The temporal dynamics of the flagellated and colonial stages of Phaeocystis antarctica in the Ross Sea

Phaeocystis antarctica in the Ross Sea forms colonies, and blooms of colonial P. antarctica often occur over large areas in the southern Ross Sea. Sites where colonies occur often have significant vertical fluxes of carbon in the form of aggregated and flocculent material. P. antarctica also is a key component of the sulfur cycle; therefore, the species is critically important in many of the biogeochemical cycles in the Ross Sea. Despite this fundamental role, the life history and temporal dynamics of this species are poorly known. This study investigated the contribution of solitary, flagellated forms and colonial cells of P. antarctica to phytoplankton abundance and autotrophic carbon, and the factors that might control the relative importance of these two morphological forms. Solitary P. antarctica cells numerically dominated the phytoplankton assemblage early in austral spring, although colony formation occurred almost immediately upon the onset of net population growth. The percentage of solitary cells relative to total cells (colonial+solitary) was high in early austral spring but decreased to a minimum during late spring; specifically, nearly 98% of the P. antarctica cells were in colonies in late spring, coinciding with the seasonal chlorophyll maximum. Significant phytoplankton mortality rates were positively correlated with high ratios of solitary: total P. antarctica cells. Highest mortality rates were observed during austral spring when solitary cells dominated the P. antarctica population. The abundance of solitary P. antarctica cells began to increase again during late summer, but colonial cell numbers were always greater than those of solitary cells during this period. This increase in the contribution of solitary forms during summer may have been a consequence of more severe micronutrient limitation for the colonies (relative to solitary cells), life history processes of P. antarctica, reduced microzooplankton grazing at that time, or a combination of these and other factors. We conclude that the relative abundance of solitary and colonial forms of this prymnesiophyte alga may be a consequence of seasonal changes in these factors. The outcome of these interactions affects the contribution of this alga to the vertical flux of carbon and the degree to which P. antarctica participates in the microbial food web of the Ross Sea. r 2003 Elsevier Science Ltd. All rights reserved.