Reproduction of the planktonic copepod Calanus finmarchicus in the Lower St. Lawrence Estuary: relation to the cycle of phytoplankton production and evidence for a Calanus pump

Due to the dynamic physical environment of the Lower St. Lawrence Estuary, the spring phytoplankton bloom in the Laurentian Channel occurs late in the season, typically in mid-June, but the high phytoplankton biomass is sustained throughout the summer months. In this study, relationships between the phytoplankton production cycle, water temperature, and the reproductive cycle of Calanus finmarchicus Gunnerus, a predominant planktonic copepod in the Lower Estuary, were investigated during spring–summer 1991. Field observations showed that the final stages of oocyte maturation in C. finmarchicus females did not begin until the onset of the spring phytoplankton bloom in mid- June. High egg production rates, as estimated by the number of eggs released by females incubated immediately after capture, commenced 1 wk later and persisted until late August. Egg production rates were significantly correlated with an index of gonadal maturity in females and were consistent with a rectilinear or curvilinear relationship with chlorophyll a standing stock. Laboratory experiments showed that: (1) in presence of food (the diatom Thalassiosira weissfloggii), maturation of oocytes would proceed and females could spawn eggs at least 2 mo before the spring bloom: (2) without food, the oocytes did not develop past immature stages, except in a small minority of the population; and (3) colder temperatures in early spring would prolong the lag between the onset of the spring bloom and the start of egg production by less than 4 d. Combined with concurrent microscopic measurements of oil sac volume, the results do not rule out the possibility that lipid reserves were used to support the early stages of oogenesis, but do show that the majority of females did not use lipid reserves for vitellogenesis prior to the spring phytoplankton bloom. It is suggested that the Lower St. Lawrence Estuary is an important region of C. finmarchicus production in summer which, because of the residual surface circulation, may act as a Calanus 'pump' to influence levels of Zooplankton biomass in the Gulf of St. Lawrence and on the shelf off Nova Scotia.