Life cycle of Calanus finmarchicus in the lower St. Lawrence Estuary: the imprint of circulation and late timing of the spring phytoplankton bloom

The life cycle of Calanus finmarchicus in the lower St. Lawrence estuary is described based on observations of female egg production rate, population stage abundance, and chlorophyll a biomass collected over 7 years (1991-199 7) at a centrally located monitoring station. The mean seasonal pattern sho ws maximum abundance of females in May, but peak population egg production rate and naupliar (N3-N6) abundance occur in early July just after onset of the late spring - early summer phytoplankton bloom. The population stage s tructure is characterized by low summer abundance of early copepodite stage s C1-C3 and high stage C5 abundance in autumn. Between 1994 and 1997, there was important interannual variation in both timing (up to 1 month) and amp litude (five- to eight-fold) of population reproduction. Patterns of season al increase of C5 abundance in autumn suggest interannual variations of bot h timing and magnitude of deep upstream advection of this overwintering sta ge. Thus, the main features of C. finmarchicus population dynamics in the c entral lower St. Lawrence Estuary are (i) late reproduction resulting from food limitation prior to the onset of the summer phytoplankton bloom, (ii) probable export of early developmental stages during summer, and (iii) adve ction into the central lower St. Lawrence Estuary of overwintering stage C5 in autumn from downstream regions. These results support the hypothesis th at circulation, mainly driven by discharge from the St. Lawrence River and its tributaries, is a key factor governing population dynamics of C. finmar chicus in this region.