Modelling the life cycle of dinoflagellates: a case study with Biecheleria baltica

The cold-water dinoflagellate Biecheleria baltica has increasingly dominated the phytoplankton spring bloom in the Baltic Sea during the past years. Life cycle transitions between bloom forming cells and resting cysts are assumed to regulate the bloom dynamics of this species. We investigate the seasonal cycle and succession of Biecheleria balticas life cycle stages using a numerical model with four different stages, vegetative cells, gametes, resting cysts and germinating cells. The transitions among the stages are functions of environmental conditions and endogenous factors. Coupled to a water column model, the model is able to represent the seasonal cycle of Biecheleria baltica with two blooms in spring. The first bloom can be explained by germination of resting cysts in winter, the second by growth of vegetative cells. Sensitivity experiments indicate that temperature is an important factor regulating the composition of Biecheleria baltica life cycle stages; increased or decreased temperature leads to fewer growing cells and more resting cysts during spring. Our newly developed life cycle model can be used to study in more detail cyst formation, cyst distribution and consequences for biogeochemical cycling in the past and future