Ecological investigations of blooms of colonial Phaeocystis pouchetti. II. The role of life-cycle phenomena in bloom termination

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Abstract
A bloom of the colonial stage of the prymnesiophyte Phaeocystis pouchetii was studied for 2 months in a 1 3-m 3 flow-through mesocosm. Phaeocystis increased in abundance for 6 weeks coincident with declining temperature and nutrient supply rates. Experiments suggested that colony growth was primarily nitrogen-limited during this period. An extended period of subzero temperatures and nutrient deprivation was associated with a mass exodus of cells from the colonies. Previously non-motile cells developed flagella, became motile and emigrated out of the colonies, accompanied by significant decreases in the chlorophyll a content and photosynthetic rates of the colonies. Concentrations of bacteria on the surfaces of such ‘ghost’ colonies were two orders of magnitude higher than on ‘normal’ colonies. Growth rate studies of field populations indicated that rapid declines in temperature induced development of motility and emigration from the colonies. Ancillary observations implied that chronic nutrient deprivation resulted in similar life-cycle events. Warming and nutrient addition did not halt release of swarmers, suggesting that, once initiated, the process proceeds to completion. The combined data indicate that blooms of colonial Phaeocystis , unlike many other phytoplankton, are not necessarily terminated by grazing or sinking out of the euphotic zone. The physiological option of motility and emigration provides Phaeocystis with an ecological alternative which has significant implications in interpreting the structure and function of plankton communities.

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