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

A bloom of the colonial stage of the prymnesiophyte Phaeocystispouchetii was studied for 2 months in a 1^3-m3 flow-through mesocosm.Phaeocystis increased in abundance for 6 weeks coincident withdeclining temperature and nutrient supply rates. Experimentssuggested that colony growth was primarily nitrogen-limitedduring this period. An extended period of subzero temperaturesand nutrient deprivation was associated with a mass exodus ofcells from the colonies. Previously non-motile cells developedflagella, became motile and emigrated out of the colonies, accompaniedby significant decreases in the chlorophyll a content and photosyntheticrates of the colonies. Concentrations of bacteria on the surfacesof such ‘ghost’ colonies were two orders of magnitudehigher than on ‘normal’ colonies. Growth rate studiesof field populations indicated that rapid declines in temperatureinduced development of motility and emigration from the colonies.Ancillary observations implied that chronic nutrient deprivationresulted in similar life-cycle events. Warming and nutrientaddition did not halt release of swarmers, suggesting that,once initiated, the process proceeds to completion. The combineddata indicate that blooms of colonial Phaeocystis, unlike manyother phytoplankton, are not necessarily terminated by grazingor sinking out of the euphotic zone. The physiological optionof motility and emigration provides Phaeocystis with an ecologicalalternative which has significant implications in interpretingthe structure and function of plankton communities.