play important roles in the bloom dynamics of this important dinoflagellate.