Modeling the bloom dynamics of the polymorphic phytoplankter Phaeocystis globosa: impact of grazers and viruses

A simulation model of a Phaeocystis globosa bloom in a large volume mesocosm has been constructed to reproduce the interactions between different cell-morphs of P. globosa as controlled by virus infection, grazing and nutrient depletion. Besides a detailed description of the processes structuring the microbial loop, a description of viral infection of P. globosa was included into the model. The model also describes P. globosa colony size class distribution, and the consequences for nutrient uptake and viral infection of the cells embedded in the colony. The model is calibrated thoroughly with a large set of data from the mesocosm experiment. With the model we can show that the enhanced growth rates, the low viral infection rate and the low grazing rate on cells in colonies, as compared to free-living single cells of P. globosa, result in a massive blooming of P. globosa colonies. However, when the controlling nutrient becomes depleted, the colonies disintegrate and liberate single colonial cells that are subject to high rates of viral infection and grazing. If the growth conditions are such that colony formation is limited or even prevented, the significance of the viral control is enhanced. The model demonstrated that the appearance of P. globosa colonies indirectly enhances single cell survival during periods of high concentrations of viruses as transparent exopolymeric particles (consisting of dead cells and mucus) decrease the effectiveness of virus infection rates.